Physics Aspects of Quality Control in Radiotherapylcr.uerj.br/Manual_ABFM/IPEM_Report_81 physics... · Fairmount House, 230 Tadcaster Road York YO24 1ES, England Legal Notice This

Physics Aspects ofQuality Control in

Radiotherapyedited by

W.P.M. Mayles, R. Lake, A. McKenzie, E.M. Macaulay,H.M. Morgan, T.J. Jordan and S.K. Powley

© The Institute of Physics and Engineering in Medicine 1999Fairmount House, 230 Tadcaster Road

York YO24 1ES

ISBN 0 904181 91 X

All rights reserved. No part of this publication may be reproduced, stored in a retrievalsystem or transmitted in any form or by any means, electronic, mechanical, photocopying,recording or otherwise, without the prior permission of the publisher.

Published by the Institute of Physics and Engineering in MedicineFairmount House, 230 Tadcaster Road

York YO24 1ES, England

Legal Notice

This report was prepared and published on behalf of the Institute of Physics andEngineering in Medicine (IPEM). Whilst every attempt is made to provide accurate anduseful information, neither the IPEM, the members of IPEM or other persons contributingto the formation of the report make any warranty, express or implied, with regard toaccuracy, omissions and usefulness of the information contained herein. Furthermore,the same parties do not assume any liability with repect to the use, or subsequent damagesresulting from the use, of the information contained in this report.

Preface

This book aims to provide a reference text to cover quality control procedures that maybe used as part of a quality assurance programme in Radiotherapy. It does not purportto deal with the quality assurance system itself, although to set it in context a summaryof the requirements of a quality system that will conform to the report of the BleehenCommittee (Quality Assurance in Radiotherapy, Report of a Working Party of theStanding Subcommittee on Cancer of the Standing Medical Advisory Committee, May1991) and also ISO 9000 or BS 5750 is given in Chapter 2. In Chapter 1 someconsiderations are set out which should form the basis of judgements made aboutappropriate tolerances for the parameters being measured.

The recommendations are based on the results of the survey carried out by theRadiotherapy Topic Group in 1992 and published in Scope in 1992. A summary of thisreport is included in Appendix B. Recommendations of frequencies of checks thereforerepresent the consensus of a wide body of physicists practising in the UK. Where thesurvey did not produce a satisfactory consensus the working party has made its ownrecommendations. A list of the many people who have contributed to the book is givenat the front.

One of the questions on the questionnaire was whether this book should make firmrecommendations or merely give advice. There was a substantial majority in favour ofthe former. However, the working party felt that there were many situations in whichthe appropriate action depended on the local situation. In order to distinguish statementsthat are considered mandatory the word ‘must’ has been used. ‘Should’ is used to indicatesomething that is considered desirable. The responsible local physicist may choosedifferent frequencies and tolerances from those contained in this publication, but insuch circumstances should be able to justify the changes made.

Contents

PagePreface

1 Quality Assurance and its Conceptual Framework (Editor: W P M Mayles) 11.1 Clinical background (H J Dobbs) 1

1.1.1 Outline of the radiotherapy process 11.1.2 Positional and dosimetric accuracy 31.1.3 Clinical evidence for the importance of accuracy 41.1.4 Clinical limitations 5

1.1.4.1 Uncertainties in location of target volume 51.1.4.2 Dose specification 6

1.2 Accuracy in radiotherapy as the basis for Quality Assurance standards 7(D I Thwaites)

1.2.1 Requirements for accuracy 71.2.1.1 Definition of accuracy 71.2.1.2 Requirements for dose accuracy 81.2.1.3 Accuracy of dose distribution 91.2.1.4 Geometric accuracy 91.2.1.5 Summary of accuracy requirements 9

1.2.2 Accuracy achievable 101.2.2.1 Standards 101.2.2.2 Practice 121.2.2.3 Uncertainty estimates 121.2.2.4 Experimental investigations of achievable accuracy 121.2.2.5 Patient dose and position measurements 13

2 Systematic Approaches to Quality Assurance and Audit (Editor: W P M Mayles) 202.1 Introduction 202.2 ISO 9000 and QART (J Garrett) 20

2.2.1 Background to the report 202.2.2 Process of adaptation 212.2.3 Basis of accreditation 212.2.4 Level of application 222.2.5 Interpretation of ISO 9002 222.2.6 Minimising the cost of application 23

2.3 Implementation of QART (S Unwin) 232.3.1 Where to start 232.3.2 Requirements affecting overall management 242.3.3 Requirements affecting the patients 242.3.4 Requirements relating to equipment 252.3.4 Requirements relating to monitoring the system 252.3.5 Summary 25

2.4 Level of detail required in physics quality systems (A McKenzie) 26

2.5 Interdepartmental audit and other methods (R J Aukett, D E Bonnett 27and J A Mills)

2.5.1 Dosimetry audit: mailed programmes 282.5.2 Central audit 292.5.3 Interdepartmental audit 30

3 Radiotherapy Imaging Devices (Editor: S K Powley) 343.1 Treatment simulators (J B Tuohy) 34

3.1.1 Introduction 343.1.2 Description of tests 343.1.3 Mechanical aspects 36

3.1.3.1 Crosswires 363.1.3.2 Optical distance indicator (range-finder) 373.1.3.3 Isocentric lasers 373.1.3.4 Field defining wires 373.1.3.5 Daily checks of crosswires, range-finder, lasers and

field defining wires 383.1.3.6 Couch isocentric rotation 383.1.3.7 Couch vertical movement 393.1.3.8 Couch position readouts 393.1.3.9 Couch longitudinal and lateral translation under load 393.1.3.10 Image intensifier carriage 393.1.3.11 Gantry angle/diaphragm angle 393.1.3.12 Alignment of shadow trays 39

3.1.4 Imaging system 393.1.4.1 Fluoroscopy system 393.1.4.2 Radiographic system 41

3.1.5 Security, safety devices and interlocks 423.1.6 Equipment required 423.1.7 Summary of test frequencies 43

3.2 CT equipment (E Thomson and S Edyvean) 433.2.1 CT scanner alignment checks 44

3.2.1.1 Alignment of internal light beam/laser with scan plane 443.2.1.2 Indication of x-axis 443.2.1.3 Couch position registration 453.2.1.4 Couch deflection under load 45

3.2.2 CT related acceptance tests on treatment planning computer 463.2.2.1 Contouring and auto-contouring 463.2.2.2 Image display on TPS 47

3.2.3 Routine quality control tests on CT images following transfer to the treatment planning computer 473.2.3.1 Distance between known points in image plane 483.2.3.2 Left and right registration 483.2.3.3 CT number/electron density verification 48

3.2.4 Acceptance tests for spiral scanners 493.2.4.1 Reconstructed slice location 49

3.2.5 Summary of CT scanner related tests 49

3.3 Magnetic resonance imaging (J P Ridgway and A Moore) 503.3.1 Introduction 503.3.2 Test objects and materials 503.3.3 Geometric accuracy (magnification) and distortion 513.3.4 Slice position 523.3.5 Slice warp 533.3.6 Signal-to-noise ratio and image uniformity 533.3.7 Image contrast 543.3.8 Suggested frequency of measurements 54

3.4 Simulator CT (I Green) 553.4.1 Introduction 553.4.2 Auto set-up positions 553.4.3 Physical inserts 563.4.4 Radiation field alignment 563.4.5 Software files 563.4.6 Computer hardware 573.4.7 Image distortion and CT numbers 57

4 Treatment Planning (Editor: H M Morgan) 604.1 Introduction (H M Morgan) 604.2 Beam data acquisition for treatment planning (J Byrne and G D Lambert) 60

4.2.1 Equipment for beam data acquisition 614.2.1.1 Detectors 614.2.1.2 Measurement phantoms 62

4.2.2 Data for photon beams 624.2.2.1 Data for entry into the TPS 634.2.2.2 Reference data – for TPS validation 65

4.2.3 Data for electron beams 674.2.3.1 Data for entry into the TPS 674.2.3.2 Reference data – for TPS validation 68

4.2.4 Brachytherapy 684.2.4.1 Data for TPS entry 694.2.4.2 Algorithm and basic data validation 69

4.3 Commissioning treatment planning systems (J Conway and N Whilde) 704.3.1 Introduction 704.3.2 Testing the TPS 71

4.3.2.1 General considerations 714.3.2.2 Basic requirements 714.3.2.3 Criteria for acceptance 724.3.2.4 Photon beam tests 724.3.2.5 Electron beam tests 764.3.2.6 Brachytherapy tests 76

4.3.3 Documentation of results 764.4 Quality control of the treatment planning system (H M Morgan) 77

4.4.1 Introduction 77

4.4.2 Hardware 794.4.2.1 Processor tests 794.4.2.2 Digitiser and plotter 794.4.2.3 Visual display unit 80

4.4.3 CT image handling 804.4.4 Software 81

4.4.4.1 Consistency of data sets 814.4.4.2 Test fields 814.4.4.3 Monitor unit calculations 824.4.4.4 Reference plans 82

4.4.5 Housekeeping 824.4.5.1 Viruses 824.4.5.2 Backing-up 824.4.5.3 Archiving 83

4.5 The treatment planning process (W P M Mayles) 844.5.1 Simulator films 844.5.2 CT scans 854.5.3 MRI 854.5.4 Target definition from X-ray films 864.5.5 Dose prescription 864.5.6 Preparation for treatment plan computation 864.5.7 Treatment plan quality control 87

4.5.7.1 Geometric accuracy 874.5.7.2 Conformance to specification 874.5.7.3 Plan optimisation 884.5.7.4 Qualitative assessment of the dose distribution 884.5.7.5 Consideration of the dose distribution in off-axis slices 884.5.7.6 Tissue inhomogeneity corrections 884.5.7.7 Corrections for beam modifiers 884.5.7.8 Machine monitor unit settings 884.5.7.9 Setting-up instructions 88

4.5.8 Transfer of data to treatment unit 894.6 Conformal Radiotherapy 89

4.6.1 Introduction 894.6.2 Geometric accuracy 894.6.3 Field shaping 904.6.4 Transfer of isocentre to the treatment machine 904.6.5 Dose calculation 904.6.6 Treatment verification 914.6.7 Intensity modulated therapy 91

4.7 Brachytherapy plan checking 914.7.1 Activities 914.7.2 Scatter and attenuation corrections 914.7.3 Accuracy of digitisation 924.7.4 Manual calculation 92

5 Megavoltage Equipment (Editor: W P M Mayles) 965.1 Introduction (M P Casebow and W P M Mayles) 96

5.1.1 General 965.1.2 Responsibilities 965.1.3 Tolerances 975.1.4 Level of checks 98

5.2 Checks on standard linear accelerators 995.2.1 Safety interlocks 99

5.2.1.1 Room protection 995.2.1.2 Movement interlocks 1005.2.1.3 Dosimetry interlocks 1005.2.1.4 Beam steering 1005.2.1.5 Filter interlocks 100

5.2.2 Indicator lights 1015.2.3 Mechanical integrity 101

5.2.3.1 Couch deflection under load 1015.2.3.2 Electrical safety 101

5.2.4 Mechanical alignment checks 1015.2.4.1 Mechanical isocentre – definitive checks 1015.2.4.2 Couch rotation axis 1035.2.4.3 Mechanical isocentre – quick checks 1035.2.4.4 Distance indicator 1055.2.4.5 Front pointer and back pointer 1055.2.4.6 Calibration of gantry and collimator rotation scales 105

5.2.5 Position of light source 1055.2.6 Optical field indication 105

5.2.6.1 Quick check 1055.2.6.2 Variation with field size 1065.2.6.3 Variation with collimator rotation 1065.2.6.4 Adjustment of optical indication 107

5.2.7 Shadow tray 1075.2.8 Couch movements 1075.2.9 Radiation alignment 107

5.2.9.1 Alignment of radiation and light field 1075.2.9.2 Alignment of radiation field – quick check 1085.2.9.3 Radiation isocentre 108

5.2.10 Interpretation of alignment checks 1105.2.11 Flatness and symmetry 111

5.2.11.1 Flatness scans 1115.2.11.2 Quick checks of beam flatness 112

5.2.12 Radiation output measurement 1135.2.12.1 Definitive calibration 1135.2.12.2 Routine calibration 1135.2.12.3 Constancy checks 1135.2.12.4 Effect of gantry rotation 1145.2.12.5 Wedge factors 1145.2.12.6 Output factors 1155.2.12.7 Depth dose and profiles 1155.2.12.8 Other checks 115

5.2.13 Beam energy 1165.2.14 Arc therapy 1165.2.15 Selection of checks and check frequencies 118

5.2.15.1 Documentation 1195.2.16 Equipment required 119

5.3 Required tests for electron beams (A McKenzie) 1195.3.1 Introduction 1195.3.2 Description of tests 120

5.3.2.1 Output calibration check 1205.3.2.2 Constancy check 1215.3.2.3 Energy check 1215.3.2.4 Beam symmetry and flatness 1225.3.2.5 Applicator factors 1245.3.2.6 Applicator interlocks 124

5.4 Machine running conditions and their effect on performance (A Iles) 1255.4.1 Performance monitoring 125

5.4.1.1 Electron gun 1255.4.1.2 RF power source 125

5.4.2 Beam control 1265.4.2.1 Input steering 1265.4.2.2 Focus 1265.4.2.3 Output steering 126

5.4.3 Beam bending system 1265.4.4 Recording parameters 126

5.4.4.1 Effects of changes in important parameters 1275.5 Special facilities 127

5.5.1 Asymmetric fields (R Clements) 1275.5.1.1 Acceptance and commissioning 1285.5.1.2 Quality control 128

5.5.2 Swept beam electrons (S Locks) 1295.5.2.1 Commissioning 1305.5.2.2 Collection efficiency 1305.5.2.3 Field non-uniformity 1315.5.2.4 Quality control 132

5.5.3 Computer control and verification (H Porter) 1325.5.3.1 Daily checks before clinical use 1325.5.3.2 Weekly checks 133

5.5.4 Dynamic wedges (M Bidmead and H Porter) 1335.5.4.1 Daily checks 1345.5.4.2 Weekly checks 1345.5.4.3 Monthly checks 1355.5.4.4 Six-monthly checks 1355.5.4.5 Software security 1355.5.4.6 Enhanced dynamic wedge 135

5.5.5 Linear accelerator monitor ion chambers (N Proctor) 1355.5.6 Multileaf collimators (W P M. Mayles) 137

5.5.6.1 Quality control challenges of multileaf collimators 1375.5.6.2 Design of MLC – Elekta design 138

5.5.6.3 Design of MLC – Varian design 1395.5.6.4 Definition of MLC fields 1395.5.6.5 Checks of beam transfer and daily set-up 1405.5.6.6 Alignment of leaf bank to the collimator jaws 1415.5.6.7 Alignment of leaf positions 1415.5.6.8 Alignment of opposing leaves 1415.5.6.9 Field alignment away from the central axis 1415.5.6.10 Relationship between optical field and radiation field 1425.5.6.11 Stability of leaf positions with gantry angle 1425.5.6.12 Library shapes 1425.5.6.13 Interlocks 1425.5.6.14 Leakage between leaves 1425.5.6.15 Check frequencies for multileaf collimators 1435.5.6.16 Dynamic multileaf collimator 143

5.5.7 Stereotactic radiotherapy (D Doughty and A P Warrington) 1445.5.7.1 Introduction 1445.5.7.2 Relocation of the frame 1445.5.7.3 Stereotactic tertiary collimator 1455.5.7.4 Radiation isocentre 1455.5.7.5 Mechanical checks 1455.5.7.6 Safety aspects 1455.5.7.7 Coordinate reconstruction 1465.5.7.8 Treatment planning system 1465.5.7.9 Summary 146

6 Cobalt Teletherapy Units (Editor: A L McKenzie) (J E Saunders) 1506.1 Introduction 1506.2 Description of checks 150

6.2.1 Radioactive contamination 1516.2.2 Radiation leakage 1526.2.3 Source position 1526.2.4 Isocentre 1526.2.5 Output calibration and output checks 1536.2.6 Beam quality and output variation with field size 1546.2.7 Timer and shutter correction 1546.2.8 Interlocks, safety devices and procedures 1556.2.9 Wedges 1566.2.10 Mechanical inspection 1566.2.11 Other checks 1576.2.12 Arc therapy 157

6.3 Test equipment required 1576.4 Statutory requirements and other guidance 1586.5 Recommended frequencies for tests 159

7 Kilovoltage X-ray units (Editor: A L McKenzie) (R M Harrison) 1637.1 Introduction 163

7.2 Description of tests 1637.2.1 Output measurements and output constancy checks 163

7.2.1.1 Medium energy X-rays 1647.2.1.2 Low energy X-rays 1657.2.1.3 Very low energy X-rays 166

7.2.2 Filter interlocks 1667.2.3 Timers and monitor chambers 167

7.2.3.1 Timers 1677.2.3.2 Monitor chambers 168

7.2.4 Beam quality 1687.2.4.1 Quality parameters 168

7.2.5 Field uniformity 1697.2.5.1 Applicators 1697.2.5.2 Units fitted with light beam diaphragms 170

7.2.6 Focal spot measurement 1707.2.7 Interlocks and warnings 1717.2.8 Mechanical fixtures 171

7.3 Dosimetry equipment 1727.4 Recommended frequencies 172

8 Dosimetry Equipment (Editor: E M Macaulay) 1758.1 Introduction 1758.2 Ionisation chambers (E M Macaulay) 175

8.2.1 Introduction 1758.2.2 Initial commissioning 176

8.2.2.1 Leakage current 1778.2.2.2 Ion recombination 1778.2.2.3 Polarity 1778.2.2.4 Stem effect 1778.2.2.5 Linearity 1788.2.2.6 Range check 1788.2.2.7 Angular dependence 1788.2.2.8 Radionuclide stability check 1788.2.2.9 Cross calibration against standard 1798.2.2.10 Second system check 179

8.2.3 Ongoing quality control 1798.2.4 Secondary standard instruments 1798.2.5 Field instruments 1808.2.6 Effective point of measurement of ionisation chambers 180

8.3 TLD and diodes (R J Aukett) 1818.3.1 Introduction 1818.3.2 Uses and limitations 181

8.3.2.1 Thermoluminescent dosimeters 1828.3.2.2 Semiconductors 1828.3.2.3 In vivo and total body irradiation dosimetry 182

8.3.3 Dosimeter commissioning 1838.3.3.1 Thermoluminescent dosimeters 1838.3.3.2 Semiconductors 184

8.3.4 Methods of calibration 1848.3.4.1 Thermoluminescent dosimeters 1848.3.4.2 Semiconductors 187

8.3.5 Frequency of calibration 1888.3.5.1 Thermoluminescent dosimeters 1888.3.5.2 Semiconductors 189

8.4 Plotting tanks (P J Rudd) 1898.4.1 Introduction 1898.4.2 General safety 1908.4.3 Regular checks on the tank and the accuracy of movements 1908.4.4 Checks on radiation detectors and electrometers 1918.4.5 Checks on software by the manufacturer and the user 193

8.5 Constancy instruments (E M Macaulay) 1938.6 Thermometers and barometers 194

8.6.1 Thermometers 1948.6.2 Barometers 1948.6.3 Hygrometers 195

8.7 Ancillary equipment 1958.8 HVL filters 1958.9 Solid phantoms 1968.10 Film dosimetry 196

9 Quality Control in Brachytherapy (Editor: R Lake) 2019.1 Introduction (A M Bidmead, C H Jones, R Lake) 2019.2 Remote afterloading (C H Jones and A M Bidmead) 201

9.2.1 Room design 2029.2.2 Commissioning of equipment 203

9.2.2.1 Source integrity 2039.2.2.2 Radiation protection 2049.2.2.3 Autoradiography and radiography of sources and

applicators 2049.2.2.4 Calibration of brachytherapy sources 207

9.2.3 Routine quality control 2129.2.3.1 Facility testing 2129.2.3.2 Machine function 2139.2.3.3 Source and applicator checks 2139.2.3.4 Radiation safety 2139.2.3.5 Dosimetry 213

9.3 Manual afterloading (A Walsh, E G A Aird and R Lake) 2149.3.1 Commissioning of sources and source trains 215

9.3.1.1 Checking sources on receipt 2159.3.1.2 Calibration of sources 2169.3.1.3 Calibration of radionuclide calibrators 2179.3.1.4 Additional notes concerning source calibrations 217

9.3.2 Commissioning an iridium-192 loader 2189.3.2.1 Checking the operation of the loader 2199.3.2.2 Radiation protection measurements 2199.3.2.3 Instructions to the operators 219

9.4 Commissioning of a treatment planning system forbrachytherapy purposes (A Walsh, E G A Aird and R Lake) 219

9.4.1.1 Examination of the delivered system 2209.4.1.2 Entry of source data into the treatment planning system 2209.4.1.3 Verification of multi-source dose distributions 2209.4.1.4 Quality control procedures 221

9.4.2 Summary of test frequencies 2229.5 Live loading (A Walsh, E G A Aird and R Lake) 2239.6 Control of sources (E G A Aird and A Walsh) 224

9.6.1 Storage of sources 2249.6.1.1 Sources in transit 2249.6.1.2 On the ward 2259.6.1.3 Discharge of the patient 225

9.7 Other sources 2259.7.1 Iodine-125 seeds (R Foley) 2259.7.2 Gold-198 grains (including gold grain (Marsden) gun)

(W P M Mayles) 2269.7.3 Beta-ray sources: strontium-90 applicators

(A Walsh and E G A Aird) 226

10 In vivo Dosimetry and Portal Verification 22910.1 In vivo dosimetry (W P M Mayles) 229

10.1.1 Introduction 22910.1.2 Indications for use 22910.1.3 Methods 230

10.1.3.1 Entrance dose measurement 23110.1.3.2 Exit doses 23110.1.3.3 Normal tissue doses 23110.1.3.4 Precautions to increase accuracy 232

10.2 Portal verification 23210.2.1 Random and systematic errors 23310.2.2 Techniques to achieve good image quality 23310.2.3 Methods of comparison with planning data 23410.2.4 Frequency of carrying out portal images 23510.2.5 Transit dosimetry 23510.2.6 Commissioning and QC of imaging devices (P M Evans) 236

11 Control of In-house Software (Editor: H M Morgan) 24111.1 General background (A B L Tyler (for the SCRAP group)) 241

11.1.1 Aims for software quality 24111.1.2 Responsibility 24211.1.3 Limitations 242

11.2 Procedures for software control – overview 24311.3 Procedures for software control – one solution 243

11.3.1 Specification 24411.3.2 Project management 24411.3.3 Programming 24511.3.4 Documentation 247

11.3.5 Project library 24711.3.6 Implementation of product 24811.3.7 Product control 24811.3.8 Backups 249

Appendix A Procedures for the Definitive Calibration of RadiotherapyEquipment 250

Appendix B Survey of Quality Control Practice in UK Hospitals Carried Out by the Radiotherapy Physics Topic Group 254

Appendix C Bibliography 273

Index 280

Acknowledgements

So many people have contributed to this publication that it is difficult to acknowledgecontributions fairly. Those who have contributed major sections of text are recorded inthe contents lists. The editors have made some alterations to the text provided and wherethe original authors feel that this has been a change for the worse, they apologise. Inaddition to these we acknowledge the advice of B. Planskoy (5.3) and J.W. Boag (5.5.2).The section on multileaf collimator QC (5.5.6) has been compiled from documentsprovided by R. Lake and E.M. Macaulay together with other published work. We arevery grateful to the MAGNET team who advised on the writing of Section 3.3 andprovided the EEC MR test object diagrams. In Chapter 8 advice from D. Carter, J. Plane,D. Doughty, S.M. Locks, K. Rosser and S. Duane is acknowledged.

Contributors

E G A Aird, Medical Physics Department, Mount Vernon Hospital, Middlesex

R J Aukett, Medical Physics Department, Leicester Royal Infirmary, Leicester

M Bidmead, Joint Department of Physics, Royal Marsden NHS Trust, London

D E Bonnett, Medical Physics Department, Leicester Royal Infirmary, Leicester

J Byrne, Medical Physics Department, Newcastle General Hospital, Newcastle-upon-Tyne

M Casebow, Department of Medical Physics, Plymouth General Hospital, Plymouth

R Clements, Physics Department, Clatterbridge Centre for Oncology, Merseyside

J Conway, Weston Park Hospital, Sheffield

H J Dobbs, Guy’s, King’s and St Thomas’ Cancer Centre, St Thomas’s Hospital, London

D Doughty, Radiotherapy Department, St Bartholomew’s Hospital, London

S Edyvean, IMPACT, Medical Physics and Bioengineering, St George’s Hospital, London

P M Evans, Joint Department of Physics, Institute of Cancer Research, Sutton

R Foley, Radiotherapy Department, St Bartholomew’s Hospital, London

J Garrett, Bristol Oncology Centre, Bristol

I R Green, Medical Physics Department, Lincoln County Hospital, Lincoln

R M Harrison, Newcastle General Hospital, Newcastle-upon-Tyne

A Iles, Bristol Oncology Centre, Bristol

T J Jordan, North Western Medical Physics Department, Christie Hospital, Manchester

C H Jones, Joint Department of Physics, Royal Marsden NHS Trust, London

R A Lake, Medical Physics Department, Cromwell Hospital, London

G D Lambert, Medical Physics Department, Newcastle General Hospital, Newcastle-upon-Tyne

S Locks, Medical Physics Department, Newcastle General Hospital, Newcastle-upon-Tyne

W P M Mayles, Physics Department, Clatterbridge Centre for Oncology, Merseyside

E M Macaulay, Radiotherapy Department, St Bartholomew’s Hospital, London

A McKenzie, Radiotherapy Physics Unit, Bristol Oncology Centre, Bristol

J A Mills, Department of Clinical Physics and Bioengineering, Walsgrave Hospital,Coventry

A Moore, Department of Medical Physics, Leeds General Infirmary, Leeds

H M Morgan, Wessex Regional Medical Physics Service, Royal United Hospital, Bath

H Porter, Department of Radiotherapy, Beatson Oncology Centre, Glasgow

S K Powley, Medical Physics Department, Lincoln County Hospital, Lincoln

N Proctor, Physics Department, Velindre Hospital, Cardiff

J P Ridgway, Medical Physics Department, Leeds General Infirmary, Leeds

P J Rudd, Medical Physics Department, St Thomas’s Hospital, London

J E Saunders, Medical Physics Department, St Thomas’s Hospital, London

E Thomson, Department of Medical Physics and Bioengineering, Norfolk and NorwichHospital, Norwich

D I Thwaites, Department of Medical Physics and Medical Engineering, University ofEdinburgh, Western General Hospital, Edinburgh

J B Tuohy, Medical Physics Department, Cookridge Hospital, Leeds

A B L Tyler, Medical Physics Department, Velindre Hospital, Cardiff

S Unwin, Radiotherapy Department, Bristol Oncology Centre, Bristol

A Walsh, Medical Physics Department, Churchill Hospital, Oxford

A P Warrington, Joint Department of Physics, Royal Marsden NHS Trust, Sutton

N Whilde, Weston Park Hospital, Sheffield (present address Harley Street Cancer Clinic,London)

CHAPTER 1

Quality Assurance and its Conceptual Framework

1.1 Clinical background

1.1.1 Outline of the radiotherapy process

Quality assurance programmes must be structured to meet the clinical requirements foraccuracy necessary to achieve optimum treatment outcome in terms of maximising tumourcontrol probability and minimising normal tissue complications. The attainment of qualityin radiotherapy treatment requires a multidisciplinary team effort as it is concerned withthe reduction of errors and uncertainties in every aspect of the radiotherapy process. Insetting up a quality assurance programme consideration must be given to each step inthis process.

Figure 1.1 illustrates the different steps that have to be taken successively in theradiotherapy process (ICRU 1993). The first step involves the confirmation of the presenceof a malignancy with review of the histological diagnosis and further investigations todefine the site and extent of the tumour and its stage according to a recognised stagingclassification (e.g. TNM, FIGO, AJCCS). This assessment will include clinicalexamination (e.g. under anaesthetic for cervical, bladder and some other tumours) andimaging by various modalities. Following this the final decision to use radiotherapy ismade and the treatment prescription given, including a statement of the aim of therapy,the definition of volume to be treated and the specification of doses, fractionation andother treatment parameters. Provision must be made for modification of this prescriptionduring treatment planning if necessary.

The next step is treatment preparation which involves consideration of theimmobilisation of the patient (and where possible the tumour with its host organ) andacquisition of anatomical patient and tumour data for dose planning with the patient inthe radiotherapy treatment position. The position chosen for treatment should becomfortable for the patient and easily reproducible. Consideration may be needed of therequirements for computed tomography (CT) imaging using non-radioopaque materials.The same fixation devices (e.g. chest wedges, arm poles, lasers) need to be available forlocalisation (whether on the simulator, CT simulator or CT scanner) and for treatment.

The process of determining the volumes for treatment of a malignant disease consistsof several distinct steps which have been well described in ICRU Report 50 (1993). Thisprovides clearly defined and unambiguous concepts to ensure a common languagebetween different centres across the world. The gross tumour volume (GTV) is thepalpable or visible extent of the malignant tumour and usually corresponds to the site ofthe cancer where the tumour cell concentration is at its maximum. A margin is thenadded around the GTV to include direct local sub-clinical microscopic spread. This marginusually has a decreasing malignant cell density towards the periphery where it shouldreach zero and with the GTV constitutes the clinical target volume (CTV). If the tumourhas been removed prior to radiotherapy then no GTV can be defined and the volume ofsub-clinical disease constitutes a CTV. Following definition of these volumes a margin

2 Physics Aspects of Quality Control in Radiotherapy

has to be added around the CTV to account for variation in size and position of tissuesrelative to the treatment beams due to patient movement, organ movement and variationin daily set-up. This volume is known as the planning target volume (PTV).

Following delineation of the target volumes, beam arrangements are selected and thedose distribution computed. This process is repeated until a satisfactory dose distributionis achieved. Where three-dimensional treatment planning is available, a three-dimensionaldose distribution can be obtained based on localisation of the target volume, normal

Figure 1.1 Steps in the radiotherapy procedure (adapted from ICRU Report 50, 1993).

➞➞

➞➞

➞➞

➞➞

➞➞

Set-up at the therapy machine

Verification–simulation

Full computation and display of theselected treatment plan

Comparison of dose distributions and selectionof optimal treatment plan

Provisional selection of beam arrangements and computationof corresponding dose distribution

Delineation of volumes

Localisation–simulationAcquisition of additional anatomical

data for dose planning

Decision on external beam therapy andprescription of treatment

Complete clinical work-up (site and extent oftumour, stage, histology, etc.)

TreatmentVerification

Follow-up

Analysis of results

Record

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3Quality Assurance and its Conceptual Framework

organs and body contour at multiple levels. The beam arrangement of the final dosedistribution should then be verified. The monitor units or treatment time is calculatedand the patient treatment chart is prepared. This section of treatment preparation isespecially demanding of good communication and liaison between all members of themultidisciplinary team.

The next phase involves treatment delivery where the preplanned immobilisation andpositioning of the patient is carried out and the parameters set up according to the treatmentplan and the first session of irradiation given. Verification should be carried out usingportal films and in vivo dosimetry to check the geometry and dose given respectively,particularly when radical doses are being given. There should be continuous feedbackbetween all the different steps outlined in the radiotherapy procedure as a difficulty atany one point may require checking and reconsideration of a previous decision. Leunenset al (1992) demonstrated errors in data transfer during the radiotherapy process andemphasised the use of verification systems for detecting these systematic errors.

1.1.2 Positional and dosimetric accuracy

There are now a considerable number of studies in the literature which have looked atvariability of patient positioning and organ movement during radiotherapy treatment.Rosenthal et al (1992) reported a study of 51 patients with head and neck malignancywho were immobilised using a bite block technique. Comparison of simulator and portalfilms showed a total median uncertainty of 7 mm with 21 per cent of portal films showingan uncertainty of more than 10 mm. Graham et al (1991) compared the immobilisationof four patients using a plastic shell system with a similar number immobilised using astereotactic frame. The mean displacement of the lateral fields using a plastic shell was1.8 mm compared with 1.0 mm for the stereotactic frame. Similarly, there have beenstudies looking at the positioning of patients for breast irradiation and Mitine et al (1991)looked at 376 portal films performed on 14 patients undergoing breast radiotherapy withtangential fields. They showed that with their original technique there was a 15.5 mmstandard deviation in the cranio-caudal direction between the portal films and thesimulator film. However, when they changed the technique to one using an arm-pole toencourage fixation of the arm position, the standard deviation was reduced to 5.5 mmbetween the portal film and simulator film.

Gagliardi et al (1992) have presented an analysis of their three-dimensional treatmenttechnique for treating patients with node negative breast cancer. They analysed the shiftin a grid drawn on the skin of a patient’s breast with movement in the position of theipsilateral arm. They found that when the arm was abducted to 130°, as for entry througha CT scanner for planning, the central mammillary plane was shifted upwards by 2 cmand the displacement was up to 4 cm in the axilla. If the arm was abducted to 90° thedisplacement in the cranio-caudal direction was small over the breast but in the regionof the shoulder joint there was a difference of 2 cm between the grids. This illustratedextremely well that the use of skin tattoos as landmarks for setting up treatment fields ishighly dependent on the arm position when used for tangential irradiation of the breast.

Griffiths et al (1991) reported a study of pelvic radiotherapy which showed that errorsin movement were greatest in the caudo-cephalic direction and that the use of positioninglasers improved lateral shift errors, particularly in prone patients. They also showed thatit was important to examine the type of mattress used on the treatment couch as this of


itself could introduce errors, as can the use of ‘tennis racket’ and other couch windowsfor under-couch treatments. These studies of patient positioning during radiotherapygive us quantitative data on the amount of movement error which occurs during treatmentand can be used to define more accurately the margin necessary for the planning targetvolume. On-line portal imaging studies are now giving additional information onmovement of field margins during treatment and these will provide us with further data.It is essential that clinician and physicist discuss the margin which has been allowed forpositional variation within the PTV and this should be correlated with the results of on-line portal imaging. For instance, if portal imaging shows a maximum deviation of 5mm on each margin of the target volume and this is the allowance that has been madewithin the PTV for movement, then one can be satisfied that the quality standard ismaintained. Further work is needed to evaluate the significance of larger deviations forindividual fractions.

1.1.3 Clinical evidence for the importance of accuracy

Accuracy in geometrical set-up is supported in the literature by studies which demonstratea higher relapse rate in patients where shielding blocks positioned in the beam to protectnormal tissues were incorrectly placed. Kinzie et al (1983) showed in the Patterns ofCare study for Hodgkin’s disease that there was a significant difference in the relapserate of patients, with 50 per cent of those with inadequate portal films relapsing comparedwith 14 per cent with adequate portal films.

There is a considerable amount of clinical evidence which indicates that a high degree

Figure 1.2. Variation of probability of effect for neutrons (left hand curves) and photons. Dotted lines representnormal tissue effects and solid lines represent tumour effects (from Batterman et al 1981). Reproduced by kindpermission of BIR from BJR 54 (BIR 1981).


of accuracy in dose delivery is essential for a successful outcome of radiotherapytreatment. This applies not only to geometrical accuracy, as described above, but also toaccuracy of the dose level delivered. Figure 1.2 shows dose response curves for bothtumour control and normal tissue complications which have a characteristic sigmoidshape. These curves illustrate that a small variation in dose level can have a considerableinfluence on the probability of tumour control and also on the occurrence of normaltissue complications. However, within the target volume there is a variation in densityof tumour cells and there also exists heterogeneity of cell type and radiosensitivity anddistribution within the cell cycle, so that clinical dose effect data curves have a reducedsteepness. Clinical studies suggest that an increase in dose of 7–10 per cent results inclinically detectable reactions (Dutreix 1984). Chassagne et al (1976) reported from theInstitut Gustave Roussy on an incident in which a radiotherapist detected enhanced skinreactions and increased diarrhoea in patients following radiotherapy for gynaecologicalmalignancy. After careful rechecking a mistake in the correction factor applied to theionisation chamber was found. An overdosage of between 7 and 10 per cent had beensustained and found to be detectable by clinical observation of normal tissue reactions.

There are also data to support a dose response relationship for tumours, providingevidence that accuracy of delivery of dose is critical for tumour response as well as fornormal tissue complications. Hanks et al (1990) reported the Patterns of Care Study forcarcinoma of the cervix, showing a clear dose response for in-field pelvic control; thehighest rate of pelvic control was with a paracentral dose greater than 85 Gy. An additionalanalysis of patients treated in 1978 showed a lower total recurrence rate for Stage IItumours with doses greater than 75 Gy and for Stage III tumours over 85 Gy, but no doseresponse for Stage I disease. Morrison (1975) reported data for bladder tumours showingthat local tumour control rates rose from 38 to 80 per cent over the range from 42.5 Gyto 62.5 Gy. Shukovsky (1970) reported clear dose response evidence for supraglotticlaryngeal tumours in 114 patients with a steep rise in control rate from 20 to 70 per centbetween an NSD of 1750 and 2000 rets. Perez et al (1980) reported a statisticallysignificant improvement in local control for non-small cell lung cancer when a dose of60 Gy was compared with 40 Gy in a randomised study.

Hence from clinical observation there is good evidence that a difference in targetabsorbed dose of 10 per cent is detectable for a number of tumours and that a differencein absorbed dose of 7 per cent can be observed for normal tissue reactions.

1.1.4 Clinical limitations

1.1.4.1 Uncertainties in location of target volume

At the moment localisation of the target volume is the weakest link in the wholeradiotherapy process because of the clinical uncertainties. The publication of ICRU Report50, with clear definitions of volumes has enabled us to have an internationally commonlanguage but more data are needed to define margins more accurately. At the momentdelineation of the GTV is very dependent on methodology and many of the imagingmethods used have not been well validated. For instance, the UICC TNM stagingclassification states that ‘imaging’ should be used to define the T stage but there are noguidelines as to the appropriate imaging modality for each primary tumour site. Thechoice of margin for microscopic spread of disease is based on characteristics such astumour type and grade and on other factors, such as lymphatic, vascular and perineural


infiltration by tumour cells. The size of this margin appears to be extremely variabledepending on the clinician’s experience and interpretation of the histological features ofthe tumour and its projected natural history. A study from Urie et al (1991) revealedquite marked differences in the size and site of a clinical target volume for treatment ofthe same nasopharyngeal tumour by two different physicians. Denham et al (1992) havereported a study of patients planned for radiotherapy of non-small cell lung cancer inwhich they showed a wide inter-clinician variation in both selection of treatment intentand choice of tumour and target volumes, even with the use of CT information. Themain causes of inter-clinician variation were the radiological interpretation, the marginfor microscopic disease and the margin for day to day variation of position. Leunens etal (1993) reported similar results for brain tumours. Studies such as these confirm theneed for more data to provide guidelines on the appropriate margins. Holland et al (1985)performed a study on mastectomy specimens for patients with Tis, T1 and T2 breastcancers and showed that, for invasive tumours less than or equal to 2 cm, 14 per cent ofpatients had invasive tumour foci at a distance greater than 2 cm from the referencetumour. These sort of data give information on which to base the size of margin used forradiotherapy to the tumour bed for patients receiving radiotherapy for breast cancer.Ideally information such as this should be gathered for all tumour sites and this could beavailable from prostatectomy, laryngectomy and cystectomy specimens as well as post-mortem data. Correlation of these surgical findings with preoperative imaging wouldinform the choice of the clinical target volume. Studies of molecular markers to definemicroscopic disease are looking promising.

1.1.4.2 Dose specification

Other clinical uncertainties add to the confusion over the dose delivered to the patient.The Royal College of Radiologists’ survey of different dose fractionation schedules(Priestman et al 1989) showed that for treatment of an early carcinoma of the vocal cord21 per cent of radiotherapists surveyed recommended 60 Gy in 30 fractions over 6 weeks,8 per cent gave 55 Gy in 16 daily fractions, 8 per cent gave 55 Gy in 20 daily fractionsand the remaining 63 per cent used 44 different schedules, varying from 33 Gy in 6fractions over 3 weeks to 70 Gy in 40 daily fractions. Analysis of these schedules usingradiobiological models shows that many of them are similar in biological effect, butthey still produce a distribution with an effective standard deviation of 9 per cent (Hendryand Roberts 1991).

In addition to this there has been no uniform method of prescription of dose and avariety of prescription methods have been used including the minimum, maximum, mid-point, modal and mean doses with a variation between the most extreme of these of up to10 per cent (Hamilton and Denham 1992). ICRU Report 50 has recommendedinternational guidelines for reporting of dose at the ICRU reference point which hasbeen chosen to be clinically relevant, unambiguous and at a dose point of physicalaccuracy avoiding regions of steep dose gradient. To fulfil these requirements the referencepoint is therefore located firstly at the centre or central part of the planning target volumeand secondly on or near the central axis of the beam. ICRU Report 50 requires that thedose at the ICRU reference point should always be reported with, in addition, themaximum and minimum doses to the PTV to reflect the homogeneity of dose across thetarget volume.

The first requirement for specification of doses for reporting is that they should beapplicable to all clinical situations and all different levels of sophistication of dose


computation. The only dose parameter that meets these criteria is the dose in the centralpart of the PTV. This contrasts with the situation with the minimum and average dose tothe PTV where, for the reported dose to be meaningful, a 3D dose computation is required.The second requirement for reporting the dose is that the dose value should be physicallyaccurate. The dose along the central axis can be calibrated with good accuracy, whereaspoints located off axis or at the periphery have greater dose uncertainty due both toinstability of the beam as well as the steep dose gradient close to the border of the field.

From a radio-biological point of view the average dose to the cancer cell populationis the parameter which is best correlated with tumour response, provided that the doseheterogeneity is not too large. However, across the CTV the tumour cell density variesto a large extent and in principle reaches zero cell density at its border. Therefore theaverage dose to the corresponding PTV does not necessarily correspond to the averagedose to the cancer cell population and may thus lose part of its biological significance.

In many centres the minimum isodose surface at the periphery of the PTV is used forreporting and prescribing dose. The objective with this approach is to ensure that alltissues containing tumour cells will receive at least the prescribed dose. However, thelocation of the periphery of the PTV depends on the size of the clinical margin, whichvaries from one radiation oncologist to another, and in principle the cell density thereshould be zero. For these reasons the minimum tumour dose cannot be considered themost appropriate single value for dose prescription.

Other dose values considered to be relevant, e.g. average dose and its standarddeviation, dose volume histograms and biologically weighted doses, when available,should also be reported as additional information. It is to be hoped that with an increasingawareness of the need for uniformity in prescribing and reporting treatments, acceptanceof the ICRU Report 50 dose specification recommendations will remove an unnecessaryuncertainty in the description and comparison of delivered radiation doses to patients.

1.2 Accuracy in radiotherapy as the basis for qualityassurance standards

1.2.1 Requirements for accuracy

1.2.1.1 Definition of accuracy

The term ‘accuracy’ in radiotherapy is often used ambiguously. Absolute accuracy mustbe distinguished from precision or reproducibility. Overall uncertainty includes bothrandom (type A, a posteriori) and systematic (type B, a priori) uncertainties. The firstcan be estimated from repeated independent observations and can be expressed as astandard deviation (SD). Systematic uncertainties on the other hand can only be estimatedby an analysis of the process under consideration, assigning reasonable uncertainty valuesto parameters where, by definition, they are not exactly quantifiable. They can beexpressed as an ‘effective standard deviation’ (ESD), being taken as the estimate of thelimits within which the correct value is expected to occur in around 70 per cent of cases.Uncertainties of both types can be combined in quadrature to provide an estimate ofoverall uncertainty (Mijnheer et al 1987, BIPM 1980). Systematic uncertainties are


associated with absolute deviations from the ‘correct’ value in any given situation, whilerandom uncertainties are linked to the precision of the process. However this distinctioncan be less than clear-cut in some circumstances (Dutreix 1984). For example a givenparameter may have a systematic uncertainty in a particular radiotherapy department,but values may be effectively randomly distributed when a number of departments areconsidered.

Within one department and one radiation modality, it is generally reproducibility, orprecision, of dosimetry that is critical. However, in transferring experience betweencentres, or in comparing results between departments or between modalities, it becomesnecessary in addition to consider some of the systematic uncertainties. Some systematicuncertainties, such as the basic physical data used in dosimetry protocols, may be commonto all departments. These would not contribute to estimates of dosimetric consistency,but would only require inclusion when considering overall absolute accuracy. In anyparticular situation it is necessary to be clear whether precision or absolute accuracy isrequired and also, if precision, exactly which contributing uncertainties are involved.

1.2.1.2 Requirements for dose accuracy

ICRU Report 24 (1976) considered at that time that ±5% accuracy was required in thedelivery of absorbed dose to the target volume, but that ±2% may be desirable in some(critical) situations. The exact meaning of these figures was ambiguous and they havebeen variously interpreted as 1 or 1.5 SD or as overall limits. Mijnheer et al (1987)considered dose–effect information for normal tissue complications and took 7 per centas a representative value for the relative gradients of the steeper curves. They concludedthat any transfer of clinical information from one centre to another will involveunacceptable risk of complication if the overall uncertainty in absorbed dose is largerthan this value. This was then assigned to the 2 SD level, resulting in an accuracyrequirement of 3.5 per cent (1 SD). Brahme et al (1988) reviewed dose–response datafor a range of malignancies and considered the general form of expressions used to modelthe relationship between tumour control probability (TCP) and dose. If a tolerance limitof 5 per cent (1 SD) is set on the uncertainty in TCP the dosimetric precision required toachieve this will depend on the steepness of the particular dose–response curve. If thesteeper tumour response curves are considered a figure of 3 per cent (1 SD) on the accuracyof delivery of absorbed dose is obtained. It should be noted in this analysis that manyclinically observed dose–effect curves exhibit steepnesses which are reduced from thatexpected for the intrinsic response due to the modifying effects of various factors. Theseinclude biological factors, such as the variation in tumour cell density within the targetvolume, the heterogeneity of cell type and distribution within the cell cycle, the variationin radiosensitivity of the patients in the group considered, etc., as well as clinical, physicaland technical factors in the studies used to obtain the data.

Thus overall, taking available information on TCP and NTCP (Normal TissueComplication Probability) into account, a figure of 3 per cent can be taken as the currentlyrecommended accuracy requirement on dose, being considered as one relative standarddeviation on the value of the dose delivered to the patient at the dose specification point.This implies that changes will be likely to be clinically observable for dose changes attwice this level, at least in the situations described by the steeper dose–effect relationships.This is consistent with the more anecdotal evidence on clinical observations followinginadvertent dose changes due to dosimetric errors (Dutreix 1984; see Section 1.1.3).


1.2.1.3 Accuracy of dose distribution

Variations in the dose distribution across the target volume may also be expected toaffect the treatment outcome and contribute to the steepness of clinically observed dose-effect curves. The largest effects on loss of TCP will be at the higher TCP values. Brahmeet al (1988) have reviewed earlier work considering the effects of dose distributionvariations on uniform tumour volumes (Brahme 1984). For simplified models representingstepped, sloped, underflattened and overflattened distributions, estimations were madeof the decrease in TCP for tumours having various dose–effect gradients. These led tothe conclusion that the standard deviation of the mean dose within the target volumeshould be between 3 and 5 per cent (1 SD). When combined with the permitted uncertaintyin the dose at the specification point a figure of 5 per cent (1 SD) is obtained for therequired dose accuracy at all other points in the target volume. Similar conclusions arereached considering dose variations over more realistic heterogeneous tumours or usingdifferent models for TCP (Brahme and Agren 1987, Thwaites and Nahum 1993, Webband Nahum 1993). It is likely that better accuracy will be shown to be necessary as morecomprehensive TCP models are developed.

1.2.1.4 Geometric accuracy

Geometric uncertainties arise from a variety of causes, including treatment machinetolerances, simulation and treatment set-up variations, patient or organ movement duringtreatment and changes of patient shape as treatment progresses. Some of the clinicalevidence for the effects of such variations on treatment outcome has been summarised inSections 1.1.2 and 1.1.3. The clinical margins incorporated in defining the target volumeimplicitly include some of the expected variations, which implies that rather largervolumes of surrounding normal tissue are being included than otherwise would be thecase and also that it may be difficult to isolate effects on tumour control due to geometricinaccuracy.

Some theoretical considerations have been applied to estimate the effect of positioningerrors on TCP (Brahme et al 1988), by varying the dose distribution at the edges of thetreatment volume, both in terms of misses (systematic) or of widened effective penumbra(random) and making assumptions about the tumour cell density distribution at the edges.Similar estimates can be made using other TCP models (Webb and Nahum 1993). Howeverit is difficult to move from such estimates to a specific requirement on geometric accuracy.

In this situation estimations of required geometric accuracy have been made byconsidering the various sources of uncertainty, combining them to give an overall valueand using the best practically achievable figures as the level to be recommended. On thisbasis the AAPM (1984) gave a figure of 5 mm (1 ESD). Mijnheer (personalcommunication) and the present author have considered a wider set of data including theoutcome of imaging studies and recommend an accuracy on positioning of field edgesand shielding blocks of 4 mm (1 ESD). This figure includes all geometric and movementfactors. The increased availability of on-line portal imaging systems is providing agrowing body of data in this area. This, and the attention it focuses on immobilisation,set-up and residual movements, may be expected to improve the estimation of thesefigures.

1.2.1.5 Summary of accuracy requirements

These recommendations concerning accuracy in radiotherapy can be summarised as:


3 per cent on the absorbed dose delivered to the specification point;5 per cent on the dose at all other points in the target volume;4 mm on the position of field edges and shielding blocks in relation to the PTV.

All these are given as one standard deviation and all are general requirements for routineclinical practice. In certain cases, for example palliative treatment, higher values may beacceptable. However it is impractical and undesirable to work to different standards fordifferent treatment techniques, so these values must be widely applied. In some specialcases smaller uncertainties may be demanded if very steep complication curves areinvolved or if tight geometric tolerances are necessary.

1.2.2 Accuracy achievable

1.2.2.1 Standards

The figures in the preceding section apply to the final stage of treatment delivery to thepatient. Greater accuracy is required for each individual contributing part of the wholeprocess if the final recommended values are to be achieved. Therefore these considerationsshould form the basis of subsidiary recommendations concerning accuracy at other stagesin the process. Where different stages are linked the requirements should be consistent.For example equipment specifications are linked to the requirements for acceptance testingand commissioning of that equipment and to the ongoing quality control and qualityaudit of the unit.

Many sets of international recommendations are available concerning quality assuranceof radiotherapy. Some of these are wide-ranging (e.g. WHO 1988, AAPM 1994b, Thwaiteset al 1995), whilst others deal only with particular areas of the process. Most provideguidance on required standards. Whilst these are based broadly on the clinicalrequirements for accuracy, they do not all use the same assumptions and thereforediscrepancies can be observed between them (Thwaites 1993a, Jarvinen 1993). As laterchapters of this book refer to specific recommendations, only a brief introductory surveyis given here, concentrating on recommendations dealing mainly with the physical andtechnical aspects of radiotherapy.

Broad recommendations were given in 1988 in a WHO document which attempted totake a consistent approach to quality assurance of the whole radiotherapy process. Morerecently, the emphasis on the application of comprehensive quality assurance programmes,or quality systems, to radiotherapy has lead to other sets of recommendations embodyinga similarly integrated and wide-ranging approach (e.g. AAPM 1994a, Thwaites et al1995). Brahme et al (1988) provided detailed discussion of the accuracy requirementsand quality assurance of external beam therapy in a document originally conceived as anICRU report. The ICRU has also produced various recommendations, mostly concerningspecific dosimetric aspects of radiotherapy, but including treatment planning (1987) anddose and volume specification (1978, 1985, 1993). The IEC have standing committeesconsidering performance specification and related matters for a range of equipment inradiotherapy, notably treatment equipment (1989). Rassow (1988) has discussed the IECapproach for setting performance standards for megavoltage equipment and it can beseen there that the underlying considerations are reasonably consistent with those outlinedabove which have been used as the basis of the recommendations for clinically requiredaccuracy. The European Commission has also given some specifications in these areas


in their document on criteria for acceptability of radiological installations. For examplethey have specified that the treatment planning system should be able to calculate thedose to within 2 per cent of the measured dose at points of relevance for the treatment,and that in regions involving very steep dose gradients, the observed position of a givenisodose curve should differ from its calculated postion by less than 3 mm (EuropeanCommission 1997).

Once performance standards are set for treatment equipment, they define the criteriato be applied to equipment acceptance and commissioning and to quality control. Inaddition they demand similar performance standards for related equipment, e.g.simulators, and in related processes. Many sets of recommendations are available fortreatment unit commissioning and quality control (e.g. NACP 1980, AAPM 1984, DIN1986, SFPH 1986, Brahme et al 1988, IPSM 1988, WHO 1988, IEC 1989). Manyprotocols are available for beam calibration as part of this, which implicitly includestandards of required accuracy (e.g. AAPM 1983, IAEA 1987, IPSM 1990, AAPM 1994,IAEA 1996, IPEMB 1996b, 1996c). There are a growing number of recommendationsfor other equipment and processes, e.g. for treatment planning (IPEMB 1996a). A numberof these are discussed in some wider-ranging publications (e.g. Brahme et al 1988,Starkschall and Horton 1991, Williams and Thwaites 1993, AAPM 1994a).

While each set of recommendations has the aim of maintaining treatment quality withinclinically required limits and therefore these limits have been considered in arriving atthe standards given, some of them have been drawn up considering a particular area ofthe process in isolation and also taking some account of the balance between technicaleffort against cost and of the uncertainties involved in test procedures. In situationswhere all uncertainties from all parts of the process combine, clinically required accuracymay not necessarily be met. Thus, for example, Rassow (1988) surveys the geometricaland dosimetric performance specifications included in the IEC documents, where valuesof generally ±2 mm and ±1 per cent are given for individual parameters. When combinedthese lead to root-mean-square figures of about ±4 mm and ±4 per cent respectivelyfrom equipment performance alone.

In setting standards two approaches are possible, both based on the clinicalrequirements:

1. To set optimum conditions based on the therapeutically desirable values, althoughthese may not be enforceable or achievable in all circumstances.

2. To set minimum conditions, beyond which performance is deemed unacceptable. Thesewould be enforceable and could be made mandatory.

These two sets can be termed tolerance levels and action levels respectively. The tolerancelevel normally equates roughly with a 1 SD requirement on the particular equipment orprocess under consideration. Action levels are often set at approximately twice thecorresponding tolerance level. However there may be instances when it is appropriate toset the tolerance and action levels to the same value. Performance within the tolerancelevel is deemed to be of acceptable accuracy in any situation. Performance outside theaction level is unacceptable and demands further action to remedy the situation.Intermediate values indicate that further testing is required, possibly leading to action.The IEC recommendations for equipment performance can be equated with approach(2). Brahme et al (1988) explicitly list performance standards for a number of types ofequipment under the two headings, where the action level standards for treatmentequipment are broadly similar to the IEC standards. The concept of tolerance and action


levels is also used in defining quality control and quality audit requirements for testingcompliance with standards.

It must be recognised that in some situations it may not be possible to meetrecommendations on standards due to limitations on equipment or techniques and this inturn should provide impetus for development. As an example, performance standardsfor treatment planning systems are frequently quoted as reproducibility of dosedistributions to within 2 per cent (or 2 mm near the edge of high dose regions). Whilethese could be achieved in many situations using simpler algorithms, they cannot bemet, for example, near interfaces or where out-of-plane anatomy changes significantly,or in complex treatment situations, necessitating the development of more complexalgorithms to cope with these circumstances.

1.2.2.2 Practice

The accuracy which can be achieved in practical radiotherapy and its relationship to therequired accuracy can be estimated in two ways:

1. By an uncertainty analysis of the various stages involved, using all availableinformation to assign reasonable practical values.

2. By experimental investigations at different stages of the process to measure theaccumulated uncertainties up to and including that stage.

The two approaches are complementary and experimental results from the latter provideinput to the former. Neither method is ideal. There are inevitably some factors omittedfrom experimental investigations, such as inter-centre dosimetry intercomparisons, andthere are areas where no hard data are available for input to uncertainty analysis andeducated guesses become necessary. The wider use of real-time in-vivo dosimetry systemsand on-line portal imaging systems is providing a growing body of information relatingto achievable precision at the point of treatment delivery to the patient.

1.2.2.3 Uncertainty estimates

Various attempts have been made to analyse the whole radiotherapy process to obtaincumulative uncertainties (ICRU 1976, Johansson 1982, AAPM 1984, Svensson 1984,Brahme et al 1988, Thwaites 1993b) incorporating varying information on different stagesand following a variety of schemes. Estimates of overall absolute dosimetric accuracy(1 SD) range from about 5 to 10 per cent for multiple field planned irradiations and onlyapproach the clinically required accuracy for the simplest techniques and under the bestconditions. However, these figures for absolute accuracy will overestimate the uncertaintyin treatment consistency because some uncertainties will affect all treatments equally(see Section 1.2.1.1). On the other hand if standards of accuracy in a specific centrewere below those estimated, even larger overall uncertainties may be present in practice.For geometric accuracy, the accuracy estimation approach produces figures of 4 or 5mm (1 SD) in the best circumstances.

1.2.2.4 Experimental investigations of achievable accuracy

Because of the uncertainties in accuracy estimation the experimental approach may yieldfirmer evidence. A steadily growing number of dosimetry intercomparisons have beencarried out, comparing doses in different treatment centres. Some of these have beenreviewed (e.g. Hanson et al 1993, Thwaites and Williams 1993, Thwaites 1993c). Most


have been undertaken in reference measurement conditions to assess consistency of basicdosimetry. However an increasing group have been designed to investigate at least somefurther parameters linked to treatment planning and some aspects of patient treatment,using multiple-field irradiations and more complex (some anthropomorphic) phantoms.It must be noted that these cannot include all parameters relevant to the real patientsituation, so all uncertainties obtained from these studies are inevitably underestimatesof overall clinical treatment uncertainties.

For recent reference point intercomparisons in Western Europe and North America,systematic deviations of the distribution of results (ratios of measured-to-stated doses)have typically been within 2 per cent of unity and the standard deviations of thedistributions have typically been within the range 1 to 3 per cent. Individual deviationsas large as 25 per cent have been observed. For example in the IPSM megavoltage photonintercomparison (Thwaites et al 1992), which included every UK centre, the mean ratiowas observed to be 1.003 and the standard deviation of the distribution 1.5 per cent,representing the measured consistency of basic radiotherapy dosimetry in the UK forMV photon beams. Some much larger standard deviations have been reported from theIAEA/WHO dosimetry service (Svensson et al 1993). For intercomparisons using morecomplex phantoms and planned multiple-field irradiations, measured uncertainties inthe defined target volume are larger, with mean deviations lying up to 4 per cent andstandard deviations lying up to 7 per cent within the target volume. In the IPSM study,for three field irradiations involving non-perpendicular beam incidence, the use of wedgesand the inclusion of inhomogeneities, observed mean deviations were around 1 per centand standard deviations around 3 per cent. For the few studies which have consideredpoints outside the target volume (e.g. Johansson et al 1987, Wittkamper et al 1987)uncertainties can be much larger.

The smallest figures obtained in intercomparisons represent the limits on currentlyachievable consistency between centres and are consistent with analytical estimates.Most results from studies with multiple fields give accuracy estimates close to theclinically required precision, but these studies still do not include patient relateduncertainties. This evidence reinforces the conclusions of uncertainty analysis that therequired dosimetric accuracy can only be achieved in the best conditions.

1.2.2.5 Patient dose and position measurements

Dosimetry intercomparisons can provide information on the precision of dosimetrybetween radiotherapy centres at certain limited selected levels in the dosimetry chain.Regular measurements of dose or positional information throughout treatment on groupsof patients can provide estimates of the relative overall precision within a given centrefor given treatment techniques and treatment units. A number of centres have begunstudies of this type. One of the more extensive investigations so far reported is thatundertaken by the Leuven group (Leunens et al 1990 a,b, 1991, Mitine et al 1991, Dutreixet al 1992). Entrance and exit dose measurements have been carried out using diodes,mainly on parallel opposed head and neck patients, but including other sites. In additionportal imaging studies have been included. The results have been used to provide ananalysis of the quality of treatment, i.e. as a form of internal quality audit. Mean deviationsfrom expected values have enabled systematic errors to be identified and remedied. Thesehave been within the range ±2 per cent on the central axis, but have been observed to besignificantly larger at other points. Standard deviations of the observed distribution ofresults give a measure of the random uncertainties associated with the particular treatment


site, treatment technique and treatment unit. For the head and neck study these havebeen around 3 to 3.5 per cent on delivered dose on the central axis and around 4 mm onposition. Similar results have been obtained in similar studies (e.g. Millwater et al 1997).Other treatment sites have also been investigated in a similar way (e.g. Heukelom et al1991, 1992), showing a range of measured values for precision depending onimmobilisation, technique, etc. Various studies have been reported on the analysis ofrepeated portal images in terms of reproducibility from fraction to fraction and also incomparison to simulator films. Some of these were summarised in Section 1.1.2. In generaldiscrepancies of between 4 and 10 mm have been observed between simulation andtreatment, depending on site irradiated and immobilisation employed. The averagegeometric precision reported from sequential portal images is around 3 to 5 mm (1 SD).

When transferring or comparing experience between centres, the appropriate precisionto be considered will be a combination of these intra-centre values with relevant inter-centre values obtained from intercomparisons. Careful consideration is required to identifyany uncertainties included twice, i.e. in both sets, or omitted from either.

References

AAPM (American Association of Physicists in Medicine) 1983 A protocol for thedetermination of absorbed dose from high energy photon and electron beams. Med. Phys.10 741

AAPM (American Association of Physicists in Medicine) 1984 Report 13. PhysicalAspects of Quality Assurance in Radiotherapy (New York: American Institute of Physics)

AAPM (American Association of Physicists in Medicine) 1994a Almond PR, Attix FH,Humphries LJ, Kubo H, Nath R, Goetsch S and Rogers DWO The calibration and use ofplane-parallel ionisation chambers for dosimetry of electron beams: An extension of the1983 AAPM protocol report of AAPM Radiation Therapy Committee Task Group No.39. Med. Phys. 21 1251–1260

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CHAPTER 2

Systematic Approaches to Quality Assurance and Audit

2.1 Introduction

In 1989 a committee of the Department of Health published a report called QualityAssurance in Radiotherapy (QART) (Bleehen 1991). This report described a system forquality assurance in radiotherapy based on an international standard for quality systemsdesignated ISO 9000, or in the UK, BS 5750. The first part of this chapter looks at thebasis of the QART document and gives recommendations for its implementation. Thesecond half of the chapter looks at approaches to audit – an inherent part of ISO 9000.

The current version of the series of standards is properly designated BS EN ISO9000:1994, which is usually abbreviated to ISO 9000. This superseded the set of identicaldocuments EN 9000, ISO 9000 and BS5750 which were published in 1987. The Bleehenreport was based on the 1987 standard which has different paragraph numbering fromthe 1994 standard. There are three major parts within the ISO 9000 series:

BS EN ISO 9001:1994 Quality systems. Model for quality assurance in design,development, production, installation and servicing.BS EN ISO 9002:1994 Quality systems. Model for quality assurance in production,installation and servicing.BS EN ISO 9003:1994 Quality systems. Model for quality assurance in final inspectionand test.

In order to avoid confusion in what follows the earlier standard is referred to as ISO9000:1987 etc. and the later standard as ISO 9000:1994 etc. ISO 9000 will be usedwhere the text is equally applicable to both versions of the standard.

2.2 ISO 9000 and QART

2.2.1 Background to the report

The Bleehen Committee was charged with producing a guide to Quality Assurance inRadiotherapy (QART). This multidisciplinary body came to the conclusion that therewere many perceptions of the task involved, many levels at which it could be addressedand that it had complex interfaces which would have different form and shape dependingupon the radiotherapy department in question.

It was necessary to adopt a structured approach with which all disciplines involved inradiotherapy could identify and adapt to suit local situations. A further requirement wasthat all aspects of quality assurance should be covered in a systematic manner such thatquality assurance would adequately ‘guarantee the safety of patients’ (p. 7).

The structured approach of ISO 9000:1987 provided a promising start. It has the built-in

21Systematic Approaches to Quality Assurance and Audit

capacity to be applied with equal rigour to a singly managed group of interacting functionsor to separately managed functions. As a Standard primarily addressed to an industrialprocess setting, its language and its emphasis on process control renders it inaccessibleand unfriendly in the medical context. In the past, medical practice has particularly reliedupon professional qualifications and associated training (i.e. the inputs) for qualityassurance. Quality outcomes in ISO 9000 are implicit but nonetheless the choice ofparameters is a matter of specification for each individual scheme. Even in its intendedsetting the standard has the reputation of sometimes being excessively bureaucratic.

Other initiatives were considered, such as the more general approach to whole hospitalquality operated in the UK by the King’s Fund. This has the merit of being designed fora health environment and therefore readily accessible. It takes up all activities within ahospital and is centrally driven. The weaknesses of this scheme for application to qualityin Radiotherapy is that its breadth and universality is the price paid for its lack of depthand specificity.

It was decided to base Quality Assurance in Radiotherapy on the ISO 9000 modelbecause of the need for a highly structured approach, with a focus upon managementprocedures and the flexibility to permit the specification of process parameters which inturn determine specified outcomes.

2.2.2 Process of adaptation

Conventionally, ISO 9000 is an assessable standard ultimately underwritten by a nationalsupervising body (the Secretary of State for Trade and Industry in the UK). The applicationof the Standard to Radiotherapy raised a number of questions which had to be addressed:

1. Was the accreditation of Radiotherapy departments envisaged?2. Which of the three Parts of the Standard would be most appropriate to Radiotherapy?3. How could the Standard be interpreted for Radiotherapy?4. How could the cost of implementation be minimised?

2.2.3 Basis of accreditation

The question of accreditation of Radiotherapy Departments also raises questions forthose groups of staff separately managed but intimately linked in a ‘customer-supplier’relationship. If not covered by an accreditation scheme, the service supplier (e.g.Pathology) has the status of ‘approved supplier’, the approval process being whatever isdefined in the local circumstance. This approval can be taken as read if that supplier isindependently ISO 9000 registered (e.g. an equipment service supplier). In the case ofthe supplier of Radiotherapy Physics services, the ISO 9000 scheme facilitates equalrigour of quality assurance when independently managed as when included within themanagement structure of an Oncology Department. Particular care in definingManagement responsibility is called for where the supplier, for example RadiotherapyPhysics, is separately managed but whose service is embraced in the accreditation of theOncology Department.

QART has been prepared in a way that is sufficiently faithful to ISO 9000 and thereforecan form a basis for external assessment to the standard, but at the same time may beimplemented without external validation.


A fundamental consideration of a quality assurance scheme is the scope of itsapplication. By definition (BS 4778: 1987), quality assurance embraces ‘all those plannedand systematic actions necessary to provide adequate confidence that a service will satisfygiven requirements for quality’. It is to this that careful attention must be paid if undueproliferation of written procedures and form filling is to be avoided. The basis of adequateconfidence is a matter of judgement but it usually implies a relevance to what is importantto the quality of treatment in its broadest sense, corroborated by some form of writtenevidence that agreed expectations throughout the chain of events have been satisfied.

2.2.4 Level of application

When the Bleehen Report was being written, in 1991, ISO 9001:1987 contained twoadditional requirements over and above those of ISO 9002:1987. These determine service‘design and its follow-up in service’. In the medical context this would cover aspects ofdisease diagnosis, the derivation of the treatment prescription and the verification of thetreatment in follow-up etc. At the time of preparing the Bleehen Report, the developmentof medical audit was progressing rapidly and hence it was considered inappropriate toinclude quality assurance requirements which were being adequately addressed by thismeans. The second level, ISO 9002:1987, was therefore adopted as the appropriate levelof application. This addresses those aspects of Radiotherapy which determine the successwith which the service delivers and meets the agreed expectations of the patient. Somehospitals have decided to implement the full ISO 9001:1994 standard.

2.2.5 Interpretation of ISO 9002

QART contains 18 requirements for quality assurance to be satisfied and they have thesame paragraph numbers as the 1987 version of the base Standard. The revision of theStandard in 1994 has resulted in some additions and amendments to the requirementsand the paragraph numbering has been altered so that sections in ISO 9001:1994 andISO 9002:1994 have the same numbers. Unfortunately this has resulted in the numberingof the Bleehen Report differing from the current numbers so in what follows referencesto both numbering schemes are given in the format: ‘4.4 (1994: 4.5)’ etc. The principalfunctional change has been the addition of the requirement that Data as well asDocumentation should be controlled.

In order to improve accessibility, QART has reproduced the Standard using languagewith which those working in or with a Radiotherapy Department will be more familiarwhile attempting to retain the rigour of the original. That was not easy and may not havebeen wholly successful; the expert reader must be the judge.

It will be noticed that the headings of some requirements have been changed. ‘PurchaserSupplied Product’ has been replaced by ‘General Patient Care’ in 4.6 – which has to bean improvement ! Where the clinical application imposes an additional important factorbecause of the special nature of the context, the requirement has been split into twoparts. ‘Identification and Traceability’ (4.7 (1994: 4.8)) has been applied both to thetreatment process and any manufactured products (e.g. alignment jigs). Similarly,‘Handling, Storage and Packaging’ has been applied both to products and to discharge ofthe patient. For each area a description of the requirements of the standard is given.Where appropriate, specific guidance on how the standard should be applied is also


given. This guidance is by no means exhaustive and serves only to provide further clarityof how the standard might be applied.

2.2.6 Minimising the cost of application

In many respects, QART broke new ground by addressing a known quality problem withan approach untried in this field. Demonstration of the feasibility of the Standard wasseen to be best served by pilot studies at the Christie Hospital, Manchester and the BristolOncology Centre. The management structure of each centre is significantly differentand the scope of application was also differently applied. The Christie applicationproceeded to registration as an all embracing scheme while the Bristol Oncology Centreis covered by three separately registered schemes covering Treatment, RadiotherapyPhysics and Equipment Management. The findings and resulting quality manuals of theseCentres should provide considerable scope for reducing the cost of subsequentapplications to radiotherapy departments.

2.3 Implementation of QART

2.3.1 Where to start

It is important when implementing QART to decide on the scope of the Quality Assurance.This can embrace as many or as few disciplines as is deemed desirable for a QualitySystem in Radiotherapy. For instance, it may include medical, radiographic and physicsinput only. Alternatively, it may include any staff group that impinges on a patientreceiving radiotherapy – i.e. nursing, clerical, secretarial, and hotel services, etc. In fact,in a hospital that is entirely dedicated to treating cancer, it may include all staff groupsworking there.

The scope may also vary depending on the management structure within the hospital.Different groups may come under different Directorates and again this may determinewho is included in the scope in order to be able to show a clear line of management. Thescope also needs to define the start and end of the system, for instance, from referral ofa patient to the discharge of the patient on completion of treatment, having given them afollow-up appointment. Defining this scope carefully, enables the system to be specificabout which activities must be included in the procedures.

The Standard suggests three main levels of documentation. The first level may becalled the Quality Manual and this is discussed below. The second level may be theProcedures Manual, enlarging on the policies described in Level 1 and using a standardformat, again discussed below. The procedures form the basis of the system, againstwhich it can be audited. Third level documentation includes detailed work instructionswhere required for specific activities, physics data sheets and local rules. Any referencedocuments essential to the safe administration of radiotherapy should also be included.

The QART document can be used as a basis for the Level 1 Policies of the system.The following outline describes a broad view of the procedures required in an averagesystem and the reference numbers given refer to the relevant sections of the QART


document. Note that following revision of the Standard, the numbers do not all correspondto those in the new standard.

Having decided on the scope, it may help in planning the system to construct a flowchart indicating all the stages through which the patient passes. This will help in selectingindividual procedures that will be necessary and in establishing the staff groups involvedand the interfaces between them. It is important when writing the procedures to highlightthese interfaces and how communication passes between them. It may also prove usefulto make an interaction chart. This can list the various staff groups and then tabulatewhich groups are involved in which of the various 18 parts (1994: 20) of the standard.For instance, all groups will be involved with Contract Review in Resource Assessment(para 4.3) and in Training (para 4.17 (1994: 4.18)), whereas physics and maintenancemay be the only groups involved in Inspection, Measuring and Test Equipment (para4.10 (1994: 4.11)).

To assist in preparing the procedures, it may help to divide the 18 requirements of thestandard into groups.

2.3.2 Requirements affecting overall management

These include Management Responsibility and Quality Policy (4.1), Quality System (4.2),and Resource Assessment (4.3). Here management review, contracts, resources andsystems must be covered. Suppliers, Purchasing and Tendering (4.5 (1994: 4.6)) is animportant section to address. If the hospital uses a central supply service for the majorityof purchasing requirements, this simplifies the system enormously, as the SuppliesDepartment can be visited for audit and each individual supplier is then approved underthis blanket (many Supplies Services are in the process of obtaining ISO 9000 themselvesand this will also help).

Any extra supplier such as those supplying radionuclides can be specified separatelyand if it is stated that a 100 per cent checking system is used on delivery, then this coversthe assessing and approval of such suppliers. Document Control (4.4 (1994: 4.5 Documentand Data Control)) includes the identifying of every procedure, work instruction, etc.with a reference number, issue number, date of issue, authorisation and issue signatures.It requires a master list of all these controlled documents and distribution lists of all therecipients. Quality Records (4.15 (1994: 4.16)) includes records of audits, minutes ofmanagement reviews, and performance analysis and identifies those records that proveconformance to the system. Training (4.17 (1994: 4.18)) describes training records kepton all staff, the use of job descriptions, identification of training needs and review.

2.3.3 Requirements affecting the patients

General Patient Care (4.6 (1994: 4.7)) may include that care before and duringradiotherapy or all care such as nursing, catering, portering and reception. Identificationand Traceability (4.6 (1994: 4.7)) relates to the labelling and retrieval of documentspertaining to the patient, of accessories such as jigs, cut-outs, plan films, treatment sheetsand case notes. It means being able to identify at what stage in the treatment process thepatient is at any time and may include referral, registration through to discharge, follow-up and archiving of records. Process Control (4.8 (1994: 4.9)) includes all activitiesinvolving the planning and treatment of the patients, i.e. teletherapy, brachytherapy and


unsealed sources, including checking monitoring and recording, at all stages leading upto discharge.

2.3.4 Requirements relating to equipment

Inspection and Testing (4.9 (1994: 4.10)) covers the inspection and testing of radiotherapyequipment, both at commissioning and routine maintenance and checks. It also includesthe checks, monitoring and inspection of patients and their records if this has not beenalready covered in Process Control. It may include records of treatment verification, ofTLD measurements, of dose calculations and summaries. Again it also includes thechecking of incoming goods and services, which may refer back to Suppliers (4.5 (1994:4.6)).

Inspection, Measuring and Test Equipment (4.10 (1994:4.11)) covers the calibrationand records of the equipment necessary to carry out the above checks and Inspectionand Test Status (4.11 (1994: 4.12)) states the need to maintain, record and identify thecalibration status of the equipment described in 4.10 (1994: 4.11).

2.3.5 Requirements relating to monitoring the system

Non-conformities (4.12 (1994: 4.13)) may be identified at the delivery of purchasedproducts that may not conform to specification, at failure of equipment, at failure ofprocedures during audit or via complaints which may be internal or external. It also isthe mechanism for recording any failure of the system such as errors. If it is necessary tocarry out a procedure in a different way to that set down, e.g. because of the infirmity ofa patient, then this can be covered by an authorised concession, which should be addressedin this section. Corrective Action (4.13 (1994: 4.14)) describes the action to take in theevent of a non-conformity or complaint. Internal Audits (4.16 (1994: 4.17)) should becarried out by staff who have received audit training and should take place at least every6 months. They are the monitoring device and test of the entire system and correctiveaction must take place in the event of failures. The results of audit and the correctiveaction must be reported to management review (4.1) and be a compulsory item on theagenda. Handling, Storage, Packaging and Delivery (4.14 (1994: 4.15)) may refer toproducts manufactured on-site, e.g. in the Mould Room or Workshop and must take intoaccount the safety and labelling of these goods. It may also refer to the care and dischargeof the patient (4.6 (1994: 4.7)). Statistical Techniques (4.18 (1994: 4.20)) may apply tothe collection, recording, analysis and presentation of data presented for performanceanalysis at management review (4.1).

2.3.6 Summary

All the above 18 headings must be addressed in the level 1 Quality Manual, which willdescribe in a few paragraphs the general policy of the area concerned, and the keypersonnel and departments involved in carrying out the activity. It will also refer at eachstage to the relevant procedures in the level 2 manual that describe how the activities arecarried out. The level 2 procedures manual will contain a set of procedures with thesame numbering system. The scope and objectives of the procedure, the persons


responsible for it and to whom they report, the documentation and forms used in carryingit out and the actual method used must be stated, including any interfaces andcommunications necessary with other disciplines to ensure delays or misunderstandingsdo not occur.

The Work Instructions, which form the third level of the documentation, are detailedinstructions for any procedures for which written instructions are considered necessaryto ensure correct implementation. They may be numbered in the same way to indicate towhich procedure they refer. For instance, regular calibration of treatment units, iridiumwire loading, treatment sheet checks, admission and discharge lists are all occasionswhere a work instruction might be considered valuable. On the other hand, such actionsas producing a computer plan of a four field bladder, setting up the patient and thetreatment unit, and monitoring the response and side effects may be considered not torequire work instructions, as being the trained actions of a professional. (Further guidanceon this is given in Section 2.4.) Physics Data Sheets and Local Rules also form part ofthis level of documentation.

The final part of the Quality System may reference documents such as RadiationProtection Legislation, Health and Safety Regulations and equipment manuals. Again itis important that these are controlled to ensure that only current issues are in use.

2.4 Level of detail required in physics quality systems

In setting the level of detail in quality manuals, consideration should be given to thebalance between the need to avoid gaps and ambiguities which could lead to error, andthe administrative load introduced by excessive detail. We have seen that it is convenientto divide the quality manual into three levels. The first-level manual is a thin document,and acts as an overview of the quality system. The greatest amount of detail at this levelwill be the description of the organisation within the department, and this will include astructural diagram of job responsibilities, traced as far as the Chief Executive of theTrust or hospital. It should only be necessary to go down to the second level ofresponsibility within the department itself, that is, head of department and those whoreport to the head. It would he unhelpful to name individuals in the manual becausethese names will probably need to be changed more frequently than the job responsibilitiesthemselves and frequent revision of manuals should be avoided. Matching of names tojobs can be done through job descriptions.

The remainder of the first level of the manual will serve as an introduction to theindividual chapters in the second level, with each chapter requiring less than a page ofintroduction on average.

In Section 2.3, the contents of the second level were divided into requirements whichcan be broadly categorised as the processes of management, the processes of monitoringthe quality system and the processes which constitute provision of the service itself. Theservice-provision processes are described in greater detail in the third level of the qualitymanual. In the second level, the aim should be to describe processes concisely enough toavoid ambiguity, particularly with regard to people’s responsibilities, and to avoidembellishments which do not affect the efficacy or safety of the processes. For example,in the case of routine output calibration of a linear accelerator, it would be sufficient at


this level to say that this measurement was carried out weekly and was the responsibilityof the head of dosimetry, but that it may be carried out and countersigned by medicaltechnical officers. If more detailed instructions exist these should be referenced in theprocedure.

The third level of the quality manual contains work instructions which are pointed toby the procedures outlined in the second level. The detail in these work instructions is,to some extent, a matter of taste. There is a perception that work instructions erodeprofessionalism, and this perception will increase according to the amount of detailcontained within the work instructions. This may be offset somewhat by asking thosewho carry out the procedures to draft the work instructions for these procedures, whichis sensible in any case. It may help to imagine that the work instructions are being writtenfor a new member of the department who has worked in another hospital. While theincomer may be familiar with the general process of calibration, it is important to knowwhether the chamber is placed at the isocentre or at a given depth below the surface of aphantom at 100 FSD, and what tolerance on the calculated output is acceptable. The aimis to anticipate branches where an incomer could reasonably make the wrong decision,and to include enough detail in the work instructions to preempt such mistakes.

The third level of the quality manual will also contain data charts and forms. Clearlythese must contain all of the data necessary for the procedures in question.

It may be helpful to keep in mind that, if the quality system is to be audited externallyby consultants more familiar with ISO 9000 than radiotherapy physics, they will be asinterested in looking for proof that processes are carried out exactly as described as bythe amount of detail in the work instructions. There is a temptation to include excessivedetail in a quality system, particularly with a view to convincing an auditor that thesystem has been well thought out. The more likely consequence is that the system willhave to be revised and updated more frequently than would otherwise have been necessary,so that the quality system will come to be regarded as a bureaucratic constraint, and thatis in nobody’s interest.

2.5 Interdepartmental audit and other methods

Part of the QART report (Bleehen 1991) underlined the need to identify and record qualityproblems in a radiotherapy service with a view to implementing solutions and changingpractice. For the physics component of the service this would include the identificationof systematic errors in both the data and the procedures in use in radiotherapy. Onemethod of achieving this is the use of both internal and external audit. Audits that havebeen used for radiotherapy physics have included both national and internationaldosimetry intercomparisons and interdepartmental audit. Thwaites (1994, 1996) hasreviewed the international approach to audit.

In the United Kingdom the Institute of Physical Sciences in Medicine (IPSM, nowInstitute of Physics and Engineering in Medicine) began a national dosimetryintercomparison in 1988 (Thwaites et al 1992). This included the checking of samplebasic dosimetry data together with the measurement of doses from a computer plannedtreatment to a patient equivalent phantom. This survey demonstrated, among other things,the value of such audits, but was not completed until 1991. The time scale of such projects


is therefore too great for regular use and the survey itself was not designed to assess theconsistency and comprehensiveness of the quality systems employed in radiotherapyphysics. A second such study in which electron dosimetry was considered was completedin 1996 (Nisbet and Thwaites 1997).

By 1991 several groups in the UK were considering the role and form ofinterdepartmental audit. The results of one such pilot study of interdepartmental audithave been reported in the literature (Bonnett et al 1994). In the same year the IPSMRadiotherapy Topic Group proposed the setting up of a number of interdepartmentalaudit groups covering the whole of the United Kingdom and a meeting of these groupsin 1993 agreed the basis of a standard audit.

At present interdepartmental audit is restricted to external beam therapy. The IPSMInterdepartmental Audit Group has agreed recommendations for photon audits but nonehave yet been made for electron beams or brachytherapy. A proposal of a possible protocolfor electron audit has been reported in the literature (Bonnett et al 1994). A joint workingparty of the British Institute of Radiology and the IPSM has completed a report onBrachytherapy Dosimetry (Aird et al 1993). Following this a number of centres are nowin the process of developing audit techniques for brachytherapy.

Both dosimetry intercomparisons and interdepartmental audit will now be consideredin more detail.

2.5.1 Dosimetry audit: mailed programmes

The first dosimetry quality assurance programme was started in 1967 by the IAEA/WHO(Svensson et al 1990) who developed a postal dosimetry service for developing countrieswhich made measurements at a reference point for cobalt-60 radiation. This service hasbeen extended to high energy X-ray beams. In 1988 the EORTC developed a postalservice to check beam outputs in reference conditions as part of a quality assuranceprogramme for European centres participating in clinical trials. This has been extendedto include centres not involved in clinical research and the range of measurements madehas been increased (Dutreix et al 1994). This programme started in 1991 and is calledthe ‘European Network for Quality Assurance in Radiotherapy’ and includes six Europeancountries: Belgium, France, Italy, The Netherlands, Sweden and the Czech Republic. Itwill soon be extended to Greece, Ireland, Portugal and Spain. The University HospitalSt Rafael at Leuven in Belgium acts as the ‘coordinating centre’ and the Institut GustavRoussy at Villejuif in France as the ‘measuring centre’. There is one national referencecentre in each of the participating countries. Derreumaux et al (1995) describe the networkin detail.

Initially, before any measurements are performed a questionnaire concerning staff,treatment planning systems, simulators, and radiotherapy and dosimetry equipment issent by the national reference centre to the local centre. This is in order to assess possiblecorrelations between department structures and measured uncertainties. Mailedthermoluminescent dosimeters are then sent to each local centre. The measurement centremails the TLDs and measures them on their return. The coordinating centre sends outthe questionnaires, supervises the organisation and analyses the results. Allcommunications with the local centres go through the appropriate national centre. Detailsof the measurements are given in the literature (Dutreix et al 1994).

The reproducibility, dose response linearity, fading accuracy and energy dependence


of the thermoluminescent dosimeters have been checked by the measuring centre duringthe first year. The energy dependence between cobalt-60 and 25 MeV has been shown tobe no more than 2.2 per cent. Accuracy is improved, as in work with primary standardsintercomparisons, by the use of fresh powder TLD.

In the first stage the TLDs are placed in a small plastic holder which is irradiated in awater tank. Measurements are made of the dose on the central axis for both cobalt-60and high energy X-ray beams. This is done under reference conditions at either 5 cm or10 cm depth depending on beam energy. For high energy X-rays the quality index is alsomeasured.

In the second stage a multipurpose phantom designed previously for the IAEA isused. This has PVC walls and two trapezoidal faces and is mailed to the centre. It isfilled with water on site and irradiated successively with three beams:

1. A large vertical square beam perpendicular to the phantom surface. This is used tomeasure beam flatness and symmetry at 4 cm depth and dose at 12 cm depth.

2. A horizontal beam incident onto the 30° oblique surface. This measures the obliquitycorrections at 5 cm and 23 cm depth.

3. The same horizontal beam but with a wedge chosen to correct for surface obliquity.

It has been proposed to extend this work by making measurements of the doses deliveredto patients.

2.5.2 Central audit

There have been a number of studies of this kind, mostly associated with clinical trials.Internationally the most extensive of these in terms of audit content have been thosecarried out for several different clinical trials by the AAPM Radiological Physics Centerat the University of Texas. In the United Kingdom the most extensive programme hasbeen the one performed as part of the trial of continuous hyper-fractionated acceleratedradiotherapy (CHART) (Aird et al 1994).

For the purposes of the CHART trial the audit was carried out by a visiting team fromMount Vernon Hospital using their own equipment. They visited all the centres involvedin the trial: 12 in the United Kingdom, two in Sweden and one in Germany. The protocolused included the following measurements: mechanical alignment of the treatment gantry,beam symmetry and uniformity, X-ray and optical field congruence, quality index, andwedge factor. Two separate groups of measurements were also made usinganthropomorphic phantoms: one corresponding to a bronchus treatment and the other toa head and neck. Slices 6 cm thick were constructed to match typical patient cross sectionsusing lung, bone and water equivalent resin. In each phantom five holes were drilled atpositions covering a typical tumour volume and two further holes within critical organs.Treatment plans were prepared by the local centre for each of the two phantoms as thoughthey were normal CHART patients. The phantom was positioned on the treatment machineby a radiographer from the local centre using the marks on the outline. A treatmentaimed at giving 1.50 Gy to the tumour was delivered and the doses from each fieldmeasured at each point. The contributions from each field were added and comparedwith doses calculated from the treatment plan.


2.5.3 Interdepartmental audit

Interdepartmental audits are designed to be carried out on an annual basis. The overallbenefit is to increase confidence in the service, to identify systematic error and to providea basis for future improvements. For the purpose of this exercise the audit shouldencompass quality assurance, dosimetry, basic data and the use of those data in therapyplanning. The first stage is to establish an audit group of participating radiotherapy centres.This group can be organised so that either one centre will audit the other members of thegroup or each member of the group will audit one other member of the group. Auditscarried out on an interdepartmental basis obviate the need for a national centralorganisation with a remit to carry out this work. Audits carried out by all members of thegroup have the advantage that they involve more personnel and thereby contribute moreto the overall learning process. It is recommended that the audit group meet annually toreview both progress and the scope of the audit.

It is obvious that systematic errors can be introduced into a radiotherapy system at allstages of the overall radiotherapy planning and beam calibration process. The system ofaudit measurements should be designed to identify these errors. The measurements thatare required can be divided into three sections:

1. Measurements of a sample of documented data.2. A simple dosimetry intercomparison.3. Measurement of an applied treatment to a test phantom for which a treatment

prescription has been prepared.

It is considered important to include a simple dosimetry intercomparison particularlywhere the ionisation chambers in use at both the participating centres are calibratedagainst different regional standards. This will also provide an additional check on thesecondary dosimetry standards. The next stage of the audit involves the measurement ofthe data used in both treatment planning and treatment calculations. This is not consideredas a repeat of the total measurement process carried out at commissioning but as asampling process to indicate the continuing integrity of the data set. If sample mechanicalmeasurements are included they also serve to indicate the current state of the machinebehaviour and to highlight any possible sources of error in the other measurements. Thecomparison of measurements in the phantom with the absorbed dose calculated fromtabulated data or, in the case of the photon beam calculated using a computerised planningsystem, provides the final overall check of the system.

In addition to the programme of measurements there should be a review of qualityassurance records and procedures. This will serve to highlight areas for improvement inthe systems in use and provide a useful means of appraisal of departmental practice.

The objectives of a standard audit have been defined by the IPSM working group asfollows:

1. To be able to demonstrate that radiation doses administered to patients should bewithin 5 per cent of that prescribed as recommended by the ICRU (1976).

2. To be able to identify gross systematic error.3. To audit one machine and one modality per year at each participating centre.

For photons it is recommended that the standard audit should include:

1. Dosimetry measurements that will encompass the accuracy of the treatment planningsystem.


2. An ionisation chamber intercomparison between host and auditing centre.3. Measurement of a sample of wedge factors.4. Measurement of beam energy (quality index).5. An examination of the quality control records and procedures for the machine being

audited.

The ‘standard’ audit defines the minimum audit that should be carried out. In additionsome audit groups have included measurements of mechanical alignment and samplemeasurements of other data such as relative output measurements, depth dose data andtray transmission factors.

For the assessment of the treatment planning system it was recommended that anIPSM type phantom (Figure 2.1 and Thwaites et al 1992) be used, either with or withoutinhomogeneity. Less complex phantoms can also be used providing additional tests arecarried out. An example of such a phantom which is based on the IPSM design togetherwith details of measurements has been given in the literature (Bonnett et al 1994). Aminimum requirement for the assessment of the planning system is that the phantom beirradiated with three fields, one of which should be a wedged field and one have obliqueincidence. Measurements made in the phantom using an ionisation chamber should thenbe compared with the calculation of absorbed dose derived from the treatment plan.Some audit groups have developed more complex phantoms to enable more complexplanned situations to be included in regular audit following an annual, but cycledprogramme (Thwaites 1996).

In order to audit records and procedures it is necessary to have a consistent set ofcriteria by which to make an assessment and examination. Currently there are nouniversally accepted standards for quality control but the recommendations of this reportmay be adopted as a guide to the acceptable limit for quality control. Alternatively itwould be possible to use another mutually agreed standard by which to judge the qualitysystem, for example IEC 976 (1989).

Figure 2.1. IPSM phantom recommended for interdepartmental audit. (Reproduced with permission of IPEM fromIPEM Report 68, 1996).


A summary of suggested criteria for the examination of records and procedures(Bonnett et al 1994) is as follows:

1. Method: Is there a clear explanation for the process?2. Instructions: Are there written instructions for the process?3. Tolerances: Are tolerances stated and how do they compare with IPEM

recommendations? [this document]4. Frequency: Is the frequency of the process stated and how does it compare with IPEM

recommendations? [this document]5. Personnel: Which staff undertake the procedure in question?6. Training: What training has been given to staff undertaking specified tasks?7. Action: How is corrective action made and documented?8. Equipment: Is the test equipment maintained and calibrated?9. Records: How are the results recorded and are they easy to understand?

Obviously not every item of this list is applicable to every procedure considered. Therecord of the audit may be recorded using a proforma.

Following the audit the auditors are responsible for presenting a report of the auditand the audit group may decide that this should be followed by a response from the hostcentre.

The report should be used to provide a clear record of the audit. As such it shouldinclude a description of the audit, both original and calculated data, a summary and a setof recommendations. The description of the audit should also include a résumé of all themeasurements made together with a summary of the procedures in use in the hostdepartment. A copy of all the original data measured in the audit together with the notesof the assessment of the records and procedures should also be included in the report.This will enable systematic errors in the departmental system to be easily traced and inaddition misinterpretations by the auditors of departmental practice or incorrectmeasurements or calculations can be easily identified. The summary of the audit shouldhighlight points of concern and recommendations which require specific action.

The response to the report is seen as an essential part of the audit process, providingthe host with an opportunity to place on record a statement correcting misinterpretationswhich may have occurred at the time of the audit, together with comments on therecommendations and an undertaking to implement recommendations where they havebeen accepted. It has also been found to be a useful mechanism for recording explanationsor disagreements with the findings of the auditors.

The report and the response together provide an important element of the audit. Theydocument the data recorded and the observations made, and set out a clear understandingof any differences between the host and auditor concerning aspects of the physics service;they also record the changes that are needed to be made to the service. Goals are thereforeset and a baseline is provided by which progress can be assessed at the next audit.

References

Aird EGA, Jones CH, Joslin CAF, Klevenhagen SC, Rossiter MJ, Welsh AD, Wilkinson


JM, Woods MJ and Wright SJ 1993 Recommendations for Brachytherapy DosimetryReport of a Joint BIR/IPSM Working Party (London: British Institute of Radiology)

Aird EG, Williams C and Mott GT 1994 The accuracy of delivery of radiotherapy asdeduced from extensive quality assurance. Proc. Int. Symp. on Measurement Assurancein Dosimetry (Vienna, 1993) IAEA-SM-330/57 (Vienna: IAEA) pp 257–266

Bleehen 1991 Quality Assurance in Radiotherapy. Report of a Working Party of theStanding Subcommittee on Cancer of the Standing Medical Advisory Committee May1991 (London: Department of Health)

Bonnett DE, Mills JA, Aukett RJ and Martin-Smith P 1994 The development of aninterdepartmental audit as part of a physics quality assurance programme for externalbeam therapy Brit. J. Radiol. 67 275–282

BS 4778 Quality Vocabulary Part 1:1987 International Terms. ISO 8402:1994 (London:British Standards Institution)

Derreumaux S, Chavaudra J, Bridier A, Rossetti V and Dutreix A 1995 A European qualityassurance network for radiotherapy: dose measurement procedure Phys. Med. Biol. 401191–1208

Dutreix A, Derreumaux S, Chavaudra J and van der Schueren E 1994 Quality control ofradiotherapy centres in Europe: beam calibration. Radiother. Oncol. 32 256–264

ICRU (International Commission on Radiological Units) 1976 Report 24 Determinationof Absorbed Dose in a Patient Irradiated by Beams of X and Gamma Rays in RadiotherapyProcedures (Washington DC: ICRU)

IEC (International Electrotechnical Commission) 1989 Medical Electrical Equipment –Medical Electron Accelerators. Functional Performance Characteristics IEC Publication976

ISO 9001 1994 BS EN ISO 9001:1994 Quality systems. Model for Quality Assurance inDesign, Development, Production, Installation and Servicing (London: British StandardsInstitution)

Nisbet A and Thwaites DI 1997 A dosimetric intercomparison of electron beams in UKradiotherapy centres Phys. Med. Biol. 42 2393–2409

Svensson H, Hanson GP and Zsdanszky K 1990 The IAEA/WHO TL dosimetry servicefor radiotherapy centres 1969–87 Acta. Oncol. 29 461–7

Thwaites DI 1994 Uncertainties at the end point of the basic dosimetry chain. inMeasurement Assurance in Dosimetry IAEA STI/PUB/930 pp 239–255 (Vienna: IAEA)

Thwaites DI 1996 External audit in radiotherapy dosimetry. in Radiation Incidents ed KFaulkner and R Harrison) pp 21–28 (London: BIR)


CHAPTER 3

Radiotherapy Imaging Devices

3.1 Treatment simulators

3.1.1 Introduction

This chapter deals with the quality control of radiotherapy treatment simulators. Testson such equipment fall into two categories – those on the mechanical performance andthose on the imaging chain. Since, however, a simulator is not a diagnostic tool but aplanning tool intended to produce precise position information for the prospectivetreatment, the emphasis in QC tests must be on the machine’s mechanical performance.In this respect, since any position error existing on the simulator may be transferred toand compounded with those already existing on the treatment machine, it is essentialthat a simulator exhibits an excellence of mechanical performance at least as good as, ifnot better than, the best treatment machine it will be called upon to simulate. For thisreason, some of the recommended mechanical tolerances will be seen to be more stringentthan the equivalent tolerances on, for example, linear accelerators. The imaging chain,however, must also be subject to tight quality control – both to ensure the production ofthe best possible quality of image but also to keep patient doses to a minimum. There isa widely held belief that because simulator patients are proceeding to radiotherapytreatment, the minimising of dosage at this stage is unnecessary – this viewpoint is notaccepted in this protocol.

Recommended performance tolerances for simulators are contained in the BritishJournal of Radiology Supplement 23 and in IEC/TR 1170 (1993); the figures used inthis protocol are based on those in these two documents. However, for an individualmachine these figures should be taken in conjunction with the performance figuresobtained during the course of a rigorous acceptance schedule. Manufacturers’ acceptanceschedules are not always sufficiently comprehensive and so each schedule needs to becarefully examined to ensure that all the aspects detailed in these documents have beencovered and, if not, extra tests added to provide the complete data set.

The frequency of testing is based on the survey of radiotherapy centres in the UK.They are given in the main text under the description of the tests and summarised Table3.1 (see Section 3.1.7). Regardless of who carries out the tests they must be under thesupervision of a medical physicist.

3.1.2 Description of tests

Many of the mechanical tests are similar to those applicable to linear accelerators and sosome test methods detailed in Chapter 5 would also apply to simulators. McCulloughand Earle (1979) give useful guidance on methods specific to simulators. For the purposesof QC tests the simulator does, of course, exhibit one overwhelming advantage overmost treatment machines in that all position aspects of the radiation field can be examinedin real time through the fluoroscopic system and full advantage should be taken of this

35Radiotherapy Imaging Devices

in devising tests. As always, the devices employed and the protocols under which theyare used should be carefully designed so as to yield a quick ‘yes/no’ result without(particularly for the more frequent checks) any need for interpretation.

Figures 3.1–3.4 illustrate some suitable devices developed either specifically formechanical QC tests on simulators or for all types of isocentric radiotherapy equipment.Some other suitable devices, intended specifically for simulators, are described by Hortonet al (1987) and others are available commercially.

Figure 3.1. Test block suitable for the daily checks on a simulator. The block is made from aluminium withembedded ball-bearings serving as radio-opaque markers for the central axis and the four corners of the field. Theoptical pattern defines the tolerance limits for alignment of the crosswires and the size and alignment of the fielddelineating wires. As the pattern is sunk in a ‘well’ this permits visualisation of the relative alignment of all theisocentric lasers with the optical field. The spirit levels are used in conjunction with the test platform shown inFigure 3.2. The axes are labelled to enable identification of the location of any errors.

Figure 3.2. Test table on which the test block is mounted for the daily optical tests. Levelling screws areincorporated in the base and used in conjunction with the block-mounted spirit levels to compensate for anylongitudinal or lateral tilt in the couch top. The height adjustment facility provides fine control independent of thecouch movement.


If the machine is equipped with a simple hard-copy device, such as a video-printer,this will provide a convenient method of producing a permanent record of test results.For the more rigorous annual tests, however, film should always be used.

3.1.3 Mechanical aspects

3.1.3.1 Crosswires

The alignment of both the optical and radiation image of the crosswires should be checkeddaily, at least with the gantry vertical (at 0˚), and less frequently at other gantry anglesand over the focus to axis distance (FAD) range available on the machine.

Figure 3.3. Rotatable test pattern which defines field sizes from 5 cm × 5 cm to 20 cm × 20 cm and can be used atthe four cardinal gantry angles. The base contains spirit levels and a levelling mechanism. The pattern is engravedin a plastic sheet and the engraving is filled with a radio-opaque paste. This device is suitable for the monthlymechanical protocol.


3.1.3.2 Optical distance indicator (range-finder)

The range-finder should be checked daily at 100 cm and gantry 0˚ and less frequentlyover its complete range. Because of the effect of flexing of the gantry arm and anypossible imprecision in the FAD movement the performance of the range-finder shouldalso be checked at different gantry angles and over the range of FADs available.

3.1.3.3 Isocentric lasers

The daily checks show whether all the laser beams are self-consistent and consistentwith other isocentric indicators. In the annual protocol, or if a need for readjustment isdemonstrated by routine testing, the position of the isocentre should be redefinedradiographically or fluoroscopically, and the lasers reset (if necessary) to this point.

A convenient method to define the isocentre fluoroscopically is to position a suitablymounted ball-bearing at a point approximating to the isocentre. Then, by successiverotations of the diaphragm system and the gantry the ball-bearing can be re-located atthe mean position of all the rotations, i.e. the isocentre. A positioning device similar tothat shown in Figure 3.4 is suitable for mounting the ball-bearing in this procedure orfor positioning the reference pointer used to establish the ‘mechanical’ isocentre by themethod described in Chapter 5. If a permanent record is required a star film as describedin Section 5.2.9.3 may be useful.

Figure 3.4. Three-dimensional calibrated positioning device which can be used to quantify positional misalignmentand for mounting the test ball-bearing referred to in Section 3.1.3.3.

3.1.3.4 Field defining wires

The integrity of the performance of the field-defining wires is probably the most criticalaspect of the simulator and checks need to examine size, alignment, symmetry andorthogonality. Note that on a simulator, field size should be measured between the centresof the shadows of opposing field wires (BIR 1989, 2.3.5)

The daily checks will cover the size and isocentric alignment of the optical andradiation image of a 10 cm × 10 cm field at 100 cm FAD and gantry 0°. The less frequentchecks will need to examine the field size and alignment under all possible conditions:


1. Over the entire range of travel of field wires:Where independent movement of the field wires is available, field wire position shouldalso be checked. Field size and alignment tolerances depend on the field size as follows:

On size a tolerance of 2 mm applies to fields from 3 cm × 3 cm to 20 cm × 20 cm.From 20 cm × 20 cm to the maximum square field the tolerance is 1.0 per cent.

On alignment the respective tolerances are 1 mm and 0.5 per cent.These checks should also look at reproducibility and test for any differential in wireposition that might appear as a result of approaching a given field size from eitherdirection.

2. Over the full FAD range.3. At all cardinal angles of the diaphragm system.4. At all cardinal angles of the gantry.

With some machines significant asymmetry can be introduced in the delineated fielddue to sag in the field wire mechanism at lateral gantry angles. A significant andsimple test that should be included in the monthly schedule is a check on the congruenceof horizontally and vertically opposed fields.

3.1.3.5 Daily checks of crosswires, range-finder, lasers and field defining wires

The daily checks on the crosswires, range-finder, isocentric lasers and field definingwires can be combined into one procedure:

1. The machine is set to the following conditions:• FAD 100 cm• Gantry 0°

• Diaphragm 0°

• Field Size 10 cm × 10 cm2. The test block (Figure 3.1) is placed on the test platform (Figure 3.2), set at an axis

distance of 100 cm using a test pointer made specifically for this purpose, alignedwith the image of the crosswires and levelled. The accuracy of the range-finder, theconsistency of the lasers and their relative alignment with the optical image of thecrosswires can be immediately assessed. In this position the size and alignment of theoptical image of the field defining wires can also be checked.

3. The diaphragm system is rotated through its range and the alignment of the crosswiresand the size and alignment of the field defining wires checked at each cardinal angle.

4. Step 3 is repeated in fluoroscopic mode to check the X-ray image of the crosswiresand field defining wires.

For the more extensive and detailed set of monthly geometric tests the rotatable testpattern shown in Figure 3.3 is suitable.

3.1.3.6 Couch isocentric rotation

Couch isocentric rotation should be checked monthly. The accuracy of this movementseems to present inherent problems on some designs of simulator couch (and treatmentcouch). If the accuracy is near to the limit of acceptability at the time of acceptance thenit is particularly desirable to ensure that it does not degrade any further in use.

Indication/readout(s) should have a simple check monthly and a full check annually.


3.1.3.7 Couch vertical movement

A simple check that couch vertical movement is parallel to the beam axis at gantry 0°

should be carried out monthly. The annual schedule should include a test with the couchcarrying a simulated load.

3.1.3.8 Couch position readouts

Indication/readout(s) of couch height and longitudinal and lateral position should have asimple check monthly and a full check annually. Indication should be within 1 mm.

In some centres couch position scales may not be used in which case tolerances maybe relaxed, but a notice should then be displayed indicating that the scale readings arenot to be used.

3.1.3.9 Couch longitudinal and lateral translation under load

Couch longitudinal and lateral translation should be fully checked in the annual schedulewith the simulated load. As well as checking the integrity of the movements in thehorizontal plane it is also desirable to test for any longitudinal or lateral tilt developingas the two movements are taken through their respective ranges and for any lateral orlongitudinal flexing of the couch top.

The recommended tolerances are 0.5° in tilt and 5 mm in vertical height. The generoustolerance of 5 mm in vertical height is in recognition of the fact that most simulators areequipped with a diagnostic (radiotransparent) type of couch top which cannot be expectedto exhibit the same rigidity as a steel-framed treatment couch top.

3.1.3.10 Image intensifier carriage

The most significant aspect of this component is the indication of the image intensifierposition along the axis parallel to the beam axis as this will be used in determiningmagnification factors. The accuracy of this indication therefore should be checked, atleast at one position monthly, and annually over the complete range.

3.1.3.11 Gantry angle/diaphragm angle

The accuracy of the indicators/readouts associated with these two parameters shouldhave a simple check monthly and a full check annually.

3.1.3.12 Alignment of shadow trays

The alignment of shadow trays should be checked at all diaphragm angles – both atgantry 0°and at one of the lateral gantry angles.

3.1.4 Imaging system

3.1.4.1 Fluoroscopy system

The quality of the fluoroscopic system can be quickly and easily checked six-monthly(or if at any time there is doubt about the performance of the system) using one of theLeeds test objects (MDD 1994) (Figure 3.5) or the phantom described in BJR Supplement23, Appendix III (BIR 1989). If the simulator is equipped with a hard-copy device suchas a video-printer then such a test can also serve as a check on this device as well.


As this check is simply a constancy test of the linear and contrast resolution of thesystem, the tolerances can only be set at the time of commissioning of the equipment.

The six-monthly checks should be complemented by a full schedule of annual testsappropriate to any diagnostic fluoroscopic system. Such tests are identified in MDD1994.

Figure 3.5a. T0.10 Leeds test object – detail layout.

Figure 3.5b. T0.10 Leeds test object – fluoroscopic image.


3.1.4.2 Radiographic system

Periodic tests appropriate to not only the X-ray equipment but also to the processingsystem are detailed in HPA TGR 32 (HPA 1980) and BJR Supplement 18 (BIR 1985).However, a number of radiographic performance aspects have more significance insimulators than in conventional diagnostic equipment:

1. Because of the geometric configuration both the precise positioning of the tube andthe alignment of the broad and fine focal spots are far more critical. In the event of achange of insert great care has to be taken to ensure precise location of the tube focus.

Because all fluoroscopy and as much as possible of the radiography will use thefine focus, alignment of the tube should be carried out with this focus. Any observeddeviation from perfect alignment should be regarded as unacceptable.

For the same reasons the alignment of the two focal spots is also much morecritical, the tolerance being an image shift with change of focal spot of 0.5 mm at 100cm focus to film distance (FFD). This feature should be checked six-monthly as wellas at the time of tube replacement.

2. Because of the high degree of magnification in simulator imaging, particularly of thefield and central-axis defining wires, replacement inserts must be specially selectedwith focal spot sizes as near as possible to their nominal values. (A focal spot havinga nominal size of 1.0 mm × 1.0 mm may, under the IEC 336 definition (IEC 1982),have an actual size of 1.4 mm × 2.0 mm. This fact needs to be considered whenspecifying focal spot size at the time of purchase.)

Figure 3.6. Test phantom for simulator quality control. Reproduced by kind permission of BIR from BJRSupplement 23 (BIR 1989). 1, Lead wire delineators; 2, 1.5 mm holes in 2 mm Al; 3, ‘Detail’ test strips in 1 mmAl; 4, Copper step wedge; 5, 2 mm lead; 6, Principal axis markers.


3.1.5 Security, safety devices and interlocks

The security of attachment of removable devices such as shadow tray systems, grids,cassettes, etc. should be checked daily before use of the equipment and any fault detectedshould be rectified immediately.

The correct operation of all safety devices and interlocks should form part of a formalplanned preventive maintenance programme which, like the QC system, should haveboth a monthly and a full annual schedule.

3.1.6 Equipment required

• Optical and radioopaque test patterns• Mounting system to enable patterns to be used at any gantry angle• Reference pointer to indicate plane 100 cm from focus• Ball-bearing device

DailyCrosswiresOptical distance indicatorIsocentric lasersOptical/radiation field size (10 cm × 10 cm)Optical/radiation field alignment (10 cm × 10 cm)

MonthlyIsocentricity of gantryAlignment of FAD movementCrosswire rotation at 90 cm and 110 cm FADIsocentricity of couch rotationAlignment of couch vertical movementOptical/radiation field sizes (all sizes)Optical/radiation field alignment (all sizes)Congruence of vertically and laterally opposed fieldsPositional readoutsAlignment of shadow traysRadiographic QC tests

Six monthlyFluoroscopic image qualityAlignment of fine and broad focus radiographicimages

AnnualRedefine isocentreCrosswires and field wires at all gantry anddiaphragm angles and FADs – all filmOptical distance indicator at all gantry anglesGantry and diaphragm anglesLoaded couch testsFull tests on fluoroscopic and radiographic system

Table 3.1. Recommended frequencies for simulator QC tests.

Test and frequency Reference Tolerance

3.1.3.1, 3.1.3.5 1 mm3.1.3.2 2 mm3.1.3.3 1 mm3.1.3.4 2 mm3.1.3.4 1 mm

3.1.3.1 1 mm3.1.3.1 1 mm3.1.3.1 1 mm3.1.3.6 2 mm3.1.3.7 2 mm3.1.3.4 2 mm/1.0%3.1.3.4 1 mm/0.5%3.1.3.4 1 mm3.1.3.4 – 3.1.3.11 1 mm/0.5%3.1.3.12 1 mm3.1.4.1 see text

3.1.4.1 see text3.1.4.2 0.5 mm

3.1.3.3 1 mm3.1.3.1, 3.1.3.4 as above

3.1.3.2 2 mm3.1.3.11 0.5o

3.1.3.7, 3.1.3.9 see text3.1.4.1 see text


• Film supplies• Fluoroscopy test phantom• Densitometer• Tape measure• Graph paper• Box-type spirit level• Weights to load couch top

3.1.7 Summary of test frequencies

Recommended test frequencies are shown in Table 3.1. The machine should be maintainedwithin the tolerances shown and these are therefore also Action levels. If it is decidedfor operational reasons to continue to use the equipment in spite of a defect a clearnotice should be displayed authorising such use.

3.2 CT equipment

This section describes a set of quality control tests for the CT equipment to be used inthe treatment planning process. All the imaging tests presented are designed to evaluatethe final reconstructed image rather than to evaluate the X-ray characteristics or thedetector system. It has been assumed that a quality control programme to evaluate basicsystem performance (e.g. kV, CT number linearity and uniformity) is in operation on theCT scanner, as described in IPSM Topic Group Report 32 (IPSM 1981) or the reportproduced by the IMPACT group (IMPACT 1998). The image quality tests described aresupplementary measurements to evaluate image quality following transfer to the treatmentplanning computer. Additional tests are presented that will evaluate the performance ofthe alignment mechanism necessary for all CT images recorded for radiotherapy treatmentplanning. Physicists responsible for quality control in Radiotherapy should assurethemselves that appropriate checks are carried out following scanner repair ormaintenance. Good liaison between them and those responsible for CT scanner qualitycontrol is essential. Checks should also be made following software updates as these canaffect the calibration of the Hounsfield Units.

The characteristics of CT images may vary with the acquisition protocols used. It istherefore essential that the tests described should be performed on images acquired usingknown scanning protocols and further, that the same scanning protocols are subsequentlyused to reconstruct patient images prior to treatment planning. For example, imagecharacteristics may vary with field of view (FOV), image matrix, patient/phantom size,reconstruction algorithm, image filters, slice thickness and scan time. These parametersshould therefore be standardised prior to establishing a quality control programme and acombination of protocols maintained for all planning scans.

Use of high resolution options on some CT scanners may considerably reduce the CTnumber range. This may affect interpretation of the images by the treatment planningcomputer. It is therefore important to avoid application of such features unless all theimplications have been evaluated.


An additional or replacement couch top, designed to represent the couch on a treatmentunit, is generally used when recording CT images for radiotherapy treatment planning.This should be present during all quality control tests.

Recommendations as to the design of phantoms used in the tests outlined in this sectionhave been kept as non-specific as possible. Each test sets down the features required ofa phantom rather than suggesting a particular design. It is recognised that dedicatedphantoms may not be accessible within a radiotherapy department but with somemodifications it should generally be possible to make use of existing resources. Ideasfor suitable phantoms and phantom materials can be found in ICRU Report 48 (ICRU1992). It is likely that more than one phantom would be required for the range of testsset out, particularly where a number of scanning protocols are used. Under thesecircumstances, phantoms representative of the dimensions of the patient section to bestudied using each protocol should always be used.

The tests described have been sub-divided into those to be carried out on the CTscanner (Section 3.2.1) and those to be carried out on the treatment planning computer(Sections 3.2.2 and 3.2.3). Additional tests are included for spiral scanners (Section 3.2.4).In general all tests on image characteristics should be carried out after data transfer andinterpretation by the treatment planning computer.

3.2.1 CT scanner alignment checks

3.2.1.1 Alignment of internal light beam/laser with scan plane

The internal light beam system is the combination of light beams or lasers that are usedto define the scanning plane.

This test would only be necessary where the internal light beam(s)/laser is to be usedto place reference marks on the patient.

Test frequency: Monthly or after engineer’s visit.Test phantom: The phantom used can be any shape but should ideally be

representative of the patient shape and dimensions. It should bemarked with a line or groove extending through at least 180˚ aroundthe upper surface. If this line is not easily visible on the CT image,a supplementary wire or radioopaque marker should be added.

Test procedure: Align reference line with the internal light beam(s) or laser. Whenthe light beam results from more than one source, check that thecomponent beams align with one another. Scan using minimumslice thickness and check that the line is visible on the image alongits full length or at regular intervals along its length.

Tolerance: ±2 mm.

3.2.1.2 Indication of x-axis

Where possible, horizontal reference marks on the patient should be aligned with the x-axis of the scanner. This ensures correlation between gantry rotation on the treatmentunit and the x-axis of the image plane. Ideally, dedicated lasers should be fitted in thescanning room to assist with the alignment process.

Test frequency: Monthly.Test phantom: As for Section 3.2.1.1.


Test procedure: Check that the lasers from each side are aligned with one anotherat a central point above the scanning couch. Attach externalradioopaque marker lines or wires to the phantom at the levelindicated by each alignment laser. Scan the phantom and evaluatethe CT coordinates of the markers. Check that the y coordinate ofeach marker is the same. Alternative image analysis facilities maybe available; for example it may be possible to measure the anglebetween the line joining the two markers and the x-axis.

Tolerance: 1˚ between the line joining the markers and the image horizontal(i.e. the x-axis). If the angle is greater than 1°, both lasers shouldbe adjusted and the entire procedure repeated.

Note: Performance of the test in this way assumes that additional checks have beencarried out to verify that the orientation of successive images is the same. Differences inthe relationship between image horizontal in successive images may exist in non-spiralscanners where the direction of tube rotation is reversed between images.

3.2.1.3 Couch position registration

Test frequency: Monthly.Test phantom: Three markers, visible on a CT image should be used for this test.

Line markers such as plastic tubing might be used as long as thetubes are aligned with the scanning plane. Ideally, these markerswould be permanently fixed, a known distance apart, on a plasticor PMMA base. The maximum dimension of each marker,orthogonal to the scanning plane, should be 2 mm.

Test procedure: The three markers should be placed on the scanning couch, 30 cmapart (i.e. total separation 60 cm). A digital radiograph should thenbe recorded, on which all three markers are visible. Transverseimage planes, using the minimum slice thickness available, shouldthen be selected and recorded at positions predicted to pass throughthe markers. Two checks should then be performed. Firstly, thedistance between markers on the digital radiograph should becompared with the true separation of the markers. These valuesshould agree within 1 mm. Secondly, the visibility of the markerson each of the transverse images should be confirmed.

3.2.1.4 Couch deflection under load

The couch on a CT scanner may twist or sag as it is extended into the scanning ring. Thiseffect is more apparent when the couch is fully laden. The significance of this effect onradiotherapy treatment planning is small as long as horizontal reference markers areplaced on the patient. Where couch sag or twist occurs, however, scan planes will nolonger be orthogonal to the couch top. This may be a noticeable effect when off-axisslices are to be used for planning (2-D or 3-D). Under these circumstances, the relationshipbetween the CT coordinate systems on successive slices would not be constant. Theeffect would have greater significance if sequential CT images are to be used forcompensator design or stereotactic radiotherapy planning.

Test frequency: Annually or following service or repair of CT couch.


Test phantom: As for Section 3.2.1.1.Test procedure: The phantom should be marked with horizontal reference lines, as

described in Section 3.2.1.2. The phantom should then be placedon the couch approximately 1 m from the end of the couch closestto the scanning ring. The test in Section 3.2.1.2 should be repeatedto confirm alignment of the laser system. A load of approximately60 kg should then be evenly distributed between the phantom andthe end of the couch and the test in Section 3.2.1.2 repeated. Thechange in the coordinates of the horizontal reference lines willthen give an indication of couch deflection under load.

The acceptable variation will be dependent on the application for which the CT imagesare to be used but a tolerance of 2 mm (change in position of the horizontal referencemarkers when under load) over the full couch extension is suggested. Precise tolerancesshould be calculated for each application.

3.2.2 CT related acceptance tests on treatment planning computer

The term ‘acceptance tests’ is used in this context to describe those tests necessary toevaluate CT images under the following conditions:1. Installation of new or revised versions of treatment planning software.2. Installation of new or revised image transfer software.3. Incorporation of a new CT scanner into the treatment planning process.4. Installation of new CT scanner software.

More regular performance of these tests, however, would not generally be necessary.

3.2.2.1 Contouring and auto-contouring

Test frequency: During the acceptance process.Test phantom: A water filled phantom or a block of water substitute material

containing regions of bone or bone equivalent material and air orlung equivalent material. (This could include representations ofdifferent bone densities.) The outside dimensions of the phantomshould ideally be representative of a patient. Ideally, two phantomswould be used, one representing a head and the other a chest. Thechest phantom should contain representative portions of lung andrib and the head phantom should contain bone sectionsrepresentative of the skull. If anatomical phantoms are notavailable, phantoms of a more regular shape may be used but theenclosed inhomogeneities should not be less than 3 cm in theirlargest dimension. To minimise partial volume effects, the phantomshould be marked with alignment lines in planes orthogonal to thescanner axis as an aid to its correct orientation on the scanner.

Test procedure: A CT scan of the test phantom should be carried out and the imagestransferred to the treatment planning computer. The auto-contouringfacility on the treatment planning computer should be used tooutline the surface of the phantom. Manual and/or auto-contouringshould be used to outline the internal inhomogeneities and


structures. The dimensions of each contour should be measured onthe screen display and on a printed hard-copy. The results shouldbe compared with the true phantom dimensions. Overall dimensionsshould be within 2 mm of the true phantom dimensions.

Note: The results of auto-contouring may depend significantly on the threshold valuesset on the treatment planning computer. The accuracy of auto-contouring will depend onthe algorithm used but in most cases an edge is detected using a preset transition inadjacent CT numbers. The transition values selected, or set within the software, willdetermine the position of the contour. In a similar way, the grey-scale settings of theimage display will affect the observed edge of the phantom structures during manualcontouring. Great care should be taken to use standard settings and grey-scale valuesduring the quality control procedure. The same values should then be used duringtreatment planning of patients.

3.2.2.2 Image display on TPS

The purpose of this test is to establish calibration of the CT images following their transferto the treatment planning computer. Generally, some form of calibration table would beused by the treatment planning computer to enable precise conversion of CT numbers toelectron density values. The precise method and phantom used for this test may varywith treatment planning software and manufacturers would be expected to provide detailedguidance on the correct calibration procedure.

Test frequency: During the acceptance process.Test phantom: A phantom should be used containing materials with a range of

electron density values. A suitable phantom may be supplied bythe manufacturer of the treatment planning computer or,alternatively, may be available from the supplier of the CT scanner.The materials used should span the range of values expected withinthe human body.

Test procedure: A CT image of the test phantom should be recorded and transferredto the treatment planning computer. The calibration procedurerecommended for the treatment planning software should then becarried out. Following this calibration, the electron density valuesindicated for the materials within the test phantom shouldcorrespond with the true values, to within 1 per cent.

3.2.3 Routine quality control tests on CT images following transfer to the treatmentplanning computer

If a special couch insert is routinely used for radiotherapy planning scans, markers canbe embedded into this insert. These may take the form of two parallel piano wires orangiocaths spaced at a known distance (e.g. 10 cm). Alternatively two PMMA rods ofdifferent sections or an additional wire may be used to provide verification of left andright (see Section 3.2.3.2). If such markers are used care should be taken to ensure thatthey do not cause artefacts and that they do not cause problems with the treatment planningsoftware.


3.2.3.1 Distance between known points in image plane

Test frequency: Monthly.Test phantom: As for Section 3.2.2.1.Test procedure: A CT scan of the test phantom should be carried out and the images

transferred to the treatment planning computer. The standardmanual or auto-contouring facility on the treatment planningcomputer should be used to outline the surface of the phantom andthe internal structures (see also Section 3.2.2.1). The dimensionsof each contour should be measured on the screen display and on aprinted hard-copy. The results should be compared with the truephantom dimensions. When comparing the printed contours withthe phantom, or a diagrammatic representation of the phantom,the reference marks on each should be aligned. Under thesecircumstances, no point in the printed contour should be more than2 mm from its true position.

3.2.3.2 Left and right registration

Test frequency: Monthly, concurrently with Section 3.2.3.1.Test phantom: As for Section 3.2.2.1.Test procedure: A radioopaque marker should be attached to one side of the phantom

during image acquisition and its position on the final imagechecked. This test should be repeated for supine and proneorientations.

Note: The left/right orientation of a CT image is subject to operator error. The operatormust generally indicate whether the patient is scanned superior/inferior or inferior/superiorand whether they are lying prone or supine. In addition the orientation of the imagewhen displayed may be selected by the user. For this reason it is advised that a left orright orientation marker should be placed on all patients, to be visible on at least one CTimage.

3.2.3.3 CT number/electron density verification

The purpose of this test is to verify that there has been no change to the CT scanner or tothe treatment planning software, since acceptance, that might result in an incorrectevaluation of electron density values. Such incorrect evaluations might result from analteration of the effective energy of the X-ray beam or modification of any calibrationtable within the treatment planning software.

Test frequency: Monthly.Test phantom: As for Section 3.2.2.1.Test procedure: A CT scan of the test phantom should be carried out and the images

transferred to the treatment planning computer. The treatmentplanning software should be used to display values for electrondensity for regions of water or water substitute material, bone orbone substitute material and air or lung substitute material. Theseshould be compared with their true values. Agreement should bewithin 1 per cent for water and within 2 per cent for lung and bone.


3.2.4 Acceptance tests for spiral scanners

Spiral scanners require specific additional quality control checks to confirm the integrityof the reconstructed slice location.

3.2.4.1 Reconstructed slice location

Test frequency: During acceptance and subsequently at monthly intervals.Test phantom: As for Section 3.2.1.3.Test procedure: The procedure outlined in Section 3.2.1.3 should be followed. The

scanner should acquire a spiral image data set covering the fulllength of the phantom. Certain spiral scanners may not be able toachieve a 60 cm long volumetric acquisition. Under thesecircumstances the maximum possible spacing of markers shouldbe used. A standard pitch and standard interpolation algorithmshould be used. Acceptable tolerance values are as set down inSection 3.2.1.3.

3.2.5 Summary of CT scanner related tests

Table 3.2 lists the recommended test frequencies for tests associated with CT scanners.Monthly tests should also be carried out after an engineer’s visit. Acceptance tests shouldbe carried out when either a new CT scanner or a new treatment planning system isinstalled. Tolerances quoted may depend on the use made of the CT data.

Table 3.2. Recommended frequencies for QC tests for a CT scanner used for generation of Radiotherapy planningdata.


MonthlyAlignment of internal light beam/laser with scan planeIndication of x-axisCouch position registrationDistance between known points in image planeLeft and right registrationCT number/electron density verification

Reconstructed slice location

AnnualCouch deflection under load

AcceptanceContouring and auto-contouringCT electron densities in image display on TPS

3.2.1.1 2 mm3.2.1.2 1°

3.2.1.3 1 mm3.2.3.1 2 mm3.2.3.2, 3.2.3 None3.2.3.3 1% for water

2% for lung and bone3.2.4.1 1 mm

3.2.1.4 2 mm

3.2.2.1 2 mm3.2.2.2 1%


3.3 Magnetic resonance imaging

3.3.1 Introduction

With the widespread introduction of CT based treatment planning, computerised dosedistributions can be accurately calculated from spatial and tissue density information.However, for a variety of tumours, the treatment volume is still typically determined onsimulation films, hence limiting the benefit from axial CT. The superior soft tissue contrastof MRI, particularly in the presence of pathological processes and three dimensionalimaging capability has resulted in attempts to include MRI in radiotherapy treatmentplanning with varying degrees of success. Because the use of MR for treatment planningis still under active development the recommendations given in this section must beregarded as drawing attention to the problems rather than providing definitive advice.

Although the physiological/biochemical basis of MR allows tumour volumes to bedisplayed more clearly than CT, this advantage is negated by the lack of electron densityinformation, longer scan times and inherent geometrical distortion, especially near theperiphery of the patient. If MR is to be used for treatment planning, an assessment ofthis geometrical distortion must be carried out and the appropriate corrections made. Anapproach to doing this has been described by Finnigan et al (1996).

MR image quality is dependent on equipment, area of interest in the body and operatordependent factors such as patient positioning and MR sequence parameter selection. Insome centres patients are imaged pre-and post-irradiation and signal intensity changeswithin tissue are interpreted as radiation induced changes. In this case it may be usefulto perform quality control of T1- and T2-related contrast. Geometric distortion, sliceposition, slice warp, signal-to-noise ratio (SNR), uniformity and image contrast have allbeen identified as important QC tests for radiotherapy treatment planning and arediscussed in the following sections. However, these last three tests for image qualitymay not be essential for radiotherapy use. Those interested in performing a full range ofquality control tests should refer to one of the general references listed at the end of thechapter (Lerski and Orr 1987, Purdy 1988, AAPM 1990, MDD 1992, Och et al 1992)

3.3.2 Test objects and materials

A number of test objects which are suitable for a range of quality control measurementsare available commercially, notably the range of six EEC MR test objects used by theMagnetic Resonance National Evaluation Team at Imperial College (Lerski and Orr 1987,MDD 1992). Alternatively, it should be possible for any reasonably well equipped medicalphysics workshop to construct a suitable range of test objects to perform the measurementslisted below. PMMA is normally used to construct the main body of the test object andits inserts, although glass plates and rods may also be used as inserts. The test objectsare normally filled with distilled water solutions of copper sulphate, nickel chloride ormanganese chloride (AAPM 1990). The concentration should be chosen to give a short-to-medium T1 relaxation time relative to the range of T1 values found in tissue (200–400 milliseconds). Some typical concentrations are given in Table 3.3. A contrast testobject which mimics a range of T1 and T2 relaxation times can be made from a mixtureof agarose gel and copper sulphate in different concentrations (Walker et al 1988).


3.3.3 Geometric accuracy (magnification) and distortion

Geometric accuracy and distortion are dependent on the magnetic field gradient linearityover large fields of view and the homogeneity of the magnetic field (shim). Introducinga patient into the field increases field variations due to tissue susceptibility and implants(e.g. hip replacements) may have an even greater effect. The former can be reduced byimaging with pulse sequences which use high bandwidth acquisitions. Errors in geometricaccuracy result in improper scaling of the distance between points anywhere within theimage. Distortion is the displacement of displayed points within the image relative tothe known location, expressed as the maximum error measured as a percentage of theactual distance.

Distortion is generally greater at the edge of the field compared to the centre and maybe quite small for brain images. For images of the pelvis the distortions at the peripheryof the patient are likely to be significant. Distortion out of the plane of the scan (slicewarp) is also likely. Depending on how the MR data are used these distortions may bemore or less significant. If, for example, relative measurements are being made at thecentre of the field of view, distortion may be negligible. On the other hand if the intentionis to use the MR data to provide the geometric shape of the patient for pelvic radiotherapy,distortion correction will be essential and adequate correction may not be achievable. Inthis case full 3D mapping of the geometric distortion as described, for example, byFinnigan et al (1996) will be essential. It must also be borne in mind that insertion of thepatient will further distort the magnetic field so that the distortion matrix measured withoutthe patient will not guarantee a distortion free image.

Measurements of accuracy and distortion should be performed for three image planeswith a multi-slice protocol and the largest matrix to maximise spatial resolution. It mayalso be necessary to image oblique planes in order to assess these planes for radiotherapy.Percentage distortions less than 5 per cent, when measured over a 25 cm or greater fieldof view, are the best that can be expected.

Table 3.3. Concentrations of agents for filling test objects.

Agent CuSO4 NiCl2 MnCl2

Concentration 3.15 mM 7.0 mM 0.3 mM

Figure 3.7a Plan view of test object TO2A. Figure 3.7b. Plates in test object TO2A for geometricmeasurements.


Two different phantoms have been used in these measurements. Images are obtainedusing the head coil and the EEC MR test object TO2A (MDD 1992) and the lineardimensions between the plates measured (Figure 3.7). Alternatively, a phantom may beconstructed consisting of a regular two-dimensional array of objects surrounded by asignal producing material (e.g. CuSO4 solution) which occupies at least 60 per cent ofthe largest field of view. The objects within the array should be of a size and spacingwhich allows the location to be measured easily (e.g. inserts of 1 cm square sectionspaced at 2 cm intervals).

Although the EEC MR test object is readily available ‘off the shelf’ the second methodis better for assessing the geometric distortion as it covers a much larger field of viewwhere the distortion will be greater. The first method would be equivalent to head andneck scanning, whereas the larger phantom would be abdomen equivalent. Measurementsmay also be made on the filmed images to assess the combined performance of the MRsystem and imager. The NEMA (Sano 1988) advice is to measure at least four pairs oflinear distances, passing through the isocentre with not more than a 45° angle betweenthe lines.

3.3.4 Slice position

The slice position can be defined as the location of the midpoint of the full width at halfmaximum (FWHM) of the slice profile. The accuracy of slice positioning can be affectedby non-uniformity of both the gradient and the static magnetic fields and to a lesserextent the RF accuracy. Typical phantoms used for this measurement consist of pairs ofopposing angled rods (AAPM 1990) which have a known angle between them (Figure3.8). The phantom should ideally contain reference pins and external scribed marks fororientation and centring. Images of the rods at several different slice locations will bedisplaced in direct proportion to the slice position. All measurements should be madealong the line defined by the magnet isocentre and the centres of the imaging planes. Anexample of a suitable phantom is the EEC MR test object TO6 (Sano 1988).

Figure 3.8a. Slice position test object TO6.


3.3.5 Slice warp

Slice warp is the deviation of the slice plane in the direction of selection over the field ofview (AAPM 1990). This can be seen visually by inspection of scans of a two dimensionalarray of crossed rod pairs. Slice warping is important because it can cause positiondependent changes in slice thickness, leading to intensity variations in normal phantomimages (Purdy 1988). A suitable test object is the EEC MR test object TO3A (MDD1992). Particular care must be taken with the orientation of this test object.

3.3.6 Signal-to-noise ratio and image uniformity

Changes in image quality can easily be assessed by measuring the signal-to-noise ratio(SNR) (AAPM 1990). This can be affected by variations in system calibration (e.g.resonant frequency), gain, coil tuning, RF shielding, coil loading, image processing andscan parameters. The measurement is performed using a ‘flood field’ phantom containinguniform signal producing material which has a diameter greater than 80 per cent of thefield of view. SNR measurements are specific to the system, phantom and scan conditions.The signal is measured using a region of interest (ROI) of 10 per cent of the FOV in thecentre of the image. The noise calculation is taken from the standard deviation of thepixel intensities within the ROI.

Figure 3.8b. Principle of slice position measurement using TO6.

mean pixel intensity

standard deviation of pixel intensitySNR =

Alternatively, the noise measurement can be taken as either the mean or standarddeviation from an ROI placed outside the test object (but not in line with the object inthe phase encoding direction). The precise method used is not crucial provided that thesame method is always used to compare measurements. The measurement of signal-to-noise ratio is a useful test to perform daily as it is sensitive to many problems which maydevelop with the MRI hardware. It should be performed regularly on all commonly usedRF coils.

Image uniformity can be performed using the same images obtained for the signal-to-


noise ratio measurement. Line profiles of the pixel values are taken in both the frequencyand phase encoding directions of the image. If possible a number of lines (up to 10)should be averaged to produce a smoother profile and the fractional uniformity iscalculated as the fraction of the profile (within the test object) which lies within 10 percent of its modal value. The measurement of image uniformity is highly dependent onthe type of coil used and is only appropriate for head and body coils.

3.3.7 Image contrast

Image contrast can be evaluated using an array of sample tubes (e.g. 20 mm diameter)containing a mixture of agarose gel and copper sulphate. The concentrations of the geland copper sulphate can be varied to provide a range of T1 and T2 relaxation timeswhich are tissue equivalent. The EEC MR test object TO5A has a series of 12 calibratedagarose gels. It is important here to use pulse sequences and timing parameters whichare used to acquire images in patients. The contrast between any two gels can be calculatedas:

(S1 – S2)

(S1 + S2)Contrast =

Table 3.4. Frequency and action levels for MR tests.

Test and frequency RF coils Reference Tolerance

DailySignal-to-noise ratio

WeeklyGeometric accuracy anddistortionImage contrast (if required)

MonthlySlice positionImage uniformity

Three-monthlySlice warp

Any coil 3.3.6 <10% variation

Head or body 3.3.3 <5% error over large FOV<2% error over small FOV

Head or body 3.3.7 <10% variation

Head or body 3.3.4 <5% errorHead or Body 3.3.6 Discuss with manufacturer

3.3.5 <2.0 mm deviation

3.3.8 Suggested frequency of measurements

The suggested frequency and action level for each of the above measurements is givenin Table 3.4. The tolerance levels given are for guidance only and should be discussedwith the MRI manufacturer or service engineer.

55

3.4 Simulator CT

3.4.1 Introduction

Modifying a simulator to allow it to acquire CT images has been performed by in-housemodifications since the late 1970s. It is only in the last few years that commercial systemshave been available from the simulator manufacturers or third party providers. The CToption comprises additional hardware in the form of pre- and post-collimators that allowa fan geometry X-ray beam to be produced suitable for CT image acquisition. This fanbeam is then scanned through the patient by rotating the gantry. The profiles for back-projection are acquired often by using the simulator image intensifier. Image acquisitiontakes around one minute during which the patient must lie still to prevent image movementartefacts. The profiles are then processed by appropriate computer hardware and softwareto produce the CT image. Image quality and CT number determination are not as good asdiagnostic systems but are of sufficient quality to be used in treatment planning. Multipleslices can be acquired but these are limited by the heat units generated in the tube.Typically these systems will be able to acquire around three slices in any session withoutany heat problems occurring.

The quality of all CT images is greatly affected by changes in the physical geometryof the acquisition process. In the diagnostic scanner much design effort is put into ensuringthat the geometry remains constant. In the simulator CT system positional toleranceswill not be as high and image quality is going to be greatly affected by the system’sability to produce repeatable set-ups accurately. Therefore additional quality control isrequired to verify set-up positions, examine the physical integrity of the inserts and toensure that during gantry rotation alignment of the collimators does not change.

3.4.2 Auto set-up positions

This is the position to which all components move automatically prior to the start of thescan, e.g. gantry rotation, image intensifier position, and collimators. Each configurationof the simulator needs to be checked to ensure that the system is consistently reachingthe same physical arrangement.

Reliance on the simulator scale readouts is not ideal as these are usually only accurateto 1 mm and such misalignments can affect image quality.

Physical markers securely attached to the simulator framework during commissioningcan be used to check that intensifier and tube move reproducibly to the same physicalposition. If these markers are not provided by the simulator manufacturer they can easilybe produced by any Radiotherapy workshop (see Figure 3.9). Markers consist of twometal strips attached to sections of the simulator that have to be moved relative to eachother. Scribed lines that match up in each auto set-up position can then be made on eachpiece. These markers can then be used to verify the positions once movement is complete.

The markers should ideally not be attached to any removable structure such as coversas these may not go back into exactly the same position following servicing ormaintenance. However this is not always possible. Use of these markers can also providea quick check of alignment, for the radiographers, each time the CT option is used.

If it is possible for the simulator to approach the set-up position from either the positive


or negative direction, then the auto set-up should be checked from both approaches tocheck for any hysteresis in the system arising from backlash in the drive mechanisms.

Auto set-up positions should be checked daily as part of the simulator quality controlprocedures.

3.4.3 Physical inserts

To collimate the X-ray beam a series of pre- and post-collimators are used. These createthe fan X-ray beam necessary for CT imaging. These are usually substantial pieces ofmetal and can be awkward to manoeuvre. They should be checked daily for signs ofphysical damage. The constant wear on the runners could introduce slop into the fittings;therefore the fit of the collimators to the simulator should be checked on a weekly basis.

Where pre- or post-collimators have adjustable delineating jaws the position of thejaws should be marked at the end of the commissioning process, if possible, by scribingaround the jaw, marking its position onto the plate to which it is attached. Movement ofthe jaws can indicate that the collimator has suffered some form of physical trauma.

Any devices such as linear diode arrays that are used to extend the field of view of theintensifier should also be checked for physical damage.

3.4.4 Radiation field alignment

The coverage of the collimated X-ray beam over the intensifier imaging slit should bechecked at all gantry angles.

This can be achieved by setting the simulator to its acquisition arrangement with thepre- and post-collimators fixed in place. With the light field turned on, the gantry can berotated and the position of the X-ray field monitored by observing the light field coverageon the post-collimators over a full rotation. The light field should show adequate coverageof the post-collimator slit for the full rotation of the gantry.

This test should be performed monthly but should be repeated if there are any problemspicked up by simulator tests described in sections 3.1.3.1 or 3.1.3.4 or any movement ofthe pre- or post-collimator blades are seen (even if this is corrected).

3.4.5 Software files

Any software files associated with the CT reconstruction process should be carefullymonitored to ensure that no corruption of the files has occurred. This can be accomplishedby using a file comparator program to compare the working files with copies kept on

Figure 3.9. Markers for image intensifier position.

57

another storage medium or by verifying that the file creation date and size have notchanged.

Software file size and date should be checked daily. If a file comparison is used thiscould be automated as part of the system’s boot-up.

3.4.6 Computer hardware

The addition of CT to a simulator system usually means the addition of a computersystem dedicated to the CT process. This computer is often of a commercial type such asan IBM compatible. This computer should be subject to whatever computer quality controlprogram is implemented in the institution for checking of disk, program and data integrity(see Sections 4.4.2.1 and 4.4.4.1).

3.4.7 Image distortion and CT numbers

These require the same checks as a standard CT system outlined in Section 3.2. Howeverit should be noted that the use of a modified simulator is much more susceptible tochanges in the physical arrangement of the source and imaging system increasing thepossibility of image distortion. It is therefore recommended that tests described in Sections3.2.3.1 and 3.2.3.3 be monitored initially weekly and then dropped to monthly if thesystem shows good stability. Visual inspection of the scans of this phantom can be carriedout daily or weekly as a useful additional check on performance.

Table 3.5. Quality control tests for simulator CT systems.


DailyAuto set-up positionsPhysical insertsSoftware files

WeeklyVisual check of phantom scanRadiation field alignment

MonthlyComputer hardwareImage distortion and CT number

3.4.2 <1 mm3.4.3 NA3.4.5 See text

3.4.7 Acceptable image3.4.4 See text

3.4.6 See text3.4.7 As Table 3.2

References

AAPM (American Association of Physicists in Medicine) 1990 Quality AssuranceMethods and Phantoms for Magnetic Resonance Imaging American Association ofPhysicists in Medicine Report No. 28 (Maryland: AAPM)


BIR (British Institute of Radiology) 1985 Criteria and Methods for Quality Assurance inMedical X-ray Diagnosis British Journal of Radiology Supplement 18 (London: BritishInstitute of Radiology)

BIR (British Institute of Radiology) 1989 Treatment Simulators British Journal ofRadiology Supplement 23 (London: British Institute of Radiology)

Dick Loo LN 1991 CT acceptance testing. Specification, Acceptance Testing and QualityControl of Diagnostic x-ray Imaging Equipment. Proceedings of AAPM Summer School1991 Ed JA Seibert, GT Barnes and RG Gould pp 1042–1066 (New York: AmericanInstitute of Physics)

Finnigan DJ, Tanner SF, Dearnaley DP, Edser E, Horwich A, Leach MO and MaylesWPM 1996 Distortion-corrected magnetic resonance images for pelvic radiotherapytreatment planning Quantitative Imaging in Oncology. Proceedings of the 19th LH GrayConference April 1995 Ed K Faulkner, B Carey, A Crellin and RM Harrison (London:British Institute of Radiology) pp 71–75

Horton PW, Deaville JL, Gamble JM and Gerard-Martin S 1987 Quality control toolsfor simulators Proceedings of the Fifth Varian European Clinac Users Meeting pp 133–136 (Zug, Switzerland: Varian Associates)

HPA (Hospital Physicists Association) 1980 TGR 32 – Measurement of the PerformanceCharacteristics of Diagnostic X-ray Systems Used in Medicine Hospital Physicists’Association (York: IPEMB)

ICRU (International Commission on Radiological Units) 1992 Phantoms andComputational Models in Therapy, Diagnosis And Protection International Commissionon Radiation Units and Measurements Report 48 (ICRU Publications, 7910 WoodmontAvenue, Suite 1016 Bethesda, Maryland 20814, USA).

IEC (International Electrotechnical Commission) 1982 Characteristics of Focal Spotsin Diagnostic X-ray Tube Assemblies for Medical Use Report 336 (Geneva: IEC)

IEC (International Electrotechnical Commission) 1993 TR 1170 Radiotherapy Simulators– Guidelines for Functional Performance Characteristics (Geneva: InternationalElectrotechnical Commission)

IMPACT (Imaging Performance Assessment of CT) 1998 Type Testing of CT Scanners:Methods and Methodology for Assessing Imaging Performance and Dosimetry. EvaluationReport Number MDA/98/25 available from IMPACT, St George’s Healthcare, LondonSW17

IPSM (Institute of Physical Sciences in Medicine) 1981 Measurement of the Performancecharacteristics of Diagnostic X-ray Systems used in Medicine. Part III. The PhysicalSpecification of Computed Tomography X-ray Scanners Institute of Physical Sciences inMedicine, Topic Group Report 32 (York: IPEM)

Lerski RA and Orr JS 1987 Practical testing Practical NMR Imaging Ed MA Foster andJMS Hutchison pp 81–93 (Oxford: IRL Press)

McCullough EC and Earle JD 1979 The selection, acceptance testing and quality controlof radiotherapy treatment simulators Radiology 131 221–320

59

MDD (Medical Devices Directorate) 1992 Assessment of the Imaging Performance ofthe Siemens Magnetom 63 SP 400 1.5T MR Imaging System Medical Devices DirectorateEvaluation Report no. 47 (London: HMSO)

MDD (Medical Devices Directorate) 1994 The Testing of X-ray Image Intensifier–Television Systems Medical Devices Directorate Evaluation Report 94/07 (London:HMSO)

Och JG, Clarke GD, Sobol WT, Rosen CW and Mun SK 1992 Acceptance testing ofMRI systems: Report of AAPM Nuclear Magnetic Resonance Task Group 6. Med. Phys.19 217–229

Purdy D 1988 Acceptance testing of magnetic resonance imagers: Which tests areworthwhile? MRI: Acceptance Testing and Quality Control – The Role of the ClinicalMedical Physicist, Proceedings of an AAPM Symposium (Maryland: AAPM)

Sano R 1988 NEMA Standards: Performance Standards for clinical magnetic resonancesystems MRI: Acceptance Testing and Quality Control –The Role of the Clinical MedicalPhysicist, Proceedings of an AAPM Symposium, April 1988 Ed RL Dixon (Wisconsin:Medical Physics Publishing Corporation)

Walker PM, Mathur de Vre R, Lerski RA, Binet J and Yane F 1988 Preparation of agarosegels as reference substances for NMR imaging and spectroscopy Magnetic ResonanceImaging 6 215–222

WHO (World Health Organisation) (1988) Quality Assurance in Radiotherapy (Geneva:World Health Organisation)

CHAPTER 4

Treatment Planning

4.1 Introduction

This chapter covers all aspects of treatment planning for radiotherapy. Most centres usea computerised treatment planning system (TPS) for calculating complex treatment plans.This involves collecting a large amount of beam data (basic data) in a specific format foreach treatment unit for which plans are to be calculated. As well as the basic data requiredfor entry to the TPS it is necessary to measure a set of reference data against which theaccuracy of the TPS can be measured. Reference data are only needed for those parts ofthe system that will be used clinically.

The extensive tests for commissioning the TPS will check that the basic data has beenentered correctly and that the data are being handled by the software as intended. Thecommissioning data will be the gold standard against which present performance can becompared, i.e. quality control. A new system can be tested using data from AAPM Report55 (AAPM 1995). This contains measured data for two machines (a 4 MV linearaccelerator and an 18 MV linear accelerator) together with the results of a number oftests. These include a selection of standard fields, an off centre plane, an irregular Lshaped field, an inhomogeneous medium and oblique incidence. The data are particularlyuseful to resolve such issues as whether an error is due to the computer algorithm or tothe measured data input to the computer. Similar test measurements can also be madeusing the centre’s own machines.

Ongoing quality control is needed on the TPS to ensure that any stored data have notbeen corrupted or that the data have not been inadvertently changed by a software update.Quality control also applies to digitisers and plotters. If CT information is to be usedthen the transfer of the CT data set needs to be assessed for correctness and this needs tobe repeated when CT scanner software is upgraded (See Chapter 3, Section 3.2.2).

The treatment planning process, from the initial presentation of the patient in thedepartment through to their final treatment, with input from several different staff groupsis outlined. Quality control of the computed treatment plan prior to being used for thepatient’s treatment is also covered.

4.2 Beam data acquisition for treatment planning

The data requirements for treatment planning and computerised treatment planningsystems can be categorised in terms of the uses to which these data will be put. Theseuses are:

1. TPS data entry: Necessary for the TPS to represent each individual treatment machine.The details and format of these data will essentially be governed by the requirementsof the individual planning system. Requirements can vary widely though the data set

61Treatment Planning

will almost certainly include measured data and user determined data which may beused simply as file headers and identifiers or may also be used in the algorithm.

2. Validation of basic data: Necessary to verify that any processing or averaging of databy the TPS algorithms does not result in unacceptable differences from the measureddata.

3. Algorithm testing: Necessary to validate the efficacy of the calculation algorithm inboth standard and non-standard conditions and to determine its applicability and/orits limitations. If measurements are not possible, published data could be used althoughit is likely that this will lead to a loss in accuracy. It may be appropriate however, touse published data (Lambert et al 1983, Das and Khan 1989) for build-down regionsand doses at interfaces.

The data required for commissioning TPSs for use with X-rays, electrons andbrachytherapy will be discussed in turn with reference to each of the above.

It is important to distinguish between the data collected for entry into the TPS anddata collected in order to validate the TPS. In this document, the following definitionshave been used:

Basic data: data entered into and used by the TPS to construct dose distributions.Reference data: data measured under standard conditions (usually in a water-filledplotting tank) to validate basic data or check the TPS algorithm.

To test TPS algorithms sensibly, it is necessary to understand them in principle, if not indetail. Tests should be designed to:

1. validate the efficacy of the algorithm under non-standard conditions; and2. determine the applicability and/or limitations of the algorithm.

Tools should be designed so that the individual components of the algorithm can betested separately in order that sources of inaccuracy can be individually quantified. Thisapplies to X-ray, electron and brachytherapy algorithms.

4.2.1 Equipment for beam data acquisition

4.2.1.1 Detectors

Careful consideration must be given to the detector type and its suitability for a particularapplication. Erroneous results can be obtained if the detector set-up is inappropriate tothe parameter being measured. In particular, the following factors should be considered:

1. Polarity effects with ionisation chambers: Collection polarity can affect the measureddata in build-up measurements with photon and electron measurements (Gerbi andKhan 1987, Aget and Rosenwald 1991).

2. Over-response in the build-up region with various designs of fixed separation plane-parallel ionisation chambers (Gerbi and Khan 1990, Mellenberg 1990).

3. The effect of detector size and type on profile resolution (Sibata et al 1991, Metcalfeet al 1993): Consideration should be given to using diodes for profile measurementsand ion chambers for percentage depth doses (PDDs).

4. The energy dependence of diodes for depth dose measurements (Rikner 1985, Riknerand Grüssel 1985).


5. The effective point of measurement (EPoM) of spherical or cylindrical ion chambersfor PDD measurements of megavoltage photon beams: The IAEA (1987) and NACP(1980) recommend an effective point of measurement 0.75 times the internal radiusof the chamber, towards the radiation source. The IPEMB (1996b) electron code ofpractice states that for a cylindrical chamber the EPoM is 0.6 times the internal radiusof the cavity in front of the chamber axis. It is recommended that this latterrecommendation be followed. A summary is given by Klevenhagen (1994) and theissue is further discussed in Chapter 8, Section 8.2.6.

4.2.1.2 Measurement phantoms

Water phantoms are preferable for making the large number of measurements requiredsince the detector can be moved automatically to a number of preset positions undercomputer control. Care should be taken in setting up any data collection system and theoperation of any motorised detector movement verified before use. Quality control ofplotting tank systems is covered in Chapter 8, Section 8.4. It is necessary to ensure thatthe plotting tank is level so that the horizontal and vertical travel of the signal probe isparallel and perpendicular to the water surface in the tank. It is also important to checkthe gantry angle with a spirit level, and to ensure that the collimator angle is square withthe horizontal travel of the signal probe. If measurements are being carried out overseveral days, the water level should be checked regularly since the water will evaporate.In order to meet the levels of overall treatment accuracy recommended by ICRU (1976)and Mijnheer et al (1987), the agreement between measured and TPS generated isodosedata should ideally be ±2 mm in regions of high dose gradient or ±2 per cent elsewhere.It therefore follows that the plotting system should be set up to greater accuracy. If datafrom only one set of jaws can be stored in the TPS, then decide at the outset which set ofjaws are to be used and be consistent. The usual practice is to measure data for beamprofiles from the lower jaws (nearest the patient). These jaws will have a narrowerpenumbra than the upper jaws, and it may be preferable to use these data in the planningplane. However, some treatment units have the wedged axis across the upper jaws andthen for consistency, it may be preferred to measure all profiles across the upper jaws forboth plain and wedged fields.

There are disadvantages of using a water tank collection system, including therequirement for waterproof chambers; and long setting up times. Since tanks tend to belarger than patients, they do not realistically represent scatter conditions in the patient(White et al 1977). In many instances solid ‘water-equivalent’ materials of more realisticgeometry are to be preferred for this purpose. Lower density blocks representing lungare also available. Such phantom materials are useful for making build-up measurementsand for verifying TPS algorithms since they have known dimensions and can be CTscanned easily. Treatments can then be planned and checked against measurements.

4.2.2 Data for photon beams

In order to discuss the relevance of data required by different TPSs, it is useful tounderstand why the algorithms used by each to calculate dose distributions require suchdiverse data sets. A brief overview of the principal algorithms is presented here to supportthe discussion of measurement requirements.


Algorithms for external beam photon therapy can be divided roughly into fourcategories:

1. Stored beam models are based on the use of measured dose distributions which typicallyare stored as fan-line matrices divergent from the source. A large number ofmeasurements are required to build up a library of beams and field sizes.

2. Beam generation models represent the beam using empirical formulae fitted tomeasured dose distributions. These models typically take the form of the product of acentral-axis depth dose function and a function describing the profile shape off-axis.

3. Scatter-integration techniques provide a semi-empirical calculation of the dose in abeam based upon the separation of the primary and scatter components and thecalculation of the contribution of each to the dose at any point.

4. Three-dimensional techniques. In recent years a number of new approaches to thedose calculation problem have been developed with the aim of fully modelling radiationtransport phenomena in three dimensions to provide accurate calculations, taking fullaccount of perturbations to dose distributions from inhomogeneities of arbitrary shape,position and density. These calculations can give accurate results for beams of irregularcross-section, under and near the edges of beam modifiers and in non-equilibriumconditions at interfaces. Foremost among these models are the class of convolutiontechniques. These convolve the three-dimensional distribution of primary energyreleased in the irradiated medium with a point-spread function which describes thetransport of energy away from a primary photon interaction site. There is a diversityof methods for implementing these techniques which will determine the precise dataset required but the principles presented in this chapter will still hold.

4.2.2.1 Data for entry into the TPS

Stored beam algorithms and algorithms based on 3-D techniques will typically require alarger amount of measured data than beam generation and scatter integration algorithms.Nevertheless, much of the user defined data will be common to all systems. Most dataare required in some form by all algorithm types. However, where data are likely to bespecific to a particular algorithm type, this will be identified in the text. Where appropriate,the following abbreviations will be used:

BG – Beam generation systemsSI – Scatter integration systemsSB – Stored beam systems3-D – Three dimensional systems

1. User defined dataUnit/machine name: Necessary as a file identifier.Beam energy: Further identifier in multi-mode machines.Source axis Distance: Geometrical requirement for isocentric and rotational therapy.Source surface distance: Reference value for measured data input.Source diaphragm distance (BG, SI, 3-D): May be used in penumbra modelling.Source block tray distance: Necessary for the calculation of penumbra widths inblocking algorithms.Collimator size range: May be different for open and wedged fields.Scale/axis orientation: The direction of positive increase of scale and any movementrestrictions may be required when three-dimensional and non-coplanar planning isconsidered.


Scales: It should be noted whether the senses of the rotation scales on the TPS agreewith those on individual treatment machines. If they do not then this should be takeninto account when transferring set-up information from the treatment plan toinstructions for the treatment radiographers. This can be a particular problem becauseof the changes to the conventions contained in IEC 1996.Machine output units/monitor units(MUs): Needed to allow monitor unitcalculations to be performed where applicable. For MU calculations with gamma rayunits, the reference dose rate, reference date and half life of the isotope will be required.It is necessary to ensure that there is consistency about whether the reference pointfor monitor unit calculations is defined at the isocentre or at the depth of dose maximum(d

max) with the phantom surface at the isocentre.

2. Basic dataBuild-up data: Requirements vary from a single surface dose value to data at 1 mmintervals from the surface to 1 cm beyond the point of maximum dose beneath theskin surface (d

max) for a range of field sizes for both open and wedged beams. Although

dmax

varies significantly with field size for high photon energies, commonly only onevalue will be accepted by the TPS and this should be chosen to minimise any errors atother field sizes.Output factors: The variation of dose with collimator setting is required to allowcalculation of treatment time/MU. Data may be required at d

max or other reference

depth with full scatter or ‘scatter free’ in air.Scatter data (SI): Scatter data are necessary for scatter integration algorithms. Apeak scatter factor (PSF) is inherently difficult to measure at high energies (Day 1983)since it requires a measurement of primary radiation in the absence of scatter. It maybe obtained by back extrapolation of normalised PSF data to zero area followed byrenormalisation. The otherwise significant uncertainty involved in this procedure maynot be significant when the PSF data occur in both numerator and denominator of thecalculation algorithm.Wedge transmission factor: This is required if wedge attenuation is shown in isodosecurves or in treatment time and MU calculations. A single value is likely to oversimplifythe physical reality but some systems may only accept a single value. Better agreementmay be obtained if more than one wedge is defined; each valid for a range of fieldsizes for which a wedge factor of acceptable accuracy can be defined. Wedge factorsshould be measured under the same conditions as the output factor (Thomas 1990).Block material transmission: The attenuation coefficients of shielding blocks mustbe specified to allow the TPS to model the dose distribution accurately under, orclose to the edge of, a block. Some TPSs require broad beam attenuation coefficientsand others narrow beam coefficients, depending on the algorithm employed.If the TPS uses the beam blocking algorithm to simulate asymmetric collimators thenan attenuation coefficient for the collimator will also be required.Source diameter (BG, SI, SB): Used in the determination of penumbral widths inmany beam blocking algorithms. It may be necessary to use an ‘effective sourcediameter’ which differs from the physical source diameter in order to overcomeinadequacies in the algorithms. The estimation of the source diameter requires themeasurement of beam block penumbral distributions by either film or plotting tankmethods.Compensator material attenuation data: These data may be expressed as


transmission data or as water equivalent thickness in cm.CT number tables: Used to correlate Hounsfield numbers with corresponding electrondensities for inhomogeneity corrections. Ideally, this should be done by calibratingthe scanner with phantoms of known electron densities. This is covered in Chapter 3,Section 3.2.2.2 and 3.2.3.3.Percentage depth dose (PDD) data: Most systems require central axis PDD data forrepresentative open and wedged fields over the range of field sizes available. Inaddition, scatter integration algorithms require zero-area percentage depth dose datawhich may be obtained by back extrapolation techniques (Bjärngard et al 1989).Profile data: The requirements for beam profiles can vary between a single profile ata single depth to the requirements of stored matrix models which require acomprehensive range of profiles at multiple depths. Three-dimensional algorithmsmay require, in addition to PDDs, sets of orthogonal profiles rather than assumingprofile symmetry.Wedge data (SI, BG): Scatter integration algorithms may require the followingphysical wedge data in order to model wedged fields:(a) Physical dimensions of the wedge filter(s) and source-filter distance.(b) Linear attenuation coefficient of the wedge filter material.(c) Factors to account for attenuation in any wedge tray and beam hardening by the

filter.

4.2.2.2 Reference data – for TPS validation

These are data that will be needed to test the TPS dose algorithms. Isodoses should bemeasured for a range of treatment distances and field sizes including rectangular fields.For stored beam systems the chosen field sizes should test the interpolation betweenstored beams. For beam generation algorithms a range of small, medium and large fieldsshould be selected. Measurements should be made for both open and wedged fields.Data for fields shaped with lead blocks should also be included.

Conditions for which validation measurements should be available are:Limiting conditions: The most extreme cases of any condition should be measuredin order to test the limits of the TP algorithm.Oblique incidence: Isodoses can be measured in a plotting tank with a gantry angle≠ 0, or against point dose measurements with an ionisation chamber in solid phantommaterial in a simulated set-up. The detection device should have good isotropic angularindependence.Glancing beams: Measured as for oblique incidence except that part of the beamshould pass through lung equivalent material or air. It may be necessary to measurethe effect of the isocentre being situated in air or lung if this is likely to occur inclinical practice.• Glancing through lung – The dose to lung and the dose to adjacent tissue should

be measured in a phantom. The effect of reduced scatter to the water equivalentmaterial from the lung should be evaluated; this is overestimated in many algorithmssince the lateral scatter contribution is inaccurately modelled.

• Glancing through air – As above, the effect of missing scatter to tissue adjacentto air should be obtained from phantom measurements.

Exit doses: The build-down region is due to the lack of backscatter and loss ofelectronic equilibrium at tissue boundaries or interfaces. This may be investigated


using a thin window parallel-plate chamber embedded in water equivalent phantommaterial with the chamber facing away from the radiation source. Thin layers ofphantom material may then be placed in front of the chamber window to measure thebuild-down effect. This effect may not be accurately modelled in commerciallyavailable algorithms.Inhomogeneities: Doses within and adjacent to block-type inhomogeneities must bemeasured. The doses at tissue interfaces are not expected to be accurately calculated.Point dose measurements in phantom material should be made for comparison withcalculations. This phantom should also be CT scanned.Re-entrant beams: Point dose measurements in a phantom should be made forcomparison with calculated distributions. As mentioned under Exit doses, the build-down and build-up regions may not be accurately modelled by the algorithm.Calculations using CT: Where the TPS supplies a relationship between electrondensity and CT number, and this is used in preference to a measured relationship,spot checks should be performed to verify the validity of the relationship.Blocked fields: Isodoses constructed from measurements in a plotting tank under ablocked field for comparison with TPS reconstructed isodoses for experimentalarrangements representative of expected clinical use should be made.Asymmetric fields: Isodoses should be constructed from measurements in a plottingtank for representative field sizes and offsets spanning the available range.Multi-leaf collimators: The data needed to test the multi-leaf collimator will dependon the algorithm used. Many computer systems use the standard blocking algorithmand cannot therefore be expected to model leakage between the leaves. While it isimportant to know the magnitude of such errors there is little that can be donepractically to correct them. A simple test consisting of a square field with the multi-leaf collimator blocking set at about 10° to the rectangular collimator edge on oneside and 45° on the other will enable tests of the modelling of the leaf ends and of theeffect of the reduction in scatter. Depending on the accuracy with which the softwareis able to model this field, further tests may be devised. Since multi-leaf collimatorsthat are part of the machine collimation system have a different effect on the collimatorscatter factor, S

c , than do blocks, it is unlikely that algorithms that treat them as the

same will correctly calculate the absolute dose.Rotational photon therapy: Point dose measurements in a phantom.Non-standard SSD treatments: The penumbra, field size and PDDs should beobtained from plotting tank measurements. Special care should be taken for both largefields at extended SSDs and small fields at short SSDs where the requested field sizesare outside the range of the basic data.Non-coplanar treatments: The following situations should be investigated, ideallyby point doses with point dose measurements in phantom material:• Collimator twist such that the field axis is not parallel to the planning plane should

be investigated to evaluate changes in geometric field size and accuracy of dosedistribution.

• Couch twist such that the field axis is not parallel to the planning plane. This shouldbe investigated to calculate the effects on field size and dose distribution as well asthe display of the location of the isocentre.

3-D distributions: These can be checked at various off-axis slices through a phantomagainst measurements made at off axis positions in a plotting tank.Multi-beam plan: Calculated point dose values should be checked against measured


dose values for a plan consisting of more than one photon beam. This should be doneon a phantom constructed to allow point dose measurements to be accurately positionedwithin the phantom (e.g. the IPSM phantom, see Chapter 2, Figure 2.1 (ICRU 1992)).This test would normally be the last to be performed on the TPS.

4.2.3 Data for electron beams

Although most electron treatments have traditionally not been planned on an individualbasis in the same way that external beam photon treatments have, there is an increasinginterest in the planning of such treatments. This has prompted the development of suitablecalculation algorithms. Most current models are based on the application of multipleCoulomb scattering theory, involving the integration of pencil beams over the field area(Brahme 1981, Hogstrom et al 1981, Khan 1984, Nahum 1985, Klevenhagen 1993).Recent work has investigated the use of a full Monte Carlo based calculation of the dosedistribution in electron beam therapy with a view to its practical clinical application(Holmes et al 1993, Bielajew 1994).

4.2.3.1 Data for entry into the TPS

1. User defined dataUnit/machine name: Necessary as a file identifier.Beam energy: Further identifier in multi-energy machines.Field size limits: For treatment units which employ variable field size applicators,the range of available field sizes must be determined. For units which use fixedapplicators, it may be necessary to define each as a separate treatment machine in theTPS.Units of output/MU: Needed to allow MU calculations to be performed whereapplicable.

2. Basic dataVirtual source distance (VSD): This is the distance between the virtual source of theelectron beam and the treatment surface at the standard treatment distance. The conceptof a virtual source can be used to facilitate computation of beam divergence and relativeoutput at non-standard distances (Klevenhagen 1993). The VSD for these twoapplications will not necessarily be the same and should be determined in anappropriate manner for the application to which it is to be put.Source collimator distance: This should normally be the distance from the virtualsource to the beam defining aperture closest to the patient.Source to isocentre distance: Geometrical requirement essential for rotating electronbeam therapy.Practical range (R

p): The practical range is the depth at which the extrapolation of

the straight descending portion of the depth–ionisation curve meets the X-raybackground level (IPEMB 1996b). It is a necessary quantity for the determination ofthe incident energy as well as the derivation of the mean energy at depth for theelectron beam.Initial beam energy (E

0): The mean incident electron energy (determined from depth–

ionisation curves) may be less than the nominal accelerator energy due to energylosses in the scattering foil and in the air path traversed by a beam before it reachesthe treatment surface.


PDD data: Most systems require central axis PDD data over the range of availablefield sizes. Some plotting tank systems may offer software to convert measuredionisation data into depth-doses.Profile data: Specific requirements depend on the TPS software. Typically four tofive profiles are required for all available square field sizes equally spaced from thesurface through d

max to a depth corresponding to a central axis dose value of 10 per

cent of that at dmax

.Beam spread: Some measurement of beam spreading in air may be required by thealgorithm to model the increase in beam penumbra effectively with an increasing airgap to the treatment surface. These data may be extracted from plots of in-air penumbrawidth against target chamber distance for each beam energy.

4.2.3.2 Reference data – for TPS validation

As with photon data, isodoses should be measured for all applicators or range of fieldsizes (square and non-square) and for all energies at the standard SSD. They should alsobe measured at extended SSD if they are to be used in this way. Special attention shouldbe paid to:

Percentage depth doseField sizeIsodose shape

To test commercially available algorithms, various experimental set-ups should be usedso as to mimic typical non-standard treatment conditions. (These may include obliquityand bone inhomogeneities as, for example, in the case of chest wall irradiation and airvolumes for head and neck treatments.)

Inhomogeneities: By suspending samples of bone equivalent tubes in a plotting tank,isodose distributions can be measured and compared with TPS calculated distributions.This should be done for beams at both normal and oblique incidence. Discrepanciescan be expected in and adjacent to inhomogeneities if the dose calculation algorithmdoes not accurately model electron scatter. (At present accurate modelling of electronscatter requires impractical computing times. As computers become faster, modellingwith Monte Carlo techniques should resolve this problem.)Rotational electron therapy: This is a specialised technique used in few radiotherapycentres to date. It has applications in treating superficial targets on curved surfaces.Reference should be made to standard texts on the subject but measurements inspecialised phantoms are essential to validate the TPS calculations.Treatment field junctions: These are difficult to evaluate by any method other thanfilm scanned by a microdensitometer. Account should be taken of the uncertainties intreatment machine set-up when irradiating the film as well as the limits in resolutionof any microdensitometer used. Checks should be performed for abutting fields,overlapping fields and small gaps between fields. Of specialised interest arenon-coplanar junctions (e.g. cranio-spinal treatments) and junctions between photonand electron fields as in breast boosts.

4.2.4 Brachytherapy

Input data requirements for brachytherapy programs differ from external beam photon


and electron software requirements in that little if any measured data are required.Typically, the algorithms used to calculate the spatial distribution of absorbed dose arebased on the integration of the contribution from individual isotropic point sources wherethis is an appropriate model or by integrating the contribution of volume elements ofactive material within the sources in the case of cylindrical sources. The results of thesecalculations may be stored in tabulated form for future ‘look up’ purposes if they applyto standard sources. Recent recommendations (ICRU 1985, BIR 1993) advocate thespecification of source content in terms of a reference air kerma rate and contain adviceon how this may be related to earlier source content specifications. (See also Chapter 9,Section 9.4.)

4.2.4.1 Data for TPS entry

The details of the data required will be specific to the algorithm but the following aretypical:

Source type identification: For example tube, seed, needle or wire, isotope and sourcecode (e.g. Amersham Catalogue Code) if appropriate.Source content: The reference air kerma rate. Alternatively, this may be specified interms of activity (equivalent or actual), equivalent radium mass or exposure rate. Ifthe system performs a decay correction then a reference date is also required.Source geometry: The external length and width, and the length and width of theactive material.Half life: The half life of the isotopes to be used.Filtration coefficients: The attenuation coefficients for both source and encapsulatingmaterial. Alternatively, in the case of small linear sources e.g. for high dose ratebrachytherapy the calculation may be based on a point source model with spatialanisotropy coefficients.Air kerma rate coefficient: The coefficient to convert air kerma rate in water toabsorbed dose in water for the isotope in question.Attenuation and scattering coefficient: The coefficients to account for attenuationand scattering of source radiation in water/tissue. These are isotope dependent andmay not be customisable in some commercial planning systems. The joint BIR/IPSMworking party report (BIR 1993) lists recommended values for common isotopes andreferences sources for less common isotopes.

4.2.4.2 Algorithm and basic data validation

The spatial dose distribution around brachytherapy sources is a specialised field becauseof the high dose gradients close to sources and the necessity to correct for detector sizeand directional response depending on the application. It is therefore difficult to validatecomputed distributions with measured data. The algorithm should, however, be checkedby comparison with a manual calculation at one or more selected points for a singlesource. Where published measured or calculated spatial distributions are available theseshould be used for comparison and major differences resolved.

The ability of the algorithm to summate the contributions of several individual sourcesshould be checked by manual calculation at single points and/or comparison withpublished data.


4.3 Commissioning treatment planning systems

4.3.1 Introduction

The following section provides recommendations for commissioning a treatment planningsystem. Protocols are presented for photon beams (Table 4.2), electron beams (Table4.3) and brachytherapy (Table 4.4). An example worksheet for the first two photon testsis presented in Table 4.6; this provides the reader with a suggested method of tabulatingthe test data to enable comparison with measured dose values.

1. Philosophy of commissioningCommissioning of a TPS is necessary before the system is to be used in clinicaloperation. This commissioning process has the following functions:(a) To compare the performance of the TPS with measured data.(b) To assess the performance relative to criteria for acceptance, i.e. limits of accuracy.(c) To observe and record under what conditions the system is acceptable or not

acceptable.(d) To monitor the use of the TPS and ensure that the standards of acceptance are

maintained.

2. Requirements of commissioning:Prior to the commissioning process the following points and suggestions should beconsidered:(a) The commissioning should be carried out or supervised by a suitably qualified

medical physicist.(b) This person should have a full understanding of the operation of the hardware

and software.(c) All necessary beam input data should be entered according to the system

documentation (see Section 4.2).(d) All generated system data should be analysed using on-screen interactive (cursor

movement) interrogation and via printed dose distributions. The TPS generateddata should be compared directly with the equivalent measured data.

(e) The uncertainties associated with the measured data, data entry and output shouldbe noted. Known inaccuracies in the planning algorithm, through publicationsand/or manufacturers’ documentation, should be understood.

3. Checks required at the commencement of commissioning(a) TPS hardware tests are required in order to ensure that both the computer and its

peripherals are operating according to specification. Most computers have systemdiagnostics that test for processor, memory and disk operation. In addition tothese the following are necessary:(i) Plotter/printer accuracy: The coordinates of various straight lines should

be entered via the keyboard. The output can then be compared against theoriginal data.

(ii) Digitiser linearity: This can be tested by placing good quality graph paperwithin the working area of the digitiser and entering a series of referencepoints. The subsequently generated coordinates can be checked or plottedout on a printer/plotter and compared against the original graph points.


(iii) Video display distortions: Computer generated VDU displays (grid pattern)can be used to check for distortions visually. Where available the accuracyof the in-built ruler should be checked by comparing the length of plotterlines (in (i)) with those measured on the display.

(b) An independent check should be undertaken to ensure that the basic input datahave been entered correctly. This can be achieved by printing out the tables ofentered tissue air ratios (TAR), tissue maximum ratios (TMR), tissue phantomratios (TPR) or percentage depth dose (PDD) values and plotting any graphicaldata that have been entered through the digitiser. These data can then be comparedagainst the original data.

(c) Any system generated data that are to be used in the beam algorithms, e.g. PDDto TMR conversion, must be checked to determine whether the conversion iscorrect.

(d) All patient identification details (e.g. name, ID) should be checked for consistencyof filing and accuracy of reproduction on hard copies.

(e) The senses of the rotation scales on the TPS should agree with individual treatmentmachines. If not, this should be taken into account when transferring set-upinformation from the plan to instructions for the treatment radiographers.

4.3.2 Testing the TPS

4.3.2.1 General considerations

The tests described in this section have a general application to most planning systems.However, some of these tests will depend on the nature and sophistication of the TPSsoftware. It is for the responsible physicist to decide which tests are appropriate to theplanning system being commissioned. Items in bold in Tables 4.2–4.4 indicate the authors’opinion of the minimum range of parameters and analysis required for that particulartest. All other items are in addition to this minimum requirement and may provide amore complete understanding of the capabilities and inadequacies of the treatmentplanning system.

Most planning systems have the facility to interrogate the displayed dose distributionby cursor movement with point dose display and/or printing of dose values at chosenintervals along a selected line through the dose matrix. The tests described below requirethe use of these facilities and it is left to the reader to decide on the most appropriatemethod.

4.3.2.2 Basic requirements

Some basic test requirements are necessary to achieve consistency in the data generatedby the TPS. The reader should decide, on the basis of the dosimetry protocols employedand the measured beam data, whether the range of parameters given in these tests areappropriate for the TPS under investigation and which method of normalisation shouldbe employed. Some general remarks and assumptions concerning the tests are givenbelow:

1. The contours in which the dose distributions will be calculated should be rectangularor square (single or multiple planes). They should be of sufficient size to allow atleast a 5 cm margin each side of the test fields and should allow a depth of at least 20


cm (e.g. a 10 × 10 cm field requires a minimum 20 × 20 cm 2-D contour or a series of20 × 20 cm planes (20 at 1 cm spacing) for 3-D contours.

2. An external beam should be incident normally on one face of the contour, unless thetest specifies an alternative geometry.

3. The contour should encompass a unit density area, unless the test specifies otherwise.4. The photon and electron tests assume a single field in a single central plane, unless

multiple fields or multiple planes are specified.5. The photon and electron tests should be performed with the SSD (source surface

distance) equal to the SAD (source axis distance) of the treatment unit. This is normallyan SSD of 100 cm, but may be 80 or 90 cm for a cobalt unit.

6. A matrix grid spacing between 2 and 5 mm (or the highest resolution possible) shouldbe employed.

7. The field dose normalisation (i.e. the 100 per cent point) should correspond to thedosimetry protocol used for the measured data.

4.3.2.3 Criteria for acceptance

The suggested levels for acceptance of photon beams are based on a consensus of variousauthors’ opinions regarding the accuracy to which relative dose calculations should bepossible. The ideal level has been given as that suggested in ICRU Report 42 (1987) andthis reflects the accuracy that is considered achievable. The levels for acceptance ofelectron beams have had little discussion in the literature and have therefore been basedon the suggestions of Van Dyk et al (1993). Requirements for brachytherapy treatmentplanning are discussed in a BIR Report (1993) and have also been considered by VanDyk et al (1993). Table 4.1 presents a summary of levels of accuracy in low and highdose gradient regions. The values are ± percentages of the central ray normalisationdose for low gradients and ± displacement errors for high gradients. The levels quotedfor brachytherapy are for point and line sources and the values are ± percentages of localdose.

Table 4.1. Acceptance levels for accuracy.

Photons and electronsLow dose gradient High dose gradient

Acceptable level1 3% 4 mmIdeal level2 2% 2 mm

Brachytherapy

Point source Line sourceAcceptable level 5% 5%Ideal level 3 2% 2%

1 Published criteria (McCullough and Krueger 1980, Dahlin et al 1983, Brahme 1988, Van Dyk et al 1993).2 ICRU Report 42 (1987).3 BIR Recommendations for brachytherapy dosimetry (1993).

4.3.2.4 Photon beam tests

The test protocol for photon beams is listed in Table 4.2 (A–N). Tests A and B verify thesystem’s capability of reconstructing both data set and non-data set open fields fromuser entered data over a range of field sizes. The subsequent analysis involves pointdose acquisition from the resulting dose distribution and generation of dose profiles at


Table 4.2. Treatment planning system commissioning tests for photon beams.

Test Range Analysis

Central axis point doses at dmax,5, 10, 15, 20, 30 cm depth2-D dose distribution comparison

Beam width between 90%, 50%and 20% valuesProfile shape agreement

Central axis point doses at dmax, 5,10, 20 cm depth

2-D dose distributions

Dose profiles at dmax, 5, 10 cmdepth

Central axis point doses at dmax, 5,10 cm depth

Central axis point doses on eitherpseudo central or defined axisBeam’s eye view at dmaxProfiles at dmaxBlock or leaf transmission

Central axis point doses at dmax, 5,10, 20 cm depthDose profiles at dmax, 5 cm depth

A. Depth dosesTARs, TMRs, TPRs or PDDs

B. Dose profilesBeam widthPenumbra

C. SAD and/or SSD changeDepth dose for isocentricand extended SSD fields

D. Wedged fieldsDepth doseMaximum doseShape

E. Oblique incidenceContour correction

F. InhomogeneitiesCT number to electrondensity conversionAttenuation correction

G. Complex field shapesBlocked fieldsAsymmetric fieldsMulti-leafcollimators

H. Tissue compensatorsBolusRemote compensators

Square fields: 5 × 5, 10 × 10,15 × 15, 25 × 25 cmRectangular fields: 5 × 10, 5 × 20, 5× 30 cm

Square fields: 5 × 5, 10 × 10,15 × 15, 25 × 25 cmRectangular fields: 5 × 10, 5 × 20,5 × 30 cm (short axis)Profiles at dmax, 5, 10, 20 cm depth

Square fields 5 × 5, 10 × 10, 15 × 15cm and 40 × 40 cm (extended only)SSD 80, 90, 120 140 cm

Square fields 5 × 5, 10 × 10, 15 × 15cmRectangular fields 5 × 10, 5 × 20 cm(short axis)Wedge angle 15°, 30°, 45°, 60° orautowedge

Square field 10 × 10 cm(a) 30° and 45° sloping surface(b) tangential field

Square field 10 × 10 cmAir, lung and bone material in ahomogeneous phantom(a) 3 cm thick slab at (dmax+2) cm

depth(b) 3 cm sided cube on central axis at

(dmax+2) cm depth

Square fields 20 × 20, 35 × 35 cm(mantle only)(a) 1/2 field block(b) 1/4 field block(c) mantle field

Square field 10 × 10 cm30° sloping surface

continued overleaf


Table 4.2. Treatment planning system commissioning tests for photon beams.

Test Range Analysis

Dose profiles at dmax, 5 cm depthMidplane and TWO off-plane dosedistributions

Central axis point doses atisocentre2-D dose distributions

2-D dose distributions comparedwith a manual dose distribution or apreviously tested TPS

Check data against manualcalculation or a previously testedTPS

Repeat Test A

Central axis point doses at dmax andsuitable points within andexternal to the contour

I. 3-D calculationsOff centre planeCollimator rotationRotation of calculation plane

J. Rotations and arcs

K. Multiple beam distributions

L. Monitor unit calculation

M. Calculation options

N. Special conditionsRe-entrant fields

Square fields 10 × 10, 20 × 20 cmSagittal, coronal, 20° oblique planes

Square fields 5 × 5, 10 × 10 cmisocentric(a) 360° rotation(b) two 180° arcs(c) 45° wedge, 180° rotation

(a) Parallel pair(b) Wedged pair with equal and

unequal weights(c) 3 field with fixed SSD and

isocentric fields

All calculation parameters10 standard plans

(a) Homogeneous andinhomogeneous calculations

(b) Matrix grid spacing of 2.0 and5.0 mm

(c) Normalisation at dmax andisocentre

Square field 5 × 5 cmC-shaped contour with the centralaxis of the field reentering

Photon fields should be checked at all energies. The minimum recommended test range and analysis are indicatedin bold. All dimensions are in cm.

given depths. For square field sizes that should reproduce the original user entered data,agreement between input and output data should be better than 1 per cent. The 2-D dosedistributions should be compared directly with measured isodose distributions.

Tests C–F check the accuracy of the system in calculating modifications to the basicopen field arrangement. These modifications include the use of wedges, oblique incidence,changes in SSD and heterogeneous tissue densities. Test H considers the effect of insertingtissue compensators into the field.

Irregular field dose distributions are often calculated using advanced algorithms wherethe effects of scatter in the 3-D volume have to be considered. The increased use ofconformal fields and half beam blocking for field matching requires that the TPS istested to determine the capability of the algorithm to model these complex shapes. TestG considers point doses on a pseudo-central axis or a defined axis which determines the


ability of the algorithm to correct for asymmetric field conditions. Dose profiles andbeam’s-eye views (BEV) provide a more thorough analysis of shaped fields includingthe algorithm’s ability to model the effect on the penumbra.

For a TPS which is capable of 3-D dose calculations, tests are required to determinethe accuracy of off-plane dose calculations. Collimator and couch rotations can result innon-coplanar fields relative to the plane of planning. Test I includes some limitedinvestigation using off-plane dose profiles and a check of the accuracy of the calculationsdisplayed for coronal and sagittal views.

Rotation and arc therapy calculations are tested in J. Values of TMR at the isocentreshould be checked with and without wedges. Tests K and L are designed to check theTPS performance using standard distributions (e.g. a parallel pair of fields) and comparethese with either manually generated treatment plans or against plans from a well testedTPS with known performance. The calculation of the final monitor units must be checkedby considering all the variables, correction factors and formulae involved at every stageof the calculation.

Table 4.3. Treatment planning system commissioning tests for electron beams.

Test Range Analysis

Position on the central axis of dmax,d80, d20, d10Beams eye view at d90

Beam width between 95%, 80%,50% valuesProfile shape agreementBeams eye view at d90

Position on the central axis of dmax,d80, d40, d20, d10

Position on the central axis of dmax,d80, d40, d20, d10

Position on the central axis of dmaxand d80Beam width at d80

Compare with point dosemeasurements in a phantom or withpublished data*

A. Depth dosesPDDs

B. Dose profilesBeam widths Penumbra

C. Oblique incidenceContour correction

D. InhomogeneitiesAttenuation correction

E. Extended SSDContour correction

F. PlansSingle field

Square fields: smallest, 10 × 10, 20 × 20cmRectangular fields: 5 × 10, 5 × 20 cm

Square fields: smallest, 10 × 10, 20 × 20cmRectangular fields: 5 × 10, 5 × 20 cm(short axis)Dose profiles at dmax, d80, d50

Square field 10 × 10 cm30˚ sloping surface

Air , lung and bone material in anhomogeneous phantom(a) 1 cm thick slab at dmax depth(b) 2 cm sided cube on central axis at dmax

depth

Square field 10 × 10 cm105 and 110 cm SSD

NoseMandiblewith and without inhomogeneitycorrectionTreatment field junctions

Electron fields should be checked at all energies. The minimum recommended test range and analysis isindicated in bold. Sizes in the table should be adjusted to suit the applicators available where appropriate. Alldimensions are in cm.* Shiu et al (1992)


Planning system options that can be altered by the user and which may affect theresult of the calculation (e.g. normalisation, grid spacing) must be considered throughoutthe tests. However, the minimum requirements are that test A should be repeated todetermine the effect of these calculation options.

4.3.2.5 Electron beam tests

The checks on electron beam dose distributions are described in Table 4.3 (A–F). Theclinical use of electron beams usually involves a single field where the energy is selectedfrom a range between 4 and 20 MeV. The tests in Table 4.3 are similar to the photonbeam tests but do not involve all the beam modifications that are available for photontherapy. Electron beam depth dose values along the central axis should be assessed byestimation of the positional error in selected isodose values compared with their measuredposition. Dose profiles at dmax and at other depths should be compared with their measuredequivalents, together with BEV distributions at one depth to determine the effect ofbeam shaping devices that produce rectangular and other non-standard sizes.

The ability of the electron dose algorithm to correct for oblique incidence andheterogeneities can be assessed by similar central axis depth dose analysis. However, amore thorough analysis of the electron beam model for off-axis doses and 3-D dosedistributions is very time consuming and is usually beyond the requirements for basiccommissioning.

Finally, to demonstrate the relative insensitivity of the central axis depth dose valuesto changes in SSD, with the corresponding increase in width and penumbra of the beam,a number of standard plans should be produced and compared with published measureddata (Shiu et al 1992).

4.3.2.6 Brachytherapy tests

A list of tests is given in Table 4.4.All entered source data must be checked against manufacturers’ and published data.

The air kerma rates and source activities will have been confirmed by measurement foreach source and the correct transfer of these to TPS data files should be checked. Pointdoses for single and multiple sources should be compared against manually calculateddoses.

The ability of the TPS to reconstruct the source positions in three dimensions is testedby constructing a polystyrene block with a known geometry of wires (or dummy sources).The end point coordinates and wire lengths should be checked for accuracy using boththe manual and automatching options of the TPS. Most planning systems allow rotationand translation of the displayed distribution. Sagittal and coronal reconstructions of thephantom should be produced and, where possible, an oblique plane should be checked.(See also Chapter 9, Section 9.4.)

4.3.3 Documentation of results

Table 4.5 is a suggested format for documenting the results of the commissioning tests.An example of the results for photon beam tests A and B are given. The ‘agreementaccuracy’ has been calculated as the mean value of the difference between the TPS andmeasured data. Any observations regarding the accuracy over the range of the data should


be noted, together with any changes subsequently made to the input data to improveagreement.

Conditions for use of the treatment planning system based on the results of thecommissioning exercise should be fully documented and conveyed to the users of thesystem.

4.4 Quality control of the treatment planning system

4.4.1 Introduction

Once the treatment planning system has been commissioned and is in clinical use, it isnecessary to perform regular quality control on the system in order to measure any possibledegradation of the hardware and to identify any corruption of the data stored in thesystem. In addition, if any treatment unit or beam parameters change it may be necessaryto incorporate these into the software, and to check that the amendments have beenmade correctly. Similarly the effect of software upgrades needs to be tested. Thecommissioning records are the gold standard against which the quality of the system is

Comparison againstmanufacturers and published data

Dose or dose rate along a linenormal to the axis of the source atdistances from centre of the sourceor sources of 0.5, 1.0, 2.0, 5.0, 7.0cm. Dose distributions comparedwith published distributions

End-point coordinates andsource length

Dose distributions compared withpublished distributions.Repeat of test B (oblique planeonly)

Table 4.4. Treatment planning system commissioning tests for brachytherapy.

Test Range Analysis

Tests should be carried out for seed, pellet and line source calculations. The minimum recommended test rangeand analysis are indicated in bold. All dimensions are in cm.

A. Source dataBasic source data

B. Point doses anddistributions

C. Source reconstructionPhantom with dummy sources

D. Coordinate translation androtation

(a) Activity(b) Air kerma(c) Tissue attenuation and scatter

factors(d) Half life

(a) Single source(b) Multiple sources(c) Standard source arrangements e.g.

Manchester, Paris dosimetry

(a) 6 line sources(b) 12 seed/pelletsManual and auto matching

(a) Sagittal and coronal planes(b) 30° oblique plane through a source

inclined at 30° to the normal planein Test B


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measured. A housekeeping programme for backing-up the system, and archiving non-current patient data is also necessary.

It is usual practice to perform checks on individual treatment plans (IPSM 1992) (e.g.checking that the printed contour matches the patient outline entered, isocentre dose,monitor unit or time settings) and this is covered in Section 4.5. These checks may identifyoperator errors and some shortcomings in the system, but are no substitute for a qualitycontrol programme. A quality control system is necessary to ensure the continued qualityof computed treatment plans and should be designed to complement the checks onindividual plans. No manufacturer of a treatment planning system can guarantee acompletely bug free system since all possible sequences of operations can never be tested.Equally, no quality control programme can guarantee to pick up every fault in a planningsystem and especial care is needed when non-routine plans are done. A quality controlprogramme should be designed to test a range of parameters that are representative of itsclinical use, on a regular basis. General recommendations for a quality control systemare made in ICRU 42 (1987) and since then other authors (Van Dyk et al 1993, AAPM1994, IPEMB 1996a) have recommended quality control programmes for treatmentplanning systems. These recommendations are all in broad agreement though it isrecognised that quality control for these systems is an evolving subject. It is importantto document the test procedures within a quality control system and to record and dateall testing. All results should be compared with the initial test which is likely to be a partof the commissioning procedure. The final step in the treatment planning quality controlprocess is in vivo dosimetry (Chapter 10). In the United Kingdom the IPEM (formerlyIPSM) has set up audit groups to include all radiotherapy centres, whereby each centrecalculates a series of treatment plans on a suitable phantom (e.g. that shown in Chapter2, Figure 2.1) using their treatment planning system. A representative from another centrewithin the audit group then visits the centre and measures the dose distribution withinthe phantom for the different treatment set-ups with their own measurement equipment.This audit is based on the dosimetric intercomparison of UK radiotherapy centres(Thwaites et al 1992).

Recommended tests are described below. The frequency of the tests will depend tosome extent on the use of the treatment planning system, the involvement of physicspersonnel in the treatment planning process and the extent of checking of individualtreatment plans, but some suggested intervals between tests are given in Table 4.6.

4.4.2 Hardware

4.4.2.1 Processor tests

Some systems automatically perform a simple memory test whenever the computer isactivated. Many commercial packages are available to perform memory exercising testsas well as other diagnostic tests. These may be used on a regular basis or to help identifyspecific problems as and when they arise.

4.4.2.2 Digitiser and plotter

Scaling accuracy of the digitiser is often checked by the computer as part of the routinefor entering a patient outline. For plotters able to produce a full size plot, contours shouldbe checked against those input for each patient. Where the plot is not life size, the scaling


factors used should be checked. A more detailed test to check resolution and linearity isrecommended. A patterned grid could be digitised into the system. The procedure shouldincorporate any different modes available for entering information (usually point or trace),and any different resolutions available.

It may be useful to plot a standard pattern as part of a quality control programme –comparison with previous plots will also demonstrate whether the ink ribbon or pensneed attention.

4.4.2.3 Visual display unit

Checks for changes in image convergence should be made. This might involve displayinga standard grid pattern. If contrast and brightness controls are easily accessible to theoperator, then a test grey scale pattern should be displayed to ensure that they are at theiroptimum settings.

4.4.3 CT image handling

This concerns the accuracy of data transfer for which a reference CT scan image may beused. However, following services and software upgrades to the CT scanner it will benecessary to check the transfer of newly acquired images.

Table 4.6. Quality control programme for treatment planning systems.


Each useInput/output devices 4.4.2.2 1 mm

MonthlyData set – using checksum 4.4.4.1 No changeor – subset of reference plans 4.4.4.4 2% or 2 mm1

Three-monthlyProcessor tests 4.4.2.1 Pass or failInput/output devices 4.4.2.2 1 mmVisual display unit 4.4.2.3CT transfer 4.4.3 1 mmSubset of external beam reference plans 4.4.4.4 2% or 2 mm1

Six-monthly (additional tests)Brachytherapy reference plans 4.4.4.4 2% or 2 mm1

Annually (additional tests)MU calculation 4.4.4.3 2%Complete reference plans 4.4.4.4 2% or 2 mm1

Following software updatesUnderstand change(s) and devise relevant test(s)Input/output devices 4.4.2.2 1 mmTest field distribution and sourceposition reconstruction 4.4.4.2 2% or 2 mmMU calculation 4.4.4.3 2%Reference plans 4.4.4.4 2% or 2 mm2

1 AAPM 1994 – comparison with gold standard.2 AAPM 1994 – comparison with measurement.


A test should be devised to ascertain that the image has the same shape as the original.Scanning a phantom of known geometry and then comparing the image and hard-copywith the original is one method. The orientation of images also needs to be confirmed.

The CT scanner should have its own quality control programme to check therelationship between CT numbers and electron density (see Chapter 3, Section 3.2.3.3).The treatment planning system look up tables for electron density need to be checked. Ifthese tables are not available to the user, then a standard plan on a CT image, usingelectron density information can be recomputed.

4.4.4 Software

Treatment planning systems have separate programs, algorithms and data sets for differentapplications: external beam (photon and electron), irregular field, and brachytherapy.All modes in use need to have quality control.

4.4.4.1 Consistency of data sets

The data set should be stored as a locked file on the hard disk and should be comparedwith the approved filed data set. This can be performed either manually by comparinghard copies or by software comparisons. This test should also be performed after softwareupgrades and whenever the hard disk is defragmented or replaced. A check sum test canbe used on stored data sets whenever the system is booted up. However, a differencemay indicate a problem with the magnetic storage media rather than a change in thestored data.

4.4.4.2 Test fields

1. Simple isodose distribution reconstructionMany computer systems rely on look-up tables for beam data, and usually store aseries of square field data for external beams. In these cases a single field for whichdata are stored (a reference field) should be recomputed. Other non-reference fields(i.e. non dataset) should also be checked on a rolling programme. These may be for 2-D or 3-D calculations. For brachytherapy applications where tables of attenuationfactors versus distance for different isotope sources are stored, a dose calculation fora single source should be recomputed to check the integrity of the calculation algorithm.In all cases the reconstructions should be checked against ‘gold’ standards, producedas part of the commissioning process or as the initial test.

2. Field modifiers – photon beamsModifier terms available on the treatment planning system should also be tested. Astandard outline may be used for these checks and the IPSM phantom (see Chapter 2,Figure 2.1) is a useful tool for this. Parameters which might be tested are as follows:• SSD variation• oblique incidence• wedge calculation• collimator rotation of 45°

• internal inhomogeneity• bolus addition/compensators• off-axis calculation• beam blocking


• asymmetric jaws• multi-leaf collimatorsAs before, these computed distributions should be compared with the gold standards.

3. Interactive optionsFor external beam this includes checking the function and accuracy of such featuresas changing field position, size, angle, weight, hot spot and point dose calculations,and calculation options for inhomogeneity corrections (i.e. on or off). These testscould all be performed on the standard outline. Comparable functions to test thebrachytherapy applications would be changing the source activity or treatment time,or deactivating (or removing) a source. The resolution of the calculation matrix forboth applications is often variable and the effect of varying the number of calculationpoints should be assessed.

4. Contour reentranceCheck for correct and consistent handling of reentrant external and internal contours.

4.4.4.3 Monitor unit calculations

An independent means of calculating monitor units is needed. This can be achievedmanually from a machine data book, or by an independent computer system. One of thetest fields (Section 4.4.4.2) should also be calculated in this way and the result recorded.

4.4.4.4 Reference plans

To test more realistic use of the treatment planning system, it is suggested that for externalbeam applications there is a set of standard plans for which the outlines are entered andrecomputed on a regular basis. These would be based on the techniques used within thedepartment and would be specific to them. For brachytherapy applications, there shouldbe reference films of sources in a known configuration, which can be digitised in tocheck the reconstruction and dose algorithms. The aim of these test plans is to confirmthe correct operation of the treatment planning system under different conditions. Theseplans should be updated to incorporate new software features as they are introduced oras techniques change. It is important that these standard plans should be compared withthe ‘gold’ standards and any differences should be explained.

4.4.5 Housekeeping

4.4.5.1 Viruses

Any new software should be checked for viruses, ideally on a separate computer, beforebeing loaded onto the treatment planning computer. This includes software upgrades, aswell as other software that you may wish to use on the treatment planning computer. Thedangers of loading unchecked software from diskette onto the treatment planningcomputer cannot be overemphasised.

4.4.5.2 Backing-up

The data on the hard disk of the treatment planning system should be routinely backedup so that in the event of a computer problem, a minimal amount of data may be lost.The required frequency will depend on the use of the treatment planning system and itsreliability.


4.4.5.3 Archiving

There should be a routine for checking that sufficient space exists on the hard disk forthe treatment planning sessions. By instigating a programme for archiving non-currentdata you will ensure that valuable time is not wasted determining which patient data canbe archived, in the midst of a treatment planning session.

Figure 4.1. The treatment planning process. The staff group normally responsible for each stage is shown in thetop of each box. These are not intended to be prescriptive.


4.5 The treatment planning process

The stages of plan definition with the various inputs that are necessary in order to producethe patient treatment are shown in Figure 4.1. Also indicated are the staff groups typicallyresponsible for the inputs. (In an individual department these responsibilities may bedifferent.) The quality assurance system must ensure that the accuracy of all inputs ismaintained. In many cases the links shown will involve the transfer of information fromone staff group to another and it is easy for misunderstandings to arise unless the processis viewed as a whole. It is not possible to define a system for quality assurance of theplanning process that is applicable to all departments because of the varied modes ofworking that operate in different departments. In what follows guidance is given pointingto aspects that need to be checked and suggesting possible approaches to carrying outthe checks.

Throughout the consideration of appropriate quality control methods it is necessaryto recall that the aim of the planning process is to deliver the dose prescribed by theclinician to the region of the patient’s anatomy specified, while keeping doses to normaltissues within specified limits. To this end it is vital that the clinician’s requirements arespecified in as much detail as possible. If possible an indication should be given of thesignificant factors in defining the target volume. This will influence the selection of themost effective approach to achieving an accurate treatment set-up. For example, if theclinician wants to give a uniform dose to a volume that is symmetrical about the patient’smidline, it will be appropriate to check that the treatment field is symmetrical about themidline. Similarly if one of the aims is to avoid the spinal cord, a simulator or portalfilm demonstrating that this aim has been achieved is strongly recommended.

Before the treatment planner begins work on the plan it is wise to ensure that the planspecification is complete and accurate. The principle that should operate is that everyinput should be checked by another person. It is often appropriate that this person is amember of a different staff group. For example, if a target volume has been transferredfrom a simulator film to an outline by a doctor it may be appropriate that this process ischecked by the treatment planner since the treatment planner may be more aware of thetechnical aspects of the transfer process.

4.5.1 Simulator films

Simulator films provide the most common source of positional data for treatment planning.A distinction should be drawn between films taken to provide data input for planningwhich should be marked to show the planning target volume and verification films whichwill show the treatment field sizes. The principle requirements of quality control relateto achieving the correct magnification and the relationship of treatment fields to thesurface markings.

Wherever possible simulator films should have more than one indication ofmagnification. Many simulators will provide a digital readout of the source film distance.This can be compared with magnification ladders placed on the film. The field size alsoprovides confirmation of the magnification.

A simulator verification film provides a permanent record of the patient set-up. Checksshould be made that the field size is correct and that any asymmetry of the beam is as


intended. The frequency of such checks will depend on experience of the accuracyachieved in the simulator. In assessing whether a simulator beam film is correct all thepossible indicators should be used. These include the alignment of the centre of the fieldand the relationship of beam edges to structures such as the midline and bony landmarks.

4.5.2 CT scans

CT scans provide two sorts of data – geometry and density. Formal methods of verifyingthe accuracy of the data involving test phantoms have already been described, but simplechecks can be applied on a patient by patient basis and if done in a systematic way thesecan significantly reduce the need for frequent formal tests. Since the radiotherapydepartment may have little control of the QC of the CT scanner this may provide auseful way forward.

In practice the dose delivered will be much less sensitive to errors in density than ingeometry. If the planning process has involved the outlining of areas of inhomogeneitythe values reported back by the planning system will give the opportunity for a quickcheck of the density information (for example bony structures will have a density closeto 1.15 in all patients). Checks of the accuracy of positional information can be aided bymaking check measurements of AP and lateral separations at the time of CT scanningand these can be compared with measurements taken from the CT patient contour.

It is essential to reproduce the patient position as accurately as possible – for examplethe patient should be scanned on a flat couch using whatever immobilisation methodsare to be used for treatment. For pelvic treatments the quality system should also includemeasures to ensure that the state of the patient’s bladder and rectum is as reproducible aspossible. Great care must also be taken in transferring treatment set-up information tothe treatment machine. It is common practice for the plan to be verified in the simulator,but comparing a simulator film to the scanogram or scout view requires considerablecare. If the operator does not consider the different divergence conditions in the twoimages a significant error can be introduced. Ideally the transfer of positioning informationshould be based on markers placed at the time of CT scanning using bed movements toobtain the isocentre. If alterations to the set-up transferred in this way are found to benecessary, the procedure should be the subject of quality audit review.

4.5.3 MRI

The use of MRI for treatment planning is currently evolving. It has been demonstratedthat the use of MR images can significantly alter the target volume defined for braintumours, but there are a number of problems of quality assurance. The principal difficultyrelates to the inevitable distortion of the image caused by non-uniformity of the magneticfields and the effect of the patient on the field, which is harder to quantify. These requirethat particular care is taken to ensure that checks are made of the patient geometry. Themost successful approach so far is to register the image with a CT data set. However, itis important to build in some redundancy into this process in order to provide a measureof the accuracy of the registration. Similar checks to those already indicated for CT dataare also appropriate (see Chapter 3, Section 3.3).


4.5.4 Target definition from X-ray films

Much treatment planning is based on volumes defined from conventional X-ray films.For planning to be possible a contour of the patient is required. Once again some referenceseparations should be measured and written down to allow subsequent checking. Thetaking of the patient contour is particularly difficult to check once the patient has goneaway so any additional measures that can be built in will be helpful. For example leadwire markers can be applied to the patient when the X-ray films are taken so that checkmeasurements can be made from these. The second stage in the planning process is thetransfer of the target volume onto the outline. This may be done by the radiotherapist oralternatively by the planning staff. In either case it is essential that the radiotherapistshould indicate clearly (e.g. by drawing on the X-ray films) what is required. This willenable the technical aspects of the transfer process to be verified. Evidently the clinicaldecision on the target volume can only be checked by another clinician.

4.5.5 Dose prescription

The dose prescription is clearly the province of the clinician. However, the physicsdepartment can contribute to the precision of its definition by providing a framework toensure that prescriptions are unambiguous. It is well known that doses prescribed for thesame condition vary widely between radiotherapy centres. In addition different cliniciansoften prescribe doses in different ways. Since much time and effort is spent by the physicsdepartment in trying to ensure that dosimetry variations between hospitals are betterthan 3 per cent, it is important to ensure that this effort is not wasted by allowing widedifferences in practice between the methods of dose prescription. For example, two centreswho prescribe the same dose with one prescribing to the maximum and the other to theminimum dose to the planning target volume will actually differ by about 10 per cent.ICRU 29 (1978) and ICRU 50 (1993) recommend that the dose to the reference point atthe centre of the target volume (or the isocentre) should always be recorded. This policyshould be actively encouraged. (One method of doing so is to provide dose distributionsnormalised to this reference point.) Some form of planning request that sets out therequirements with regard to such things as the maximum dose to sensitive tissues andthe preferred beam orientations (if any) will assist the treatment planner to get the planright first time and will also make it possible for the person checking the plan to have aninformed view as to whether the plan meets the clinical requirements.

4.5.6 Preparation for treatment plan computation

The first step in computation of the plan should be to ensure that all the necessary inputdata are correct and complete. Much time can be wasted by doing a careful plan on thewrong data. In addition, in following through the procedures that have been carried outto arrive at the plan definition the planner will reach a better understanding of the clinicalrequirement. The plan is then created following the department’s preferred procedures.


4.5.7 Treatment plan quality control

Table 4.7 gives a list of items that should be checked. The procedure for the creation ofa treatment plan should include provision for checking all of these, although they neednot be carried out by the same person or at the same time. It is, however, important thatitems be checked by someone different from the person who originated that item. Forexample it may be appropriate for the check of the plan input data to be checked by theperson carrying out the plan if that person was not involved (or only peripherally involved)in the preparation of that data.

Table 4.7. Items to be checked on a treatment plan.

Are the outline and structures geometrically correct?Does the plan meet its specification?Is the plan the best that can be achieved (within resource limitations)?Is the dose distribution sensible?Has appropriate account been taken of 3-D variations?Have the appropriate inhomogeneity corrections been applied?Have the appropriate corrections been made for wedge filters, compensators, lead blocks, etc.?Are the machine monitor unit settings correct?Are the machine setting up instructions correct?Transfer of data to treatment unit?

The following is written as if all the checks were carried out at one time. In practice,when each individual item is checked will depend on the system established in a particularhospital. The ideal should be for items to be checked before any work is done whichwould have to be redone if a mistake were subsequently found. There should be no needfor items to be checked more than once, although it is clearly important that the personchecking has the appropriate expertise. If inexperienced staff are doing the planningdouble checking of important items may be advisable.

4.5.7.1 Geometric accuracy

For plans done from manual outlines the printed plan should be checked against theoriginal outline. For all plans, measurements made at the time the outline was taken orthe CT scan done should be checked against the plan. The position of the target volumeshould also be checked against the X-ray film or CT scans. Particular attention shouldbe paid to whether the correct side of the patient is being treated.

4.5.7.2 Conformance to specification

The treatment plan should meet the doctor’s specification. In some situations this willbe impossible and compromises will be necessary. Often the specification will besomewhat vague and require interpretation by an experienced treatment planner. In suchcircumstances it is obviously necessary to refer the plan back to the doctor. However, itis the responsibility of the physics department to provide appropriate advice to the doctorsand it is therefore important that an experienced person should confirm that thespecification can indeed not be met.


4.5.7.3 Plan optimisation

In principle a plan that meets the specification is adequate. However, as alluded to abovethe specification is often somewhat vague and a simple change may significantly improvethe dose distribution. How much effort can be devoted to such improvements will dependvery much on the total dose that is to be given and on time constraints in the planningdepartment. Much time can potentially be wasted by differences of opinion betweendifferent members of the planning staff. To minimise this, clear protocols for commontreatments are required which indicate the preferred beam arrangements and define thecriteria for plan acceptability.

4.5.7.4 Qualitative assessment of the dose distribution

However thorough the quality control programme for the planning system, it is likelythat bugs will remain undiscovered. It is therefore important that dose distributions andbeam parameters produced by computers should be treated with caution. Such problemsmay only be spotted by someone who is very familiar with what might be expected. (Aconcrete example of such a bug is a planning system in which inhomogeneity correctionswere not correctly applied if the inhomogeneity was not present on the most inferiorslice.)

4.5.7.5 Consideration of the dose distribution in off-axis slices

It is common practice for plans to be done showing the dose distribution on the centraltransverse slice. The patient contour and tissue densities may vary considerably betweenthis and parallel slices. Consideration needs to be given as to whether it is appropriate toignore such variations or whether they need to be accounted for by appropriate tissuecompensation. This will be particularly important in the head and neck and upper thorax.

4.5.7.6 Tissue inhomogeneity corrections

Most planning systems have a choice of tissue inhomogeneity correction algorithm. It isimportant to be consistent about which of these is used as the results may differsignificantly – particularly if one of the algorithms is not normally used and consequentlyhas not been fully tested. (Ideally in such circumstances some method should be devisedto make the use of the untested algorithm impossible.)

4.5.7.7 Corrections for beam modifiers

If any form of beam modifier such as blocks, wedge filters, or compensators have beenused an appropriate correction must be made.

4.5.7.8 Machine monitor unit settings

Evidently one of the most important parts of a treatment plan is the machine monitorsettings. To check this it is helpful to have a calculation programme that uses a differentalgorithm from that used for the original monitor unit calculation and that uses anindependent data set.

4.5.7.9 Setting-up instructions

Each department will have its standard way of providing setting-up instructions to theradiographers. A check should be made to confirm that the setting-up instructions have


been prepared in accordance with the treatment plan. Consideration should be given tothe design of a standard printed format that has specific places to record each instruction.Items which are essential for beam data entry to the treatment machine are unlikely to bemissed, but if there is an unusual instruction written in an obscure corner there is a realpossibility of error.

4.5.8 Transfer of data to treatment unit

To complete the quality loop some system is required for checking that the correct dataare transferred to the treatment machine. The most secure system is for electronic transferto take place and then for the radiographer to have to confirm the data by typing it inagain. Manual transfer of data and data made available after electronic transfer shouldbe checked by a second person. From time to time the correct operation of this system(including the human elements) should be checked by the physics department as it ispossible for errors of interpretation to creep into a system if such checks are never carriedout.

4.6 Conformal radiotherapy

4.6.1 Introduction

‘Conformal radiotherapy’ is used to describe those techniques in which the high dosevolume is tailored to conform to the internal target volume. In many ways the qualitycontrol requirements are no different from those of conventional radiotherapy, butadditional care needs to be paid to the geometric aspects of the process. Some of therelevant issues are collected together here and references given to other parts of thebook where more information can be found. A useful summary of the issues in conformaltherapy is given by Dahl et al (1996) and considerations of the infrastructure necessaryto be able to deliver conformal radiotherapy safely are given by Kolitsi et al (1997).

4.6.2 Geometric accuracy

The geometric accuracy of the CT scan data on which the plan is based has been consideredin Chapter 3, Section 3.2.1 and this Chapter, Section 4.5.2. Reproducibility of planningtarget volume definition can be improved by the use of computer generated marginsaround the gross tumour volume. Growing the volume is a three-dimensional problemand the computer can consider the effect of the variation in the target volume betweenslices (Belshi et al 1997, Stroom and Storchi 1997). The accuracy of the marginingalgorithm should be tested as part of the commissioning process and for each new versionof the software. Consideration must also be given to the way the algorithm extends thevolume in the sagittal dimension. The departmental system must incorporate clearguidelines to doctors so that there is no confusion as to how this is being done.

The field edge will usually be designed based on a beam’s eye view (BEV) of thetarget. It is important to verify the accuracy of BEV displays. Here again it is necessary


to consider the special case of the sagittal margin which should take account not only ofthe need to project the volume beyond the location of the last slice, but also of the factthat for coplanar fields the fall-off at the edge of the beam will be faster than is generallythe case in the transverse plane. Checks that an appropriate margin has been allowed canbe made by plotting the dose distribution in a sagittal plane, by inspecting the dosevolume histogram or by looking at a 3D view of the dose distribution.

4.6.3 Field shaping

The accuracy of MLC block shaping is covered in Chapter 5, Section 5.5.6. For blocksthe quality control may require more effort. Quality control must include checks atmultiple gantry angles of the tray holder system (see Chapter 5, Section 5.2.7) and checksto ensure that the individual blocks are the right size and are correctly oriented. It isrecommended that some form of tray coding system be used so that it is difficult to usethe wrong blocks for the individual beams. A light beam device set up in the mould roomto simulate the accelerator provides a convenient method of checking the shape of theblocks in advance of treatment.

4.6.4 Transfer of isocentre to the treatment machine

It is common practice for a simulator check to be made prior to the treatment. For thispurpose a digitally reconstructed radiograph (DRR) is a useful aid (see Chapter 10, Section10.2.3). It is important that the accuracy of the DRR is checked when the software versionis commissioned. This can be done using a scan of a phantom containing an object ofknown dimensions.

4.6.5 Dose calculation

True 3D dose calculation algorithms should take into account the effect of inhomogeneitiesand of the patient contour in neighbouring slices. However, it should be borne in mindthat any dose calculation is a model of reality and it is unlikely that the calculation willbe correct in every respect. The purpose of testing the algorithm is to establish itslimitations. A 3D planning system will provide a 3D view of the dose distribution so thatit is possible to verify that the treatment plan adequately covers the volume in threedimensions. This can be checked by creating multiple transverse slices and comparingthe results to both the 3D view and any multi-planar reconstructions. Dose volumehistograms (DVH) which show the proportion of the target and other organs raised to aparticular dose are a particularly useful check that there is adequate coverage of thetarget volume and for comparing plans. The accuracy of these should be checked. Thiscan be done by careful summation of the areas on different slices, taking account of theinterpolation between slices. Panitsa et al (1998) have suggested two methods ofcomparing the DVH with the calculation in a transverse plane. A volume is defined inthe middle of a single plain field. This allows checks of the maximum and minimumdose. For the second method a second volume is defined in the penumbra of a pair ofparallel opposed fields. This produces a cuboidal volume at the edge of the field inwhich the dose falls uniformly from 80 per cent to 20 per cent. These methods allow


comparison of the DVH with the output on single slices, but do not look at the absoluteaccuracy of the calculation. An approach to this is through the use of gel dosimetry(Maryanski et al 1996). In considering the accuracy of DVH calculations it is alsonecessary to establish how the computer closes the volume at the superior and inferiorextent of the volume. Considerable variation can be found if inadequate resolution isused for the calculation.

4.6.6 Treatment verification

When doses higher than those given using standard methods are to be delivered withtighter margins the in vivo quality control of dose (Chapter 10, Section 10.1) is desirableand portal verification (Section 10.2) is essential.

4.6.7 Intensity modulated therapy

Intensity modulation is in its infancy, although dynamic wedge treatments (Chapter 5,Section 5.5.4) are already widely used, and it is not therefore possible to make prescriptiverecommendations at this stage. Some suggestions are given in Chapter 5, Section 5.5.6.16as to how checks can be made of the accuracy of dynamic multi-leaf collimation. 3Ddosimetry techniques with film and gel dosimetry (Maryanski et al 1996) should be usedto check the accuracy of the predicted dose distributions. For individual treatment plansit is useful to have a phantom of the same part of the body for which a similar plan canbe executed. A discussion of the issues involved is given by Grant (1997).

4.7 Brachytherapy plan checking

There is such a wide variety of brachytherapy systems currently in use that it is notpossible to give a comprehensive guide as to what should be checked. However, theprinciples are the same as for external beam planning. The following particular checksshould be made.

4.7.1 Activities

A check should be made that the correct activities and activity date have been assignedto each source. This should include checking that the appropriate decay correction andconversion factors to reference air kerma rate (RAKR) have been used.

4.7.2 Scatter and attenuation corrections

The appropriate corrections for attenuation and scattering should be used. The BIR/IPSMreport on brachytherapy dosimetry recommends that the data of Sakelliou et al (1992)should be used, but this may not be possible with every planning system. Each departmentshould have a defined policy.


4.7.3 Accuracy of digitisation

The accuracy of digitisation is fundamental to accurate dosimetry. Unfortunately it isnot easy to check that the digitisation has been performed correctly from the printedoutput. However, provided that the source lengths are known, the accuracy may bechecked. Software producers should be encouraged to print the calculated length of anysources as part of the output. For seed implants checks are even more difficult. Forsurface implants a simple measurement could be made at the time of the implantation.

4.7.4 Manual calculation

Depending on the dosimetry system in use it may be more or less easy to do a manualcalculation of the dose (or to use another computer with an independent algorithm).Such a check is strongly recommended.

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CHAPTER 5

Megavoltage Equipment

5.1 Introduction

5.1.1 General

Linear accelerators have undergone considerable development since their introductionin the 1950s. Almost all such machines are now isocentrically mounted. The conversionof the machine between X-ray and electron treatment modes is now fully automated,thus considerably reducing the risk of exposures with incorrect treatment modalities.Technical advances such as the widespread use of interlocks ensure that the modernlinear accelerator is intrinsically far safer than earlier machines. However, although themachines are generally safer, device failures can lead to inaccuracies in treatment.Indications given by in-built monitoring systems can sometimes be misleading. Forexample beam flatness and symmetry faults which have not been detected by themachine’s monitoring system have been experienced on accelerators of more than onedesign.

Many of the tests covered in this chapter were described in IPSM Report 54 (1988)on the Commissioning and Quality Assurance of Linear Accelerators. However, sincethat book was published in 1988 further developments have taken place and technicalimprovements have also opened up new opportunities for errors. The emphasis here ison the ongoing quality control procedures. Special problems associated with developmentssuch as scanned beams (Section 5.5.2), multileaf collimators (Section 5.5.6) and movingcollimators (Section 5.5.4) are considered at the end of the chapter.

In this chapter each area of quality control will be considered and tests appropriate tothat area described. Summaries of when those tests should be carried out are tabulated.In many departments regular periods of time for quality control are allocated and theseare often of equal duration. Since checks are recommended at different frequencies it isnecessary to create a detailed annual programme of QC checks so that the less frequentchecks are carried out at the appropriate intervals. It some cases it may be preferable tocarry out checks of related items on a rota basis. In this way the common parts of thesystem are checked more regularly than if all the checks are carried out together.

5.1.2 Responsibilities

The quality control of the treatment machines is rightly the responsibility of a clinicalphysicist who must coordinate the quality control checks carried out by different groups.He or she should interpret deviations in terms of the effect that they will have onradiotherapy treatments and advise the radiotherapists accordingly. There is often adelicate balance between the need to treat patients and the rectification of a machineproblem. There is growing evidence that gaps in treatments, particularly for rapidlygrowing tumours, can have a deleterious effect on patient outcome (Hendry et al 1996,RCR 1996) and it may be better to allow treatment with a sub-optimal beam than to

97Megavoltage Equipment

interrupt a patient’s treatment. Radiotherapists should be fully involved in determiningthe policy for such situations.

Radiographers, as the regular users of the equipment, should always be aware of thecurrent performance and of any problems with the machine. They should therefore beinvolved in the QC programme in order to have day-to-day confidence in the equipmentthey operate. Radiographers frequently undertake daily constancy checks together withfunctional safety checks. Other tests may be done by physicists or medical physicstechnicians as appropriate. The radiotherapy physicist must retain the role of coordinatorfor the various groups involved in the quality control programme. If this is not possibleit is necessary to base the physics quality control programme on the assumption that noother checks are carried out. It is essential that when work is carried out on a linearaccelerator a suitably qualified physicist is involved in the decision to return the machineto clinical use. This rule may be relaxed if the work has been of a purely mechanicalnature, but where there is any element of doubt the default should be to consult a physicist.Because of the complex nature of linear accelerators, application of this rule is perhapsmore important than for other equipment. A useful discussion of maintenance issueswith regard to accelerators is given by Colligan and Mills (1997).

5.1.3 Tolerances

Alignment and dosimetry checks require criteria against which the measured parametermay be compared in order to decide what action, if any, needs to be taken. These criteriamust be clearly identified in departmental protocols. Routine quality control checks maytake place when it is not convenient to take the machine out of service to makeadjustments. It is therefore appropriate to define an ‘action’ level at which the machinemust be taken out of service should the error exceed that level. This may be coincidentwith the manufacturer’s specified tolerance, or in some cases greater latitude may beallowed. The action level must be locally determined based on the types of treatmentbeing carried out. A tolerance limit may also be defined which is the error which isacceptable on completion of maintenance. When a particular parameter is set up, however,it should be better than this tolerance limit.

In deciding the tolerances to be set, it must be borne in mind that the manufacturer’sspecification indicates the design tolerance of the equipment. Although it may be possibleto maintain tighter tolerances than this, to do so is likely to require more frequentpreventive maintenance than is economically justifiable. In this document recommendedtolerance levels are given. They are based on documents such as IEC Publication 977(1989b) and WHO 1988, although it must be recognised that the IEC specification is adisclosure standard and the values given are ‘guidance on the values that may beexpected’. The European Community has defined standards which should be met by alltreatment machines (European Commission 1997). These figures are given in Table 5.1.(The figures given for calibration and constancy do not have the same meaning as thoseelsewhere in this book. Calibration refers to the difference between measurement andabsolute dose and constancy refers to daily variation rather than to the measurementobtained with a constancy meter as in Section 5.2.12.3).

It is important to monitor trends in performance rather than just to state that a givenparameter is within specification. Recording the value measured provides better evidencethan a tick in a box that the check has been conscientiously performed. It may then be


Table 5.1. Criteria for acceptability of linear accelerators (European Commission 1997).

Test Remedial action level

Gantry and collimator rotation indication ±1°

Yoke rotation ±0.2°

Isocentre diameter ±2 mm

Source distance and beam axis indicators ±2 mm

Numerical field size indicators ±2 mm

Light field compared to radiation field ±2 mm

Treatment couch scale 2 mm

Couch deflection under load 5 mm

Immobilisation devices (e.g. casts, etc.) ±2 mm

Patient alignment devices ±2 mm

Light field indication – field size ±2 mm

Light field indication (density measurements) ±1 mm per edge

Dose calibration at reference point ±3% for photons and ±4% for electrons

Output constancy (including accelerators,cobalt and orthovoltage ±2%

Timer of cobalt unit ±0.01

Electron/photon beam type Correct type

Beam flatness and symmetry ±3% (photons and electrons and cobalt)

Orthovoltage beam symmetry ±6%

Transmission factor of wedge or compensator ±2%

Dose monitoring system

Precision ±0.5%

Linearity ±1%

Dose rate effect ±2%

Stability ±2%

Variation with gantry angle ±3%

possible to predict future problems and to take action in a planned way before the actionlevel is reached.

5.1.4 Level of checks

Many of the functions of an accelerator can be checked using a rapid procedure forroutine use with the more rigorous check being reserved for occasional checks or whena problem is identified. However, quick checks must be reliable to avoid unplanneddowntime resulting from faulty measurement equipment and quick check devicesthemselves must be included in the quality control programme. In what follows quickchecks and more rigorous checks will be described for many parameters. Table 5.2


(Section 5.2.15) distinguishes between quick checks and more thorough checks eitherdirectly or by reference to the paragraph number of the checks.

5.2 Checks on standard linear accelerators

5.2.1 Safety interlocks

As already indicated machines contain safety circuits designed to protect staff and thepatient against malfunction. However, these interlocks may fail and in some casesconsiderable harm to the patient or damage to the equipment may be caused when a faultgoes undetected. It is important to make an assessment of each particular machine toestablish what is monitored and whether there are ways in which a fault can be simulated.Checks that are generally applicable are described below. Many interlocks are checkedimplicitly every time the machine is used. Even in such cases a record of the check beingcarried out must be made at regular intervals.

Ideally a formal written analysis should be carried out of all the possible faultconditions. It is then possible to establish which interlocks need to be checked explicitlyand to ensure that appropriate records are made of any implicit checks. In some casesphysical intervention may be necessary to check an interlock’s operation. The risksassociated with such an intervention must then be balanced against the potential damagethat can be caused if an interlock fails to detect a fault. The failure of a water flow relay,for example, may result in considerable damage to the accelerator in the event of thefailure of the water pump.

It is to be expected that interlocks will trigger from time to time in normal use. It isimportant that carefully thought out procedures are in place for control of the resettingof such interlocks, so that fault conditions are brought to the attention of the physicsstaff. On rare occasions it may be considered necessary to override particular interlocks.This practice is to be strongly discouraged, but in any event should only be carried outwith the full knowledge of the Head of Physics who should normally discuss this withthe Clinical Director and the Superintendent Radiographer. Where it is necessary tooverride interlocks in order to carry out particular tests, it is essential that clear writtenprocedures are in place to ensure that their function is restored, and these proceduresmust be carefully followed.

5.2.1.1 Room protection

It is clearly of fundamental importance that the operation of the interlocks on the mazeentrance should be satisfactory and this should be checked daily. The barriers may befitted with two safety switches in series. If this is the case the operation of both switchesshould be checked independently at monthly intervals. Secondary access doors, such asto plant rooms, etc. and emergency-off switches should be checked monthly. Allemergency-off switches should be tested monthly. They may be tested as a series circuitby pushing all emergency-off switches in the circuit (which will include switches insidethe room, at the control area and in plant rooms) to their latch-open position and thenunlatching each in turn while checking the power at the linear accelerator.


5.2.1.2 Movement interlocks

The most important movement interlock is the ‘deadman’s switch’. The action of thisshould be checked at least weekly by checking that when it is released at least one of themachine movements is disabled. On some machines there may be several such switchesrelating to different movements. On a monthly basis a systematic check should be carriedout that none of the machine movements (other than those designed such as the collimatorjaws) will move without the appropriate switch being pressed.

The touchguard operation should be checked weekly. A check should also be carriedout to ensure that the touchguard operation can be cleared. Each electron applicator willhave a touchguard. The operation of these should be checked monthly, ideally through amore frequent check on a rota basis.

A monthly check should be carried out of the operation of all limit switches.Most accelerators will have applicator and lead tray interlocks. These should be

checked monthly.

5.2.1.3 Dosimetry interlocks

A check must be made before treatment commences that the machine will switch off atthe set monitor units although this check will usually be carried out as part of the dailyconstancy checks. It is also necessary to check that the machine will switch off with thesecond dosimetry channel. To check this it will probably be necessary to change thecalibration of one of the dosimetry channels and this check can therefore be limited toan annual check. However, if the dosimetry has to be adjusted anyway it is sensible toperform this check at the same time. On computer controlled accelerators a temporarychange in the calibration can be made and this will permit more frequent checks of thesecond channel. The backup timer interlock can usually be tested without interferingwith the machine’s internal controls by setting too short a time and checking that themachine terminates at the set time. This test should therefore be done monthly.

Accelerators should also have a high dose rate interlock. Testing this can be tricky,but it is advisable to do so at least annually as it provides the ultimate protection againstserious dosimetry faults. The manufacturer’s advice should be sought on how to do this.

5.2.1.4 Beam steering

All modern accelerators will have flatness and symmetry interlocks. These need to becalibrated so that the indicated flatness and symmetry is approximately correct. As thisrequires that the beam be steered away from the ideal such checks can only be carriedout occasionally. If, however, a significant change is made to the steering, such as whena monitor ionisation chamber is changed, the calibration of these devices and the operationof the interlocks should be checked.

5.2.1.5 Filter interlocks

Wedge filters will have some interlock to show that they are in place and, in the case ofmachines with multiple wedges, to identify which wedge is in use. The way in whichthese interlocks are intended to operate should be ascertained and a systematic checkcarried out monthly to ensure that all potential faults are covered. For manually operatedwedges this will include checking that the machine will not run:• if a wedge is selected and no wedge is in place;• if the wrong wedge or the wrong orientation is selected; or


• if a wedge is in place and no wedge is selected.

Sometimes the wedge storage is interlocked so that a positive indication that a wedge isin the store and not in the machine can be given. This indication should also be tested.For automatic wedges it will be necessary to examine the control system to identifypotential errors in operation and then to establish a test regime appropriate to the risk.

Multi-energy accelerators have systems to ensure that the correct combination of filtersor scattering foils are in place for the energy and modality in use. The advice of themanufacturer should be sought as to methods of verifying these systems.

5.2.2. Indicator lights

Beam on indicators and all other lamps and indicators on the control panel should beworking. To test some of these it may be necessary to set up fault conditions and acarefully written procedure should be defined if this is the case.

5.2.3 Mechanical integrity

Checks should be made on all the attachments to the machine to ensure that fittings havenot come loose, etc. Checks should also be made that the free play in the couch, collimatorrotation, etc. is not excessive. Brakes should also be checked. Such items should be partof the manufacturer’s maintenance schedule.

5.2.3.1 Couch deflection under load

A test of the couch deflection under load should be carried out. The IEC standard (1989a,b)specifies that this should be conducted by placing a 30 kg weight on the couch at theisocentre and distributed over 1 m with the couch fully retracted over its support. Theheight of the couch should be noted. The couch should then be fully extended and a 135kg load spread out symmetrically on either side of the isocentre to represent a patient(i.e. over a 2 m length). The difference in the height of the couch at the isocentre underthese two conditions should be less than 5 mm. The angle of roll of the table top as it ismoved laterally from the centre to one side should also be measured. This should be lessthan 0.5˚.

5.2.3.2 Electrical safety

A linear accelerator has a number of high voltage circuits and these may be moreaccessible than in conventional equipment and are also likely to include charge storagedevices. Tests for electrical safety should be carried out regularly. Particular emphasisshould be placed on the integrity of the earth circuits and of the provision for dischargeof high voltages. A check should also be carried out to ensure that all covers are in placeand that appropriate warning signs are fitted.

5.2.4 Mechanical alignment checks

5.2.4.1 Mechanical isocentre – definitive checks

The isocentre is the common centre of rotation of the gantry, the collimator and patient


support system (treatment couch). Because of mechanical limitations, the isocentre isnot a single point and is therefore defined as the centre of the smallest sphere that containsthe axes of rotation of these components. The diameter of this sphere should be notgreater than 2 mm, but will depend on the design and age of the accelerator. The isocentricaccuracy in older machines may well limit the types of treatment set-up that can becarried out and in such circumstances clinical practice should be reviewed to ensure thatset-up methods achieve the best results possible.

Isocentric techniques rely on being able to rotate the machine about the isocentreplaced at the centre of the tumour without having to move the patient between fields.The initial setting of the isocentre is usually guided either by a distance indicator fromthe anterior field or by the laser isocentre markers. The point of rotation of the gantrymay move in the manner indicated in Figure 5.1 as the gantry is rotated. To achieve thebest possible geometric accuracy of treatments in such a situation it may be appropriateto set the indicated isocentre position fractionally below the true isocentre position asindicated. It should certainly not be set to the centre of rotation from the anterior direction(Woo et al 1992). It is also quite common for there to be a misalignment of the centralaxis in the longitudinal direction between the gantry 0˚ and gantry 180˚ positions.

The most satisfactory way to establish the isocentre is to attach a sharp rigid pointerto the front of the accelerator. This pointer needs to be adjustable both in the distance ofits tip from the source and in the other two orthogonal directions. Figure 5.2 shows sucha device. It is first set up so that the centre of the pointer does not move when the collimatorsystem is rotated. The collimator is rotated through its complete travel and the centre ofrotation noted. (A convenient way to achieve this is to use a spring loaded pointer and apiece of carbon paper. As the collimator is rotated the point is pressed onto the carbonpaper to mark out its movement.) Accurate collimator rotation is relatively easy to achieve;it should therefore be possible to adjust the pointer so that its movement is less than 0.5mm.

Next a second fixed pointer is set up (Figure 3.4 shows a suitable mounting), usuallysupported on the couch. The distance of the tip of the gantry-mounted pointer from thesource is then adjusted to give the minimum movement of its tip relative to the fixedpointer as the gantry is rotated. The fixed pointer is moved to the optimum positionbearing in mind the importance of the sector close to the horizontal as discussed above.The tip of the fixed pointer now defines the mechanical isocentre, and the optical and

Figure 5.1. Position of pointer (indicated by •) with gantry at 0°, 90°, 180°, and 270°. Note that with the gantry at0° the centre of rotation falls outside the 2 mm diameter sphere. It is suggested that in such a situation theindicators should be set to indicate the centre of the 2 mm diameter sphere as illustrated.


mechanical indicators can be adjusted accordingly. It is convenient if a reference pointercan be kept for each machine as this allows for rapid and accurate checking, and ifnecessary adjustment, of the isocentre indication.

Once the fixed pointer has been located the size of the sphere containing the isocentre(isosphere) can be determined. Observation of the relative movement of the two pointerscan be very useful for diagnosis of problems if the movement is found to be excessive. Asimple test of the alignment of the optical system can also be carried out by watching therelative movement of the shadows of the pointer and of the cross wires as the gantry isrotated.

5.2.4.2 Couch rotation axis

The axis of rotation of the couch should also be contained within the isosphere. Thisconsideration is often forgotten when measuring the isocentre position. If the fixed pointeris rigidly fixed to the couch the couch can be rotated and the movement of this pointerrelative to the gantry mounted pointer measured. The indication of the isocentre should,however, not be altered to take the centre of rotation of the couch into consideration;rather this should be adjusted if necessary to match the gantry and collimator definedisocentre. The accuracy of the couch rotation is important when treatments in inclinedplanes are being carried out using couch rotations or with multiple arc stereotacticradiotherapy; errors in couch rotation in such techniques can be corrected for if necessaryby moving the patient, although this makes the treatment more time consuming.

5.2.4.3 Mechanical isocentre – quick checks

Several simpler methods can be used for a quick check of isocentre indication. Theseuse the optical indication of the isocentre. It should be noted that problems with the

Figure 5.2. Reference pointer suitable for mechanical isocentre checks. The position of the tip of the pointer can bemoved in and out and sideways. The knob on the left allows rotation of the pointer. (photograph courtesy ofClatterbridge Centre for Oncology).


optically indicated isocentre will often be caused by a misalignment of the light bulbrather than by a misalignment of the gantry. Some of the tests are complementary and acombination should be used for routine checking:

1. The collimator is rotated with the gantry at 0° and the crosswire indication is markedon a piece of paper at different collimator angles. All the points should be containedwithin a 1 mm diameter circle. This test can also be carried out with the referencemechanical front pointer (see Section 5.2.4.1). Adjustment of the crosswire is coveredin Sections 5.2.6 and 5.2.10.

2. The gantry is rotated through a small angle and the couch height altered until theshadow of the crosswires does not move across the couch. The surface of the couchthen contains the rotation axis of the gantry. However, this test can be misleading asit emphasises the position of the rotation axis for gantry angles close to 0°. For thereasons explained in Section 5.2.4.1, this is less important than with the gantry closeto horizontal.

3. A white plate marked with a black cross mounted so that it can be rotated about ahorizontal line in its surface can be constructed (Figure 5.3). This is then placed withits rotation axis at the indicated position of the isocentre and the central cross alignedwith the crosswires. The gantry is then rotated and the alignment of the crosswires tothe cross on the test tool can be observed. This test is particularly recommended. Bysetting the field size so that the field is smaller than the plate, movements of thecollimator jaws with gantry angle can also be monitored. By rotating the collimator atdifferent gantry angles it can be confirmed that the crosswire gives an accurateindication of the axis of rotation of the collimator at all gantry angles.

4. A square block with crosses marked on its faces can be used. This is placed with itsupper surface half the thickness of the block above the isocentre and centred on the

Figure 5.3. Device similar to that shown in Figure 3.3 (Chapter 3) for assessing optical isocentre accuracy.(Photograph courtesy of Clatterbridge Centre for Oncology).


crosswires. When the gantry is rotated to give a horizontal beam the centre of thefield should be aligned with the centre of the block. The block can be mounted in atrack on a levelling plate. By moving the block horizontally the alignment of thelateral lasers can be checked as well as the indication of 90° and 270° on the gantryrotation scale.

5.2.4.4 Distance indicator

Once the position of the isocentre has been established the mechanical and optical distanceindicators can be set. For a quick check methods (3) and (4) in the previous section canbe used. The mechanical distance indicator is likely to be more stable than the opticaldistance indicator and should only be altered following accurate establishment of theisocentre position as described in Section 5.2.4.1. Having set the distance indication forthe isocentre distance the linearity of the distance scale should be checked by movingthe couch a known distance.

5.2.4.5 Front pointer and back pointer

The front and back pointers can also be checked once the position of the isocentre hasbeen established.

5.2.4.6 Calibration of gantry and collimator rotation scales

The gantry rotation scale can be checked using a spirit level placed on a surface knownto be perpendicular to the axis of the beam. The resolution of the scale may be only 1°,but this should not be more than 0.5° in error. The collimator rotation scale should alsobe checked in a similar way.

5.2.5 Position of light source

If, as is commonly the case, the light source rotates with the collimator, the followingmethod can usefully be used to check that it is accurately on the axis of rotation of thecollimator. The gantry is set to the vertical position and the couch top retracted so that asmall field, such as 50 × 50 mm can be projected onto the floor. A convex lens of focallength approximately 400 mm is placed in the beam at about 500 mm from the source.The height of the lens is adjusted so that an image of the filament is projected onto apiece of paper on the floor with a magnification typically of about 3. The paper is markedat the centre of the filament and the collimator rotated through 180°. The paper is markedagain and the correct centre is midway between the marks. The light source may beadjusted as necessary. A 3 mm distance between the marks will correspond to a 0.5 mmerror in the filament position.

5.2.6 Optical field indication

5.2.6.1 Quick check

A quick check of optical field size indication for a 10 cm × 10 cm field should be carriedout daily. With a suitably marked dose checking device this check can be carried outsimultaneously with the daily dose check.


5.2.6.2 Variation with field size

The field size scale should be checked monthly over its full range. This can beconveniently done using a piece of squared paper placed in the plane of the isocentre.The collimator is set successively to field sizes of, say, 4, 10, 20 and 30 cm and the sizeof the field noted. If adjustments are found to be needed the radiation field size shouldfirst be set correctly and the optical indication adjusted to match it using the optical fieldtrimmers. For small fields the tolerance usually used is 2 mm in the field size and in theposition of individual jaws. For modern precision radiotherapy a tolerance with regardto individual jaw positions of 1 mm is usually achievable. The optical indication may beless accurate for the 30 cm field. An occasional check should also be made that the fieldsize is correctly indicated at extended SSD (see Section 5.2.10 Item 2).

For these checks a sheet of accurately ruled graph paper approximately 30 × 40 cm isuseful. This should be marked with a central cross and additional lines corresponding tofield sizes of 5, 10, 20, 30 and 40 cm. It is convenient if the paper is laminated with atransparent plastic coating to give some protection against damage. This also stiffens thegraph paper so that it may be held vertically on the couch top, using lead blocks assupport.

5.2.6.3 Variation with collimator rotation

A check should be carried out that the positions of the field edges do not change as thecollimator is rotated to the four cardinal angles. This can be carried out at the same timeas checking the crosswire rotation. This test should also be carried out at gantry angles

Figure 5.4. Misalignment of the crosswire or light source can be compensated for in one plane, but not in all.

➝

Image of crosswire does not move withcollimator rotation in this plane

➝

Isocentre plane

Large movement of image ofcrosswire in this plane


other than 0°. The alignment of the field indicated at gantry 0° with that at gantry 180° isa useful check. If the light bulb is loose a greater than normal discrepancy will be seen.

5.2.6.4 Adjustment of optical indication

If it is found necessary to adjust the optical system the first step is to establish that thelight source is accurately positioned on the axis of rotation of the collimator. This can bedone as described in Section 5.2.5 or using test (1) described in Section 5.2.4.3. This testshould also be carried out at extended SSD (e.g. on the floor) as it is possible for amisalignment of the light source to be made to compensate for a misalignment of thecrosswire if the test is only carried out at one SSD (see Figure 5.4). To establish that thelight source is at the same distance from the isocentre as the radiation source the fieldsize indication at extended SSD can be checked.

5.2.7 Shadow tray

It is increasingly common to use individual shielding trays with pre-mounted lead blocks.Such systems rely on all the lead trays fitting accurately into the shadow tray holder. Iftransferability between machines is required it is also necessary that trays should fitcorrectly in all machines. A reference lead tray should be kept for the purpose of testingthis. The tray should have a cross scribed through the point that should coincide with thecentre of the beam (which will not necessarily be the centre of the tray). The correctalignment of the tray can then be checked by comparing the shadow of this cross withthe crosswires. It is also important that blocks should be correctly aligned when thegantry is horizontal and that they should not move relative to the centre of the beam asthe collimator is rotated. To test this, a straight sided lead block can be mounted on asecond reference tray so that one corner of the block is exactly aligned with the axis ofthe beam. With the gantry horizontal the collimator can be rotated and the position of theblock corner compared to the centre of the beam. Films should occasionally be taken ateach orientation of the collimator to verify the relationship between the lead tray and theradiation centre of the field.

5.2.8 Couch movements

Couch lateral and longitudinal movement scales should be verified against marks madeon a sheet of paper on the couch. Couch vertical movement should be checked by notingthe position of the crosswires at different couch heights. Gantry sag would result in anapparent misalignment of the couch and this can be verified using a plumb-line. Thescales will indicate absolute distances from a particular point and should be within 2 mmof their stated value. In practice relative movements of the couch need to be more accuratethan the absolute indication (depending upon the way the equipment is used).

5.2.9 Radiation alignment

5.2.9.1 Alignment of radiation and light field

The alignment of the radiation field is, of course, fundamental to accurate radiotherapy.


In most situations (except when matching fields) the radiographers will set a field sizeusing the numerical field size indicators and will assume that this field is centred on thecrosswires.

The basic test is to place an envelope wrapped film at the isocentre, set a mid rangefield size, mark the position of the light field and the crosswires and irradiate the film.Marks can be made either by applying pressure to the film (before exposure) with a biroor hard pencil, or by pricking the corners of the field with a pin, or using a jig withradioopaque inserts. The dose given should be such as to give an optical density ofaround 1.5. A slow film should be used as, with short exposures, undue weight will begiven to any transient misalignment of the radiation beam before the beam steering servoshave time to act. The positions of the field edges are then identified with a densitometer.Misalignment of the radiation and light fields will be caused either by a misalignment ofthe optical system or by misalignment of the radiation beam. For monthly checks it issufficient to perform this test for one representative field size (e.g. 10 cm × 10 cm or15 cm × 15 cm). Every six months, or when adjustments are made, the alignment ofsmall and large fields should also be checked. In analysing these tests the relationship ofthe radiation field to the field centre (as indicated by the crosswires) and the size of theradiation field are more fundamental than the alignment with the optical field per se.

Treatment verification film is conveniently envelope wrapped but has a small lineardose-density range. Fine grain graphic line films have better linearity and backgrounddensity but require reusable black plastic envelopes. They are also thinner than X-rayfilm and can get stuck in some automatic X-ray film processors; this can be avoided byattaching a leading X-ray film, using special metallic adhesive tape. A slab of PMMA ofsize 25 cm × 25 cm × 5 mm is useful for checking the agreement between X-ray andlight beams. The slab is marked with a central cross and fields of 5 cm × 5 cm, 10 cm ×10 cm, 15 cm × 15 cm and 20 cm × 20 cm. Lead markers (e.g. thin solder) can beinserted into holes drilled on the central cross and along the edges of each field.

The radiation and light field should agree within 2 mm. The centre of the radiationfield should also be within 2 mm of the central axis of the machine. The latter should bereadily achievable and for machines being used for precision radiotherapy a tolerance of1 mm should be achievable.

5.2.9.2 Alignment of radiation field – quick check

Several commercial devices are available with multiple detectors designed for checkingbeam flatness (see Section 5.2.11.2). These typically have diodes or ionisation chambersat the centre of the field and at the edges of a 16 cm square. For flatness and symmetrychecks a 20 × 20 cm field would be set, but if a 16 × 16 cm field is set the dose recordedat the edges should be 50 per cent of that on the central axis. Since the variation of dosewith distance from the centre between the 20 per cent and 80 per cent points on the beamprofile is roughly linear and the beam penumbra is typically 6 mm, a 10 per cent differencefrom 50 per cent will indicate an error in the position of the field edge of 1 mm. Asdevices are not normally designed for this purpose they should be individually calibratedagainst the field edge position indicated by film.

5.2.9.3 Radiation isocentre

Assessment of the radiation isocentre is inevitably a complex procedure that does notlend itself to a quick check. The simplest method is to expose a film which is marked in


the usual way with the gantry vertical and then irradiated with the gantry at 180°. Bycomparing the alignment of this film with one taken in the manner described in thepreceding paragraph it is easy to establish the relative movement of the radiation field.This will usually be along the direction of the gun target axis due to sag of the gantry. Itis evidently important that the film should be placed in the plane of the isocentre asotherwise an error in the gantry angle will be incorrectly seen as an error in the radiationisocentre.

A useful method of looking at the radiation isocentre is the ‘star film’. A film is mountedvertically in a PMMA holder in the plane of rotation of the gantry with its centre at theindicated position of the isocentre. As narrow a field as possible is set and the filmirradiated with the gantry at 0°, 120° and 240° (these directions are not prescriptive). Theperspex holder may have holes drilled at the centre and at 120° increments around thecentre to coincide with the directions of irradiation. The holes are used to pierce the filmto assist analysis. The image on the film will be a star shape and it is relatively easy todraw lines on the film indicating the centre of each beam. These should all meet at thefilm centre. An example for which the increments were 50° is shown in Figure 5.5. Itwill be noted that this method gives no indication of errors in the direction orthogonal tothe plane of rotation.

Figure 5.5. Star film in which the film has been irradiated at 50˚ intervals. A small dot, indicated by the arrow,marks the isocentre.


With the development of stereotactic radiotherapy a very precise estimate of thefundamental isocentric accuracy of the gantry becomes possible. With a circular applicatorattached to the head of the machine and a ball bearing suspended at the isocentre thegantry may be rotated and films exposed at appropriate angles. The apparent movementof the image of the ball bearing relative to the circular field is easily observed. This testmay of course be carried out with a small field produced by the ordinary collimator.

5.2.10 Interpretation of alignment checks

The results of some of the preceding tests may indicate a problem in alignment of theunit. The possible error situations are described below. The first step in aligning thesystem is to ensure that the crosswires do not move when the collimator is rotated.

1. The light source is not on the rotation axis of the collimatorThe light source rotates with the collimator; it is therefore possible to set the opticalpenumbra trimmers and the crosswire so that the crosswire position stays static andthe collimators are symmetric at all collimator rotations (see Figure 5.4). However,the alignment of the crosswires with the axis of rotation will only be correct at oneSSD and an extended SSD check of collimator rotation will demonstrate this.

2. The light source is not at the correct distance from the isocentreIn this case the field size at extended SSD will not be in correct proportion to the fieldsize at the isocentre and coincidence of the radiation and the light field will not becorrect at the extended SSD.

3. The light source mounting is looseIf the light source moves as the gantry is rotated the alignment of the light source willbe different at different orientations of the gantry. This can be detected by checkingthe rotation of the collimator with the gantry at 0° and 180°.

4. The optical field trimmers protrude too much or too little into the beamIf the optical trimmers are incorrectly set this will be manifested by a difference infield size between the radiation and the light field.

5. The optical field trimmers are asymmetricThe light field will be asymmetric and the asymmetric jaw will move round as thehead is rotated through 180°. If there is doubt about this it is useful to adjust the lightfield trimmers so that they do not protrude into the beam at all and then ensure thatthe correct radiation field size is set before adjusting the light field. If the opticalfield is symmetric in spite of the trimmers being asymmetric this indicates that thelight source or the radiation source (see Item 7) is incorrectly set up.

6. The radiation source is not on the central axis of the machineThe radiation source does not move with collimator rotation. If it is not on the axis ofrotation of the collimator there will be an asymmetric radiation field whose asymmetrywill be equal and opposite for opposing jaws and will not change as the collimator isrotated.

7. The electron beam is striking the target at an angle other than 90°

If the electron beam strikes the target at the wrong angle this may result in anasymmetry in the penumbra. The flatness scans will also show a misalignment of theminimum in the centre of the beam with the geometric centre. For this reason it isimportant that flatness scanners are set up so that the collimator central axis positionis known. There may also be an energy change brought about by the need to compensate


for the incorrect angle by a change in energy to match the bending current. This willresult in an asymmetric beam profile at the edges. Because of this possibility it isimportant to check the beam profile before adjusting the optical field.

8. The collimators are asymmetricThe asymmetry of the radiation field will change sides as the head is rotated through180°.

9. The collimators are not parallel to the beam edgeIf the collimators are not aligned with the edge of the diverging beam an increase inthe width of the penumbra will be seen. It should also be possible to detect significantangulation by looking at the collimator. This may also make it difficult to set up theoptical penumbra trimmers.

10. The gantry sags or there are distortions in the drum or gantry bearingsGantry inaccuracy will be demonstrated by a misalignment of the beam at 0° and180°. This can be demonstrated with a mechanical front pointer or by marking up afilm with the gantry at 0° and exposing it with the gantry at 180°. The former is theonly way to demonstrate problems with the gantry bearings.

11. The collimator bearings are faulty so that it does not rotate correctly about its axisErrors in the collimator bearings will be demonstrated by erratic movements of thecentral cross when rotating the collimator.

12. The rotation axis of the gantry and collimator differThis will result in a shift in the A–B direction (i.e. perpendicular to the gantry rotationaxis) as the gantry is rotated through 180°.

5.2.11 Flatness and symmetry

5.2.11.1 Flatness scans

From the point of view of quality control the establishment of an appropriate standardfor beam profile measurements is problematic. Beam symmetry on the one hand is easilydefined and is not very dependent on the depth of measurement. On the other hand theflatness of the beam depends on the size and shape of the measurement phantom. Atcommissioning of an accelerator it is necessary to establish that the beam profile conformsto the accelerator’s specification. This must be done in a water phantom makingmeasurements at the depth or depths (usually at the peak and at 10 cm deep) at which theflatness is specified. Subsequently it is important to ensure that the profile does notchange significantly from when the beam data were measured. This can be satisfactorilydone using an ionisation chamber contained in a small block of PMMA. The profilesobtained in this way should not be regarded as absolute measurements of flatness andshould be compared to a similar measurement made at the time of commissioning. Forthese ‘in air’ scans the dose at the periphery of the beam should be higher than at thecentre. This will be affected by the energy of the beam and by the focal spot size on thetarget. A change in the beam profile is an indication that there is something wrong withthe way the accelerator is set up and it is not really appropriate to apply the same criteriaas at commissioning. With monitor ion chambers that monitor the whole of the beam achange in the profile may affect the dose calibration of the accelerator and this will oftenbe the cause of differences between the dual dosimetry channels. The profile shouldtherefore be maintained within 2 per cent of its shape at commissioning. Asymmetry ofthe beam will be related to the beam steering.


The IEC definition of flatness (IEC 1989a) is illustrated in Figure 5.6. Flatness andsymmetry are measured only within the ‘flattened area’ which is defined in Figure 5.6a.Symmetry is then the ratio of the dose rates at symmetrical points on either side of thebeam axis and flatness is the ratio of maximum to minimum dose anywhere in the beam.For X-ray beams the minimum is usually close to the central axis of the beam. Althoughthe IEC specification allows an asymmetry of 3 per cent, it should be easier to maintainbetter beam symmetry with a modern accelerator. However, 3 per cent is sufficientmeasurement accuracy for a quick check. A very convenient method of measuring thebeam profile is with a multi-element array of diodes or ionisation chambers. Not onlyare such devices faster to use, they also provide a method of observing the moment tomoment stability of the beam and are particularly convenient for setting up beam steering.

When assessing the cause of a beam asymmetry it may be useful to carry out a scanwith the flattening filter removed. (With a computer controlled accelerator, removingthe flattening filter can often be achieved by typing a command at the keyboard.) Theresulting scan will show clearly whether the peaked dose distribution is correctly centredand whether it is symmetrical.

5.2.11.2 Quick checks of beam flatness

For a quick check of flatness it is adequate to measure the dose at the centre and twosymmetric points on the beam profile. A number of rapid check devices are available for

Figure 5.6a. IEC 976 (1989a) definition offlattened area (shaded). W is 80% of thefield width for field sizes between 10 cmand 30 cm. For sizes below 10 cm it is 2cm less than the field size and for sizeslarger than 30 cm it is 6 cm less than thefield size. Along the diagonal the distancefrom the corner (d) is 20% of the fieldwidth for field sizes between 10 cm and 30cm. For smaller sizes it is allowed to be 2cm and for larger sizes it is 6 cm.

Figure 5.6b. IEC 976 (1989) definition of flatness andsymmetry. Flatness is the ratio of Dmax to Dminexpressed as a percentage. Symmetry is the absolutemaximum value of the ratio of D-x to D+x (where D-xrepresents the value of the dose rate at a distance x onone side of the central axis of the beam and D+x is thedose rate at the corresponding point on the other side ofthe beam axis). The maximum symmetry may be at anypoint within the flattened area i.e. it will not be at thepoint indicated in the diagram.


this purpose. Film can also be used, but the reduction in the required machine time mustbe balanced against the cost of film and the time taken to read it out. An electronic portalimaging device can provide a very rapid constancy check.

5.2.12 Radiation output measurement

5.2.12.1 Definitive calibration

It is recommended that a definitive calibration (Appendix A) should be carried outfollowing a major repair – such as replacement of a monitor ionisation chamber – and atleast once a year. The definitive calibration should follow the relevant IPEM dosimetryprotocol exactly using a dosemeter which has been directly calibrated against a secondarystandard. Consideration should be given to the fact that the dose per pulse varies fromone machine to another and this will affect recombination corrections. If using the 2560/2561 secondary standard dosemeter it is not possible to measure the recombinationcorrection directly and it is therefore necessary to know the dose per pulse. This may beobtained by dividing the dose rate by the pulse repetition frequency (PRF). It is advisableto check this with an oscilloscope as machines sometimes have frequency dividers whichmake the actual PRF different from the indicated value (e.g. the Elekta SL25 at 25 MV).For the annual calibration the requirement for a second calibration with another dosemeteris not necessary, but the measurement of a dose in a non-standard condition should beincluded.

5.2.12.2 Routine calibration

Ideally the routine calibrations should also be done following the protocol exactly.However, it is common practice to use a small water or PMMA phantom for convenience.In order to be classed as a calibration rather than a quick check the measurement must becarried out with a calibrated thimble ionisation chamber and any differences betweenthe dose measured in the calibration phantom and the 30 cm cube water phantomrecommended in the code of practice should be measured so that the result can beexpressed in terms of dose. If a plastic phantom is used for weekly (or daily) calibrationsit may be appropriate to use water on a monthly basis.

It is important to record the monitor units indicated by both dosimetry channels. Onmany accelerators the two channels are derived from different areas of the beam and ifthis is the case a difference between them can be a sensitive indication of a change inbeam profile. A check should also be carried out of the machine dose rate indication.This is not critical from the point of view of patient dosimetry, but dose rate is an indicationof the general health of the machine and if the calibration is inaccurate this can bemisleading.

5.2.12.3 Constancy checks

A number of devices are available which enable a rapid check of dose to be made. Thesemay use a parallel plate ionisation chamber with a minimal amount of built-in build-up.This allows measurements to be made at the peak of the depth-dose curve by adding anappropriate thickness of build-up material. In principle, provided that the build-up isappropriate the calibration factor should be dependent only on the beam quality of theaccelerator, but in practice it is found that the calibration may be different for two


machines of the same energy. This makes such devices inappropriate for calibrationpurposes, but they are suitable for daily constancy checks. Diodes may also be used forthis purpose. Whatever device is used it is important that it should be checked regularlyagainst a calibration measurement. The frequency of such checks will depend on thestability of the constancy device. The minimum frequency is every 6 months, but it maybe appropriate to make a check every time a calibration is carried out. It is commonpractice for one particular dosemeter to be associated with one treatment machine. Whileergonomically sound this approach does not benefit from the added security that comeswhen an instrument is used on several machines, all of which are unlikely to develop achange in dosimetry in the same direction at the same time.

5.2.12.4 Effect of gantry rotation

If the ionisation chamber becomes damaged it is possible for the plates to move as thegantry is rotated. It is therefore important to check the stability of machine output withgantry angle. This can be done using a thimble ionisation chamber (with an appropriatebuild-up cap) placed on the axis of the machine. The variation should be compared tothe value obtained when the gantry is at the orientation used for calibration. It should bepossible to maintain the absolute calibration with 2 per cent. Quick check devices maybe susceptible to RF pickup which should be borne in mind as a possible explanation oferratic readings. Where a cobalt-60 unit is available it can provide a useful method ofverifying the stability of calibration of the constancy meter.

5.2.12.5 Wedge factors

Wedge factors should be measured by placing the axis of the thimble ionisation chamberparallel to the unwedged direction and making two measurements with the collimatorrotated through 180˚ between them. The mean value should be used to calculate thewedge factor. It is recommended that measurements should be made at the referencedepth (5 or 7 cm). In calculating the wedge factor allowance should be made for thedifference in depth dose with the wedge according to the method adopted by thedepartment for that purpose. The two measurements of the wedge factor should agree tobetter than 3 per cent. Differences may be associated with the alignment of the beamcentre with the wedge or with the accuracy of set-up. The difference in the wedge factorwith different orientations of the collimator may cancel out provided that the wedgefactor is stable with orientation of the gantry.

Wedge factors should also occasionally be checked with a horizontal beam as thewedge position may have some backlash. Errors in the wedge factor with a horizontalbeam may cause a significant error in the dose delivered to the patient. The calibrateddose at any orientation of the collimator or gantry angle should be within 3 per cent.

The frequency of wedge factor measurements will be critically dependent on the designof the accelerator. On some Elekta machines with automatic wedges the wedge factorcan be adjusted electronically and should therefore be checked as frequently as thecalibration is carried out. Automatic wedges generally require more frequentmeasurements than fixed external wedges which can be seen by the operator. In this caseit may be appropriate to establish a constancy check procedure where the dose is measuredwith the wedge in the beam for one orientation of the collimator only. Figure 5.7 showsan internal automatic wedge that had been damaged when the bearings became worn.This damage was barely detectable on the central axis and it is suggested than aninspection of the internal wedge should be carried out from time to time.


5.2.12.6 Output factors

The variation of machine output with field size is one of the measurements required atcommissioning. Although this variation is likely to be stable over the life of the acceleratorit is wise to verify a few field sizes on a regular annual basis. This may be most usefullydone by using the machine data as used for routine calculations to calculate the monitorunits required to deliver a dose for a few randomly selected depths and field sizes. Ifdifferences are detected further measurements will be needed to determine the cause ofthe discrepancy.

5.2.12.7 Depth dose and profiles

Depth doses and beam profiles will also be measured at the time of commissioning.Machine characteristics may change over the life of the machine and it is recommendedthat a subset of these measurements be repeated on an annual basis. Such measurementswill also provide added confidence in the original measurements and provide a measureof the stability of the machine performance. The measurements should show that machinedosimetry is consistent within 3 per cent. A change of any measurement from thecommissioning measurements greater than 2 per cent should be investigated.

5.2.12.8 Other checks

Occasional checks should also be carried out to verify the consistency of dose output.This may be done by taking a series of ten consecutive measurements. The standarddeviation should be better than 1 per cent. It is also necessary to check the linearity ofthe dosemeter. It may be found that for short exposures the dose delivered is not inproportion to that delivered with longer exposures. This can be caused by instability ofthe beam before servos are able to stabilise it or because of end errors in the circuits thatswitch off the beam. A differential diagnosis can be obtained by plotting dose delivered

Figure 5.7. Automatic wedge showing damage to upper surface caused by wear.


against dose set. The possibility of variation of delivered dose with output dose rateshould also be considered. However, changes in dose rate are usually associated withchanges in beam steering, etc. so it is impracticable to measure the variation with doserate routinely. Alteration of the PRF does not usually change the dose per pulse and sowill be unlikely to affect the dose calibration unless there are problems such as a leakyionisation chamber. Causes of such variability include recombination in the ionisationchamber and changes in beam characteristics with dose rate.

5.2.13 Beam energy

The energy of the beam can be affected by the frequency of the RF waves. This willaffect the ‘ears’ or ‘horns’ at the edge of the beam because the effect of the flatteningfilter will be changed. Beam energy can be defined in terms of the tissue phantom ratiobetween 20 cm and 10 cm deep, or TPR20/10. This is measured directly by measuringthe dose delivered at 10 cm deep in a water phantom for a fixed number of monitor units,say 200, and then placing a further 10 cm of water equivalent material on top of thephantom without moving the position of the ionisation chamber relative to the source. Inpractice it is convenient to measure the 5/15 ratio since this can be done immediatelyfollowing a calibration at 5 cm deep by simply adding 10 cm of solid water on top of thephantom. A change of 5/15 ratio greater than 1 per cent which corresponds to a changein energy of about 0.5 MV should be investigated. Some quick check devices (Section5.2.11.2) have one detector which can be placed beneath a steel absorber so that anenergy measurement can be made at the same time as flatness and constancy.

5.2.14 Arc therapy

Accurate arc therapy requires three factors to be correct:1. The transmission ion chamber reading must be independent of gantry rotation – this

is checked as in Section 5.2.12.4.2. The gantry must start and stop at the required point.3. The servo relationship between gantry rotation and dose must be correct.

Of these the third is the most important. It may be checked in a number of ways:1. A film may be exposed in a cylindrical phantom.2. An asymmetric phantom may be used and the expected dose calculated for a given

arc. It can then be verified that this dose is actually given when the arc is carried out.3. If outputs of arc angle are available the delivered dose may be plotted directly against

arc angle. (It should be noted that the delivery of a constant dose rate is not necessaryprovided that rotation speed is altered to compensate appropriately.)

It should be possible to deliver the required dose to an asymmetric phantom to within 2per cent.

In many centres arc therapy will be used only very occasionally or not at all and thefrequency of testing can be adjusted accordingly. If the facility is not regularly testedsteps must be taken to ensure that it is not used without the knowledge of the physicsdepartment. Arc therapy should not be omitted from acceptance testing even if it is notused clinically, especially on a computer controlled accelerator where arcing may providea useful test of the control system.


Table 5.2. Check frequencies for linear accelerators.


DailyOutput constancy 5.2.12.3 ±5%Light field size (10 cm square field) 5.2.6.1 2 mmCrosswires and laser axis lights coincident 5.2.4.3 2 mmOptical distance indicator (compared to lasers) 5.2.4.3 3 mmMaze entrance interlock 5.2.1.1 FunctionalAudio visual system FunctionalMachine log 5.2.15.1

WeeklyDeadman’s switch 5.2.1.2 FunctionalTouchguard 5.2.1.2 FunctionalDistance indication at different SSDs 5.2.4.4 3 mmPointers (if used) 5.2.4.5 2 mmCalibration 5.2.12.2 ±2%Wedge factor constancy (machines withadjustable wedge factors) 5.2.12.5 ±2% of expected

Two–weekly

Flatness and symmetry at gantry 0° (quick check) 5.2.11.2 ±3% of expected

MonthlySecondary access interlocks 5.2.1.1 FunctionalEmergency-off switches 5.2.1.1 FunctionalMovement interlocks (full check) 5.2.1.2 FunctionalCoded lead tray interlocks 5.2.1.2 FunctionalBackup timer interlock (where set by user) 5.2.1.3 FunctionalGantry and collimator rotation scales 5.2.4.6 ±0.50

Optical field size variation for different field sizes 5.2.6.2 2 mm (small sizes)Isocentre quick check 5.2.4.3 2 mm diameterShadow tray alignment 5.2.7 1 mm from centreDistance indication at different SSDs 5.2.4.4 2 mmCouch movement calibration 5.2.8 2 mm relativeCouch vertical movement 5.2.8 2 mmGantry angle indication 5.2.4.6 10

Radiation field versus light field (one field size) 5.2.9.1 2 mmCalibration in water 5.2.12.2 ±2%Energy check using dose ratio 5.2.13 Ratio ±2%Arc therapy (if used) 5.2.14 Dose ±2%

Three-monthlyIndicator lights 5.2.2 FunctionalFilter interlocks 5.2.1.5 FunctionalBackup dosemeter and timer interlocks(computer controlled) 5.2.1.3 FunctionalField size indication at extended SSD 5.2.6.2 3 mm at 150 cmOutput calibration variation with gantry angle 5.2.12.4 Calibration ±2%Wedge factor variation with gantry angle 5.2.12.5 ±3%Dose check for non standard field size and SSD 5.2.12.1 ±2%Optical field versus radiation field (small andlarge fields) 5.2.9.1 2 mmFlatness and symmetry full scans at all gantry angles 5.2.11.1 3%

continued on page 118


5.2.15 Selection of checks and check frequencies

Table 5.2 lists the recommended checks for linear accelerators with photon beams. It isnot claimed that the checks and frequencies on this list are either necessary or sufficientfor proper quality control of each and every machine, but they represent a level consideredappropriate as the basis of a quality control system. Individual physicists have theresponsibility of varying the tests and frequencies to suit the requirements of theirparticular equipment. It is necessary to balance the complexity and difficulty of carryingout a test against the consequences of a fault occurring and perhaps giving rise to atreatment error. In choosing to differ from these recommendations it is wise to recordthe reasons for the change both for medico-legal purposes and as an indication of thethinking behind the quality control system in use for the benefit of other physicists inthe department. For example low energy accelerators with a standing wave guide do nothave a bending system and flatness and symmetry are unlikely to vary. On the otherhand a machine with servo controlled bending that is known to be unstable may requirechecks at very frequent intervals.

The frequencies of the tests have been compared with those in IPSM Report 54 (IPSM1988), the WHO publication, Quality Assurance in Radiotherapy (WHO 1988), the surveyof UK practice (IPSM 1992) and the report of AAPM Task Group 40 (AAPM 1994).

It is assumed throughout this publication that a test that is required to be carried outdaily should also be carried out at the other frequencies except where some more thoroughtest is prescribed. The tolerances given for daily checks are mostly wider than would beappropriate for more extended monthly checks and in such cases the check has beenexplicitly repeated. It may be appropriate for the simpler test to be carried out at thesame time as the more thorough test to provide a baseline. Daily checks will often becarried out by the radiographers and this is to be encouraged as it gives them a feel forthe accuracy of the equipment. Other checks may be carried out by technicians orphysicists as appropriate.


Six-monthlyRadiation isocentre 5.2.9.3 2 mm diameterFlatness interlocks 5.2.1.4 2% of intendedRadiation check of shadow tray alignment 5.2.7 1 mm (at isocentre)Calibration of constancy check device (if used) 5.2.12.3 Device dependentLinearity of dosimetry system 5.2.12.8 Within 1%

AnnualVerification of water flow and other interlocks FunctionalCouch deflection under load 5.2.3.1 5 mmHigh dose rate interlock 5.2.1.3 FunctionalDefinitive isocentre check 5.2.4.1 2 mm diameterDefinitive calibration 5.2.12.1 ±2%Backup dosemeter and timer interlocks (notcomputer controlled) 5.2.1.3 FunctionalPlotting tank measurements of depth dose and profiles 5.2.12.7 >2% changeCalibration of flatness monitor 5.2.1.4 2% of intended

For electron tests see Table 5.3.

Table 5.2. Check frequencies for linear accelerators (continued).


5.2.15.1 Documentation

It is essential that adequate records are kept both of the checks carried out and of anyfault conditions that may occur (NRPB 1988). These records should be inspected regularlyand follow-up action taken where necessary. It is important that records made byradiographers should be readily accessible to the physics and technical groups and viceversa.

5.2.16 Equipment required

The following is a list of equipment that will be found useful for quality control checks.Some of the equipment is related to specific test methods and where this is the case areference is given to the paragraph in which the test is described:

• ruler (this should be accurate to 0.5 mm);• plumb-line;• spirit level (sensitivity 0.2° ≡ 3 mm/m) for horizontal and vertical use);• carbon paper (Section 5.2.4.1);• free-standing pointer, such as an engineer’s scribing block (Section 5.2.4.1);• sheet of accurately ruled graph paper approximately 30 × 40 cm (Section 5.2.6.2);• film phantom (Section 5.2.9.1);• photographic film of width at least 25 cm (Section 5.2.9.1);• simple film densitometer to measure optical density at a point;• thermometer and barometer (see Chapter 8, Section 8.6);• stop-watch;• thimble ionisation chamber and electrometer (see Chapter 8, Section 8.2.5);• daily output check dosemeter (Section 5.2.12.3);• 40 cm focal length lens (Section 5.2.5);• scanning densitometer;• beam profile scanning equipment (Section 5.2.11.1); and• dose plotting water tank.

5.3 Required tests for electron beams

5.3.1 Introduction

Many of the quality control checks performed on linear accelerators are not appropriatefor the electron modality. Some of these, however, will be necessary for the operation ofboth X-ray and electron beams, such as safety interlock checks and mechanical checkson the couch. Since these are considered elsewhere, we shall discuss only those checkswhich are exclusive to electron use.

There are essentially only three electron beam parameters to check, namely output,energy and beam uniformity (symmetry and flatness). Different workers recommenddifferent frequencies for performing these tests. For example, in IEC publication 977(1989b), it is recommended that field symmetry and flatness be performed monthly using


a different scatter foil each month, and using only the maximum energy appropriate forthe foil. If this protocol were followed, beam uniformity at some energies would only bechecked annually, when the publication recommends a check at all energies. In contrast,the World Health Organisation (1988) recommends that the symmetry and flatness ofeach energy used should be checked twice per month.

Differences in the check frequencies recommended by different groups will dependupon experience. Over the years, technological improvements have led to increasedreliability and stability of electron beam characteristics, and check frequencies whichwere appropriate 15 years ago may no longer need to be so stringent. We consider thatthe following programme of checks and their frequencies will give confidence thatelectron beam characteristics will remain within specification, and thus be fit for theirclinical purpose.

In producing our programme, we have taken account of recommendations from IECpublication numbers 976 and 977 (1989a, b), the World Health Organisation (QualityAssurance in Radiotherapy, 1988), the Institute of Physical Sciences in Medicine ReportNo 54 (1988), the National Radiological Protection Board’s Guidance Notes for RadiationProtection (1988) and the American Association of Physicists in Medicine (Task Group40, 1994). The Institute of Physical Sciences in Medicine survey on quality controlreported in Scope (1992) was also consulted.

When electrons are used for patient treatments the set-up frequently differssubstantially from the measurement situation. A regular programme of in vivo dosimetryis particularly appropriate in this situation and if such a programme is in place thefrequency of some checks may be appropriately reduced.

5.3.2. Description of tests

5.3.2.1 Output calibration check

Each week, a check should be made to verify that, for each available electron energy, aknown dose will be delivered to a reference depth in water (normally the depth ofmaximum dose, dmax) through a standard applicator (with a standard insert if appropriate).This should be done at a gantry angle of 0°, and, typically, a 10 cm × 10 cm applicator/insert would be used as the standard for each energy. It is usual for the linear acceleratorto be calibrated to deliver 1.00 Gy at dmax in water for a setting of 100 monitor units atthe standard treatment SSD. The measured dose should be within 2 per cent of thecalibrated dose, determined using the IPEMB (1996) protocol.

A temperature and pressure corrected ionisation chamber, cylindrical or parallel-plate,must be used for the calibration check. The effective point of measurement of the chambershould be at the reference depth for the energy of the beam to be measured (see Chapter 4,Section 4.2.1.1). Water-equivalent material is preferred for the measurement phantom(the water equivalence must have been demonstrated), but other materials such aspolystyrene or polymethylmethacrylate (PMMA) are suitable if the measurements canbe related to those which would have been made in water. Polystyrene or PMMA phantomsshould consist of slabs no thicker than 12 mm in order to minimise charge storage effects.Both cylindrical chambers and parallel-plate chambers may be used at higher electronenergies (i.e. greater than a mean surface energy of about 10 MeV). At lower electronenergies, the steepness of the depth-dose gradient makes cylindrical chambers less suitablefor output calibrations, and parallel-plate chambers should be preferred, although, if


care is taken to reproduce the conditions under which the cylindrical chamber wascalibrated at low energy, it may be used if a parallel-plate chamber is not available.

In addition to the weekly check, a check on the variation of output with gantry angleshould be done annually, choosing one electron energy and noting the ionisation chamberreadings at 0°, 90° and 270°. The ratio of the largest to the smallest of these readingsshould not exceed 1.02.

5.3.2.2 Constancy check

Output constancy checks should be performed at every energy or at each energy whichmay be used during the day. Consideration should be given to using a different applicatoreach day for the constancy check. This scheme would not only check the output, butwould also be an indirect check that the collimator settings for each applicator are correctlyinterlocked (see Section 5.3.2.6). For a set of five applicators, such a scheme wouldprovide a check on each applicator every week. The reading of the constancy checkdosemeter should be within 5 per cent of the expected reading.

Constancy checks are meant to give reassurance that no serious malfunction oroversight has occurred of such a nature that a patient would be seriously under- oroverdosed in a single fraction. In the context of electron treatments, a satisfactory resultin a constancy check would demonstrate, for example, the integrity of the scatter foilsystem. Although no check can guarantee that the next exposure will also be safe, aconstancy check carried out at the beginning of each treatment day should provide thenecessary reassurance, when part of a quality control programme as set out in Table 5.3.

A tolerance level of 5 per cent is coupled with the knowledge that weekly outputcalibration checks will ensure that the output remains within 2 per cent of specification.Experience with different types of electron-beam linear accelerators shows that outputsdetermined in the weekly calibration checks cluster randomly about the mean with astandard deviation of less than 1 per cent, and exhibit a long-term drift which is sogradual that intervals between recalibrations may extend to many months. The role ofthe constancy check is therefore different from that of the calibration check, and thetolerance level may safely be relaxed to 5 per cent.

The reason for wishing to relax the tolerance level for the constancy check is againpragmatic: constancy-check equipment may be a radiation diode or ionisation chamberwithout temperature and pressure correction. The intrinsic accuracy of the constancy-check equipment is therefore likely to be less than that of the calibration equipment.Experience with such devices shows that a 5 per cent tolerance level is triggered veryinfrequently, and then most likely because the device itself needs to be recalibrated.

It should be noted in passing that the National Radiological Protection Board’sGuidance Notes (1988) recommend that electron-beam apparatus be calibrated at leasttwice weekly, with a tolerance level of ±5%. Having regard to improvements in thereliability of this modality over the past two decades, we believe that the recommendationsof checks and frequencies set out in Table 5.3 more than fulfil the aims of the GuidanceNotes for checks of electron output.

5.3.2.3 Energy check

Checks of electron energy are based upon the penetrative property of electron beams:the higher the energy, the deeper the penetration. The simplest way to do this, havingdetermined the energy of an electron beam according to an appropriate protocol, is to


take the ratio of readings of a detector at two depths in a phantom irradiated by the beamat fixed SSD. One of the depths should be close to the maximum ionisation depth, andthe other on the descending part of the ionisation curve, ideally around the 50 per centionisation depth, although anywhere within the 30 – 80 per cent range will be satisfactory.

Either parallel-plate or cylindrical chambers may be used for energy checks usingthis method, although the same remarks which applied to cylindrical chambers at lowerelectron-beam energies in measuring output apply here also. Other types of detectormay be used, such as diode detectors, and a compound parallel-plate ionisation chamberis available commercially which will verify electron energy constancy in a single reading,provided that the reading is normalised with the measured output. Phantom material iscommonly water-equivalent resin, but polystyrene and PMMA are also useful.

Whichever method is chosen, the indicated energy should be within that correspondingto a shift of not more than 2 mm in the chosen ionisation level on the descending part ofthe curve. It is recommended that each electron energy be checked monthly at a gantryangle of 0°. In addition, each electron energy should be checked annually at gantry anglesof 90° and 270°.

5.3.2.4 Beam symmetry and flatness

In IPSM Report No 54 (1988) and also in IEC 976 and IEC 977 (1989a, b), it isrecommended that electron beam flatness should be determined by finding the ratio ofthe highest dose at the depth of dmax anywhere in the radiation field to that on the centralaxis at the same depth. Using this definition, it is easy to put limits on the size of any

Table 5.3. Summary of checks for electron beams.


DailyOutput constancy check 5.3.2.2 ±5%

WeeklyOutput calibration check of each available energy 5.3.2.1 ±2%

MonthlyEnergy check at each nominal energy, gantry at 00 5.3.2.3 dz correct to within ±2 mm

where z is any value between30 and 80

Applicator interlocks 5.3.2.6 Functional

Three-monthlySymmetry and flatness for each available energy 5.3.2.4 Symmetry: max/min ≤1.03

Flatness:0.97≤1.00≤1.03

Jaw positions for each applicator and energy 5.3.2.6 ±2 mm

AnnualApplicator output factor check for a small number

of representative applicators and energies 5.3.2.5 ±2%

Output constancy at gantry angles of 0°, 90° and 270° 5.3.2.1 Max/min output ≤1.02Energy check at each nominal energy, gantry at

90° and 270° 5.3.2.3 As for monthly check

Symmetry and flatness, gantry at 90° and 270° 5.3.2.4 As for three-monthly check


peaks of intensity in the radiation field relative to the central axis (the IPSM Reportrecommends 1.03) but it seems that regions of low intensity are not proscribed.

In order to remove this ambiguity, it is suggested instead that electron-beam flatnessis checked using the largest available applicator (20 cm × 20 cm or greater) and measuringdose at depth dmax across both major axes on either side of the central axis up to adistance of 0.4 times the field width from the central axis (see Figure 5.8). (The fieldwidth is the distance across the field on a major axis between the 50 per cent points onthe surface of the measurement phantom.) The ratios of doses so measured to the doseon the central axis should be determined: for example, at A in Figure 5.8, the ratio is1.03, and at B, the ratio is 0.96. It is recommended that the flatness should be defined asthe value of whichever of these ratios differs most from unity (in this example, it wouldbe 0.96). It is recommended that the flatness should be within 3 per cent of unity, i.e., itshould lie within the range 0.97–1.03.

Figure 5.8. Profile at dmax for an electron field illustrating the definition of flatness and symmetry.


With some combinations of electron applicator and scatter foil and energy, thistolerance may be exceeded, even if the beam is symmetric: for example, with sometypes of linear accelerator, the beam profile may exhibit symmetric dose ‘horns’ whichare 5 per cent more intense than the dose on the central axis. In such cases, there may beno adjustment left apart from major redesign of the system, and it may be judged wiserto accept a higher ratio in these instances.

Beam symmetry is checked by comparing the doses at points equidistant from thecentral axis on dose profiles taken along each of the two major axes at the depth ofmaximum dose. The dose at each point is averaged over 10 mm of profile, and the ratioof the larger to the smaller of the doses in any of the sets should not exceed 1.03. Theprofile need only be extended towards the beam edge as far as the 90 per cent isodose.

Beam symmetry and flatness may be measured using ion chamber readings at discretepoints at the depth of maximum dose, or using densitometry on film exposed within thelinear range of the response characteristic. The largest available applicator should beused. Water equivalent resin or material such as polystyrene will be found convenientfor these measurements, which should be done with the surface of the phantom at thenormal treatment distance from the source. The measurements should be done at a fixedgantry angle, usually 0°, but an additional check at 90° and 270° should be done for arepresentative energy annually, at the same time as the three-monthly check.

It is recommended that the checks be performed on a three-monthly basis (when eachof the available energies should be checked).

5.3.2.5 Applicator factors

Applicator factors relate the central-axis dose delivered per monitor unit at a given energythrough a given applicator (with insert, if appropriate) at depth dmax to that deliveredthrough the standard applicator/insert at dmax and at the same energy. In general, theconvenience of using the same, standard value of dmax for all applicators at a givenenergy does not introduce significant error. However, at very small applicator sizes, thedepth of dmax can differ significantly from that of the standard applicator, so that thetrue value of dmax may have to be used instead for such applicators.

Since the applicator factor depends upon two parameters, the particular applicator/insert and the electron beam energy, it is common practice to construct a table with theenergy and applicator/insert as two axes of a table. Great care should be taken to clarifywhether an applicator factor has been defined as the ratio of doses delivered per monitorunit or the ratio of monitor units required to deliver a given dose.

Since measuring and checking applicator factors simply involves determining theratio of doses under applicators at dmax at the same energy, it is sufficient to take theratio of uncorrected readings if an ionisation chamber is used for the check. A selectionof applicator factors should be checked annually. It is not necessary to check allcombinations of applicator, insert and energy – on a machine with six available electronenergies and 30 inserts, this would mean checking 180 factors; it is sufficient to checkonly a representative few. Checking should be done at a gantry angle of 0°.

5.3.2.6 Applicator interlocks

A check should be made to verify that, if applicator interlocks are provided on thetreatment unit, then radiation can only be delivered when the correct applicator code hasbeen entered on the treatment console. This should be done monthly, taking a differentapplicator each month, and cycling through the available applicators.


The position of the diaphragms at different energies and applicator sizes will affectthe uniformity and output of the electron field. These positions should therefore bechecked on a regular basis, at least three-monthly. In modern accelerators it is commonpractice for the diaphragms to be set according to the applicator and energy automatically,based on detecting which applicator is attached to the machine. If the detection circuit ismaladjusted this can result in a significant error in the diaphragm settings with aconsequent significant error in the delivered dose. In some designs this would be detectedby the verification system and treatment would be prevented. However, in others theerror would go unnoticed and in such cases it is important that more frequent checks ofdiaphragm setting are carried out. One way of doing this is by using a different applicatorfor the daily constancy checks as described in Section 5.3.2.2.

5.4 Machine running conditions and their effect onperformance

5.4.1 Performance monitoring

In a linear accelerator which was originally installed correctly a useful rule of thumb isthat each variable will give a peaked dose rate when optimally adjusted. Any deviationfrom the original setting could therefore be expected to result in a drop in dose rate tosome extent, as well as any more specific changes in the beam.

The main functional blocks of a linear accelerator are electron gun, RF power source,beam control (input steering, focusing, output steering) and beam bending system.

5.4.1.1 Electron gun

Electron gun variations can alter the number of electrons injected into the acceleratorand the position of their effective source. For example, an increase in filament temperaturewill lead to increased numbers of electrons. For a given waveguide power, each of theseelectrons will have a smaller amount of the available energy, resulting overall in lowerbeam energy. A change in the position of the effective source of electrons can lead totheir trajectory deviating from the centreline. The subsequent changes in flatness andsymmetry will depend on the type of beam bending system fitted, if any.

5.4.1.2 RF power source

RF power source changes are in power output, frequency and spectrum. All theseparameters alter the energy absorbed by the electrons and hence flatness and symmetry(see beam bending). The spectrum may change as the power source (magnetron, klystronor RF driver) ages or when a new one is installed. Incorrect tuning (AFC) of the powersource is a common cause of flatness problems.

In the case of magnetron machines where the gun is pulsed with a proportion of thesame pulse fed to the magnetron then a reduction in the magnitude of this pulse iscompensated for by a reduction in the number of electrons injected. The resultant dropin the electron beam energy is therefore less than expected because of the smaller numberof electrons available to share the energy.


5.4.2 Beam control

Beam control may not be required in a machine with a short accelerating waveguide.

5.4.2.1 Input steering

Input steering is intended to set the electron beam into the centre of the accelerator.Errors can lead to symmetry problems, but as the electrons tend to follow a spiral paththe plane of symmetry change is different from that of the control change, i.e. a changein the transverse control current may alter the symmetry in a diagonal or even radialplane.

5.4.2.2 Focus

Focus solenoids retain the electron beam in the centre and hold the beam together againstmutual repulsion. Variations may make the focal spot shape and size change causingchanges in dose rate. Severe errors in input steering or focusing can also cause the beamto clip the accelerator structure, leading to increases in leakage and vacuum degradation.

5.4.2.3 Output steering

Output steering aligns the beam with the centre of the beam bending system and hencethe target. It has a strong effect on symmetry and flatness and can also cause changes inthe position of the output beam. The output steering is usually servoed by a feedbacksystem monitoring the beam symmetry. Failures in the servos can of course force changesin the steering currents applied.

5.4.3 Beam bending system

Beam bending systems are not fitted on all designs of linear accelerator. Those whichhave them can have one of several different types, each of which have their owncharacteristics. The energy sensitivity of a 90° bend causes the position and angle of thebeam striking the target to change. An energy change therefore causes a large change insymmetry and flatness with some interaction with the radial output steering control. The‘slalom’ and 270° bending systems are designed to give little or no change in position orangle of an electron beam of varying energy. There will however be marked changes inthe dose rate and the flatness.

Variations in flatness or symmetry are caused by changes in the relationship betweenthe shape and position of the raw beam profile and the shape and position of the flatteningfilter profile.

5.4.4 Recording parameters

The running values of important parameters can be recorded regularly at run-up. Normalvalues can be established over a period of time, preferably starting when the equipmentwas accepted. This collection of data will also build up a picture of the stability andreliability of the machine.


5.4.4.1 Effects of changes in important parameters

Table 5.4 shows the expected effect of changes in machine parameters. Most will affectthe dose rate as the machine would normally be running at a peak in photon mode.

Table 5.4. Effects of changes in important machine running parameters.

Parameter Causes changes in:

Electron gun Energy; flatness; symmetry (depending on beam bend type)Lower energy = bigger ‘ears’ (effect of flattening filter)

RF power Energy; flatness; symmetry (depending on beam bend type)magnetron, klystron/RF driver

Input beam steering Symmetry in both axes; leakage; vacuum(1R, 1T)[buncher radial, buncher transverse]

Focus Electron spot size/shape; leakage; vacuum

Output beam steering Flatness; symmetry; vacuum; beam position with respect to centre(2R, 2T) axis and light field[radial position, transverse position]

Beam bending system Energy; flatness

Parameter names as used by Elekta are shown in round brackets () and the equivalent Varian name in squarebrackets [].

5.5. Special facilities

5.5.1 Asymmetric fields

Most modern linear accelerators have independently movable jaws. When opposing jawsare positioned at different distances from the axis of rotation of the collimator system,an asymmetric beam is produced. Asymmetric beams have some useful properties, butdo introduce some geometric and dosimetric complications, and additional tests arerequired with the machine in asymmetric mode.

There are two main clinical uses for asymmetric beams:

1. Matching adjacent treatment volumes.2. Modification to a treatment volume while maintaining the original centre.

To match adjacent treatment volumes the geometric properties of asymmetric beamsmay be used. The angle of divergence of the edge of a beam is proportional to the distanceof that edge from the axis. A field edge with zero divergence is produced by positioningone of the collimators at the axis. Adjacent irradiated volumes can be prevented fromdiverging into each other by matching non-divergent beam edges. This technique has theadditional advantage that a single isocentre is used to treat the adjacent areas, thusavoiding moving the patient between fields. It is important that the jaw positions be


accurately calibrated at the zero position, and that the isocentre be confined to a smallsphere, to avoid any overlap or gap between the irradiated volumes.

Khan et al (1986) showed that the dose at the centre of an asymmetric field can becalculated by applying an off-axis factor (OAF) to the symmetric field of the samedimensions. The off-axis factor is simply the off centre ratio (OCR) for a large field atthe off-axis distance and depth of interest. All other factors are for the symmetric field.Depths are measured parallel to the axis and not along ray lines.

Khan’s results or any other calculation schemes intended for use with asymmetricfields should be checked for the individual accelerator. Wittgren et al (1993) reportedthat the monitor chamber of some accelerators is sensitive to asymmetric back scatterand in these cases predictions based solely upon symmetric field measurements areunreliable.

5.5.1.1 Acceptance and commissioning

Additional tests are required with the jaws in asymmetric mode. Any tests for jaw positionshould be extended to include the full range of positions for each jaw and the zero positionin particular. The maximum allowable position of each independent jaw with each wedgein place must be checked. If a verification system is used, then its ability to correctlyrecord or set asymmetric fields must be checked.

Additional isodose, profile and point measurements must be made to verify theplanning system output for asymmetric fields. These measurements and calculations mustinclude wedged asymmetric fields.

A system of notation must also be decided so that asymmetric fields may be definedin an unambiguous way. The easiest way to do this is to ensure that the treatmentparameters include the position of each jaw and the collimator rotation.

5.5.1.2 Quality control

Additional tests to be performed as part of the monthly QC programme:

1. Light field positionThis test may be used as a replacement for the standard test. Instead of checking thelight field size in symmetric mode each jaw position may be checked independently.This test allows the full range of positions for each jaw to be checked.

The field should be set to maximum and then each jaw in turn moved progressivelyto its minimum position and out again, checking the indicated position when the lightfield edge reaches predefined positions on a template. Care must be taken not toovershoot positions as they must be approached from the correct direction to checkfor any ‘slop’ in the position indicator. Note that the minimum position will be anegative distance from the axis and may not be the same for each set of independentjaws.

2. Junction homogeneity filmsThe jaw position at zero for half-blocked fields is particularly important since anyposition errors along a junction can lead to very large dose heterogeneity. To checkfor any displacement between adjacent half-blocked fields, a film is exposed twice,with opposite jaws set to zero.

If the 80–20% line is approximately straight then the amount of misalignmentmay be estimated from the maximum or minimum dose at the junction, and the 80–20% width.


Let the penumbra width 80–20% = P mmLet the region of overlap = Q mmLet the junction percentage dose = J %Then the junction dose from each field is:

Giving:

For a fairly typical 80–20% width of 6 mm, this corresponds to approximately 10 percent per millimetre misalignment. The tolerance limit should be ± 2 mm, correspondingto a dose at the junction of between 80 and 120 per cent of the field centre dose.

The X and Y jaws may be checked separately, with the other pair of jaws insymmetric mode, or simultaneously, with adjacent jaws set to zero. In either case,four exposures are required, and these may be fitted onto a single film.

Occasionally the adjacent fields should be exposed from opposing gantry angles.In this case the overlap or gap is a measure of the combined errors in the jaw positionsand the movement of the collimator axis.

A similar junction check may be made at positions other than the axis by exposinga number of thin ‘strip’ fields, with the jaws positioned for each strip such that adjacentstrips abut each other.

3. Light-to-radiation field coincidenceThis test may be performed less frequently than the others, although it should beconsidered if either of the other tests show poor results, or following adjustmentswhich may affect the light to radiation field coincidence.

The standard test should be extended to include asymmetric fields.To check each jaw at the zero position, four 10 × 10 cm fields with each jaw in

turn at the central axis are set on a single film. Each field is positioned on a clear partof the film, and the light field edges marked. An exposure is given and the processedfilm analysed with a suitable densitometer. Each field size should be as indicatedwithin ±2 mm, and each edge should coincide with the light field to within ±2 mm.

If jaw positions beyond the central axis are to be used, then the test should berepeated with each jaw at its minimum position.

5.5.2 Swept beam electrons

As electron energies become higher, the coefficient for energy loss due to bremsstrahlungproduction increases. Thus, use of a scattering foil system increases X-ray contaminationof the beam. Collision interactions in the foil broaden the electron energy spectrumresulting in a shallower fall off to the depth dose curve. To alleviate these problemssome linear accelerators provide a ‘swept’ or ‘scanned’ electron beam facility, especiallyat high energies. In swept beam electron mode no scattering foils are used and the electronsexit the linear accelerator head in a narrow Gaussian profiled beam. In order to achievefield uniformity for clinical beams this narrow beam is swept in a zig-zag raster patternover the full aperture of the primary collimators. The therapeutic advantages of the cleaner

Q = (J-100) × P

60

( )J

2

Q

2× 60

P= 50 +


high energy beam achieved in this manner are accompanied by some dosimetricdifficulties which are outlined here and addressed in detail in the references.

5.5.2.1 Commissioning

The commissioning measurements required are the same as for scattered electron modesexcept for a measurement of the half-width of the Gaussian beam (see below).

There are two dosimetric problems associated with swept electron beams. Firstlybecause of the high intensity per pulse of an unscattered electron beam, the collectionefficiency of an ionisation chamber diverges significantly from unity when the centralaxis of the Gaussian beam is near the chamber. The collection efficiency also varies withtime because the beam is moving around the field, exposing the chamber to differentintensity pulses depending on the relative positions of chamber and Gaussian beam. Thesecond problem is the non-uniformity of the field over short intervals of time, i.e. ahomogeneous field is only developed over an extended time period.

The problem of collection efficiency is mainly a problem connected with absolutedosimetry and the derivation of an effective calibration factor for a field instrument. Thenon-uniformity problem is one of how to obtain representative profiles and depthionisation data without integrating readings for an inordinate length of time at eachmeasuring point.

5.5.2.2 Collection efficiency

A detailed account of the derivation of collection efficiency of an ionisation chamber ina swept electron beam is given by Boag (1982) and experimental measurements byMajenka et al (1982). These references are important reading for anyone needing tomeasure collection efficiency under these circumstances. A brief description of a practicalmethod for determining collection efficiency is given below:

1. First it is necessary to find the width of the Gaussian beam.Turn off the scanning magnets. Place a film at the depth of interest and irradiate. Ifthe film is at d

max, three-monitor units should be sufficient for Kodak X-Omat V film.

Measure the half-width, a, of the Gaussian beam, i.e. the distance from the centralaxis of the beam to the point where the intensity is e-1 times maximum intensity.Calculate k, where:k = (equivalent radius of the rectangular field swept out by the beam axis at the

treatment distance)/a.(It should be noted that the mathematical treatment requires the ‘field swept out’, i.e.the total field without any collimation rather than the field defined by the collimators,in the definition of k). If k≥2, which should be true unless the field swept out or theenergy is small, then the calculations below may be used in their simpler approximatedform, and Boag (1982, Fig. 2), which uses this approximation, may be used.

2. Place the ionisation chamber at the centre of the field at the measuring depth. Turn onthe scanning magnets. Measure the ratio of charges collected (Q

1/Q

2) at polarising

voltages V1 and V

2 , where V

1 is the normal operating voltage of the electrometer. The

comments given by Conere and Boag (1984) should be noted when choosing V1 and

V2 values. The number of monitor units chosen should be large enough to give

reproducible readings (see Section 5.5.2.3).3. If k≥2, then the collection efficiency at voltage V

1 (symbolised as φ(ζ

1)) can be read

from Boag (1982, Fig. 2). Alternatively the following equations may be used, which


can be solved iteratively to find the unknown quantity ζ1 and hence φ(ζ

1). (ζ is a

function of the maximum charge density seen by the chamber, the effective electrodespacing, the polarising voltage and a physical constant.) These equations are takenfrom Boag (1982) and use the same notation.

where:

A good approximation when k ≥ 2, is:

The following two expressions are also needed:

4. For the normal operating voltage V1, a number of V

2 values should be chosen to find

the consistency range of the collection measurements (Majenka et al (1982).

5.5.2.3 Field non-uniformity

In order to achieve reproducible readings of a point dose in a scanned field it wouldnormally be necessary to measure each point for approximately one minute to bring thereproducibility to within 1 per cent. However, it is possible to reduce the integrationtime per point significantly by triggering charge integration measurements on the startand end of the sweep cycles. It should be pointed out that only a few manufacturers ofbeam data acquisition systems offer this facility. The pattern swept out by the electronbeam will repeat after a number n of complete sweep cycles given by the expression:

where fy/fx = ratio of the y and x sweep frequencies (fy > fx) (Ertan et al (1984).Thus, it will only be necessary to integrate for a number of complete sweep cycles

equal to m × n where m is an integer. Typically n = 5 and fx = 0.615 Hz. Thus the integrationtime per point should be a multiple, m, of 8.1 seconds. This contrasts well with integrationtimes of one minute for non-triggered charge collection.

φ (ζ) = 1ζ

ψ (ζ)

∑ ∑(ψ (ζ) =∞

n=1

1n

Vnn

i=1

1i )

V =ζ

1 + ζ

Q1

Q2=

φ (ζ1,k)

( V1

V2

φ ζ1

,k )

( )n = 1 +1

fy

2fx– 3

φ (ζ,k) = ψ (ζ) – ψ (ζ e )

ζ (1 – e )

–k2

–k2


The value m will have to be determined experimentally. Although the pattern sweptby the beam repeats after n cycles, the individual Gaussian pulses on this pattern willnot in general recur at the same positions. This only becomes important at higher energieswhere the Gaussian half-width is small. Profiles and percentage depth ionisation curveswhich are fairly smooth at, for example, 15 MeV, for m = 1 might look jagged unless mis increased to 2 for higher energies.

5.5.2.4 Quality control

The normal quality control tests will be required. However, the non-uniformity problemwill imply large measuring times unless special measuring techniques are used. For energymeasurements and field flatness/symmetry, the simplest technique is to use filmdensitometry. Irradiation times will have to be experimentally determined to be longenough to achieve field uniformity. Because this will result in a dark film the characteristiccurve of the film should be known and utilised during densitometry to relate opticaldensity to absorbed dose.

A possible procedure for routine energy measurement is to measure the chargecollection ratio with a probe at two fixed depths under standard conditions, which can berelated back to the same measurement performed during commissioning.

For flatness and symmetry across the whole field a diode detector array assembly thatintegrates readings triggered from the sweep cycle is a useful tool for on-line adjustments,although obviously more expensive.

5.5.3 Computer control and verification

Computer controlled linear accelerators will reduce the possibility of random set-uperrors, but there remains the possibility that an error will go unnoticed and will be repeatedmany times. Careful checking of data entered is vital and staff should be encouraged toremain alert to the possibility of error.

Computer hardware errors are likely to have catastrophic effects so that they areunlikely to go unnoticed, but it is wise to implement some sort of simple checkingprocedure to ensure that basic functions are operating as they should. The followingsections describe tests that can be carried out on a daily and weekly basis to address themost likely problems. These tests are not intended to be prescriptive.

Most computer controlled accelerators have a service mode and a clinical mode. It iswise to carry out at least some of the test dosimetry measurements in the clinical modeto ensure that any optional settings in service mode have not affected the output. It isparticularly easy to forget to save changes to the database before leaving service modeso that on returning to clinical mode changes made are forgotten.

5.5.3.1 Daily checks before clinical use

A test patient should be stored in the database of every machine. This patient is called upas the last item in the morning preparation and the following tests are performed andrecorded on the appropriate forms in the quality control documentation. The test patienthas two beams: an open field and a wedged field.

1. The correct prescription is loaded – checked by examination.2. Interaction with the control software is correct – checked by examination.


3. The auto set-up function is tested on the first beam – by comparison with the expectedset-up.

4. For accelerators with software wedges, the loading of the correct software codes forthe software wedge is checked. This is confirmed by checking that the beam deliverytime is correct (within an agreed tolerance) when the beam is run.

5. The auto acquire function is tested on the second beam. This should be done by slightlyvarying the machine settings – gantry, collimator, couch vertical, couch longitudinaland lateral; auto acquiring; then comparing the verification display with the machinedigital display.

6. After both beams have been delivered the verification printout is examined to ensurethat the date is correct and that the beams are recorded correctly including the fractionnumber (and elapsed day count if appropriate) and the accumulated doses.

The test patient record in the machine database is checked weekly by a physicist.For a machine with a multileaf collimator (MLC), an additional MLC field should be

run for the morning test patient. This ensures that the MLC parameters are downloadedfrom the network correctly. Ideally the test MLC outline should be for a rectangularfield employing all MLC leaves. This permits the accuracy of leaf position to be quicklyassessed by inspection.

The essential principle is that the daily tests are designed to test that the record andverify system does what it should do, not to test that it does not do what it should not do.This aspect is tested on a weekly basis.

5.5.3.2 Weekly checks

Using a different test patient (programmed for once a week treatments) the followingtests are performed and recorded on the appropriate forms in the quality controldocumentation. The test patient again has an open field and a wedged field.

1. Auto set-up is used, followed by a change in a parameter. An attempt is made to treatthe beam. This should fail and the failure to switch on is recorded.

2. This is repeated with a different parameter. Appropriate parameters are gantry angle,collimator angle, one of the field diaphragms and the couch vertical. For only one ofthese parameters an override is actioned. It is then checked that this override is correctlyrecorded (and that it is cleared for the next beam).

3. Check that interrupted beams are recorded correctly. To do this, for example for asoftware wedged beam, the beam is interrupted during the beam. This should berecorded as a partial treatment in the verification system with the correct number ofmonitor units. (This is a very important test, as it is an operation that seldom occursin practice and problems may not be spotted until a real and often confusing situationoccurs.)

5.5.4 Dynamic wedges

When using the dynamic wedge facility, the wedged beam shape is produced by thecomputer controlled movement of one Y-collimator jaw across the radiation field. Thejaw movement produces a series of fields which progressively decrease in width. Themonitor units given per field segment are automatically adjusted to give the wedge shapedisodose distribution.


As the use of dynamic wedge fields is a relatively new treatment modality there isparticular need for quality control measurements to verify the dynamic wedge treatmentdelivery. The following is a summary of various quality control checks and a suggestedfrequency for their performance. Familiarity with the dynamic wedge and its performancewill eventually mean there can be a reduction in the frequency of the quality controlchecks performed on a routine basis.

5.5.4.1 Daily checks

1. Run-upThere is a unique instruction table for each square field size (in 5 mm increments)which specifies the delivered dose as a function of the moving jaw position. Thesetables are called segmented treatment tables (STT). It is desirable to include twoSTTs (for each energy) for specified dynamic wedges during the running up (‘morningcheckout’) procedure. One of these should be for the orientation Y1 and the other forthe orientation Y2. The computer stores a ‘morning checkout log’ which contains atable which enables comparison of dose delivered and jaw position with planned values.Standard deviations of the monitor units and jaw positions can be recorded. (Differentwedge angles and field sizes may be used in rotation.)

2. CalibrationThe effective wedge factor for a typical dynamic wedged field should be determined,using a calibration phantom and ionisation chamber or a beam constancy meter. Adifferent wedge angle should be checked each day. The value obtained should bewithin 2 per cent of the standard. This can either be done using the run-up beam orseparately.

The function of the dynamic wedge selection interlock should be checked, inrotation.

3. Procedure for patient treatmentBefore the patient is treated for the first time (and without the patient present), all theproposed treatment fields should be checked through to ensure the correct orientationof the collimator and dynamic wedge on each field. The use of Record and VerifySystems is very helpful as once the information has been entered correctly and verifiedit is there for the duration of the treatment.

For each field the monitor units to be given before the Y collimator movementcommences should be calculated and the value recorded on the treatment sheet. Thefinal collimator position should be recorded after each field has been treated. Theseparameters can then be checked during each treatment.

5.5.4.2 Weekly checks

1. ORNT interlockThe function of the orientation of dynamic wedge (ORNT) interlock should be checkedby trying to ‘confirm’ with an incorrect orientation selected.

2. DPSN interlockThe function of the interlock which checks that the intended dose and position andthe actual dose and position are in agreement during treatment (the DPSN interlock)should be checked.

3. IPSN interlockThe function of the interlock that checks that the Y collimators are within ±1 mm ofthe start position (the IPSN interlock) should be checked.


5.5.4.3 Monthly checks

1. Interrupted dynamic wedge exposuresAn interrupted wedge exposure should be given for the same dynamic wedge fieldsize as used for the wedge factor check. The wedge factors should be compared andshould agree to better than 2 per cent.

An ‘edge’ film should be exposed to an interrupted dynamic field and comparedwith a standard dynamic wedge field.

2. Dynamic wedge factor variation with gantry angleThe effective dynamic wedge factor for all wedges at each angle in monthly rotationshould be measured and examined to ensure that gravitational effects are insignificant.

3. Dynamic wedge profilesIf the necessary equipment is available, it is useful to measure dynamic wedge profilesat gantry 0° at 50 mm deep for all wedges for a chosen standard field size. These canthen be compared with previously scanned profiles.

4. Reverse compensator testSome form of ‘reverse compensator’ (a wedge, designed to compensate for the shapeof a dynamic wedged field) can be constructed. This is then fixed to the treatmenthead and used to check wedge profiles at different gantry angles.

5.5.4.4 Six-monthly checks

An ionisation chamber at 50 mm deep in an IPSM phantom (Chapter 2, Figure 2.1)should be treated using a computed four field dynamic wedge plan (with all fieldsdifferent). The monitor units are calculated from the plan to give 2 Gy to the isocentre.

5.5.4.5 Software security

To help ensure the integrity of the STT tables these files should be given the ‘read only’attribute in DOS. In addition a simple batch program can be written to check that the filelengths have not changed.

5.5.4.6 Enhanced dynamic wedge

The enhanced dynamic wedge allows the use of asymmetric fields and provides a widerrange of wedge angles. Similar quality control procedures are appropriate, but shouldinclude at least an asymmetric wedge field.

5.5.5 Linear accelerator monitor ion chambers

The ionisation chambers used to monitor the dose delivered can be usefully categorisedas sealed or unsealed and thick walled or thin walled. The problems that have beenencountered can thus be divided into four groups, together with those common to alltypes.

Thick walled sealed ion chambers, by virtue of the thick walls, cannot normally beused with accelerators producing electron beams. (However, in the SL75/14 the walls ofthe ionisation chamber are used as part of the scattering foil system.) When only photonbeams are being monitored, they have the advantage of being unaffected by variations inthe temperature, pressure and humidity of the atmosphere. It has, however, been foundthat any oxygen in the gas filling the chamber is depleted over a period of time; presumablyby chemical combination with the materials of which the chamber is made, assisted by


the polarising voltages and the presence of ionised atoms. It is therefore necessary toensure that, as far as possible, the chamber is filled with pure nitrogen. The reduction inoutput signal because of the fall in internal pressure is thus avoided.

Thick walled unsealed ion chambers have no particular advantages. However, a sealedionisation chamber may develop a leak so that it is effectively unsealed. It would thenhave generally similar characteristics to a thin walled unsealed chamber but will nottransmit electron beams.

Thin walled unsealed ion chambers can be used to monitor both photon and electronbeams. As variations in both the atmospheric pressure and the ion chamber temperaturewill affect the density of the gas filling the chamber, and hence the output signal, somemeans of making corrections for these variations must be provided. If this is doneautomatically, by having a thermistor attached to the ion chamber and a pressure sensorin the accelerator control, the stability of readings can be very good. This approach isparticularly suited to computer controlled accelerators. It is necessary to ensure that theinsulating properties of the insulators supporting the internal structures of the ion chamberare not affected by humidity that might penetrate. It should be borne in mind that changesto the calibration of the pressure or temperature sensors will affect the calibration of theion chamber.

Thin walled sealed ion chambers are not affected by humidity variations but flexingof the thin walls allows variations in the density of the gas filling. It has been observedthat atmospheric pressure variations can be transmitted with virtually no reduction inamplitude. It is therefore necessary either to correct the output in a similar fashion tothat for an unsealed chamber or to overcome the problem in some other way. Onemanufacturer does this by applying an excess pressure to the internal gas. The walls thenflex to their limit and little or no further change in dimensions (and hence internal gasdensity) occurs as atmospheric pressure fluctuates. The obvious danger in this approachis the possibility of a leak causing a large drop in output signal. Should this occursimultaneously to both channels of the dual dosimetry system then the change would notbe detected by the dose difference circuitry. As with thick walled sealed chambers it isnecessary to use a chemically inert gas filling such as nitrogen.

With any type of ion chamber it is possible for the electrodes to cause problems. Thisis particularly true with the more sophisticated systems where beam distribution ismonitored as well as total dose delivered. In the simple designs it is likely that theelectrodes will be made of metal foil and the only problem arises when the connectinglead becomes detached. This causes a total loss of output signal and is instantly observable.Where different areas of the beam are monitored independently the collecting electrodes,from which the output signal is obtained, are commonly made of mylar film on which aconducting metal surface is deposited, with narrow insulating lines delineating the areasof interest. In this case not only can the leads become detached, with total loss of signalfrom an area, but it is also possible for a small area of the metallising to become isolatedby a hairline crack from the rest of the electrode. In this case only a small reduction insignal output may occur but it can seriously disturb any beam steering servo system andmay, therefore disproportionately affect dosimetry.

The result of an ion chamber becoming subject to uncompensated fluctuations in itsinternal gas density, either due to a leak developing or to a failure in a compensationcircuit, is that a variation in delivered dose per monitor unit will occur. As ion chambertemperatures tend to be governed by the accelerator running temperature rather thanroom temperature, it is the variation with atmospheric pressure which is most noticeable.


It is therefore beneficial with all types of ionisation chamber to monitor the variation ofthe output calibration with atmospheric pressure which, if the chamber is effectivelyunsealed, will show an inverse correlation.

If water vapour penetrating the ion chamber has an adverse effect on insulation thereis likely to be a noticeable leakage current, probably appearing as a dose rate reading inthe absence of any radiation. It is also possible for the leakage current to cause a reductionin polarising voltage. In the latter case there may be a reduced collection efficiencyresulting in an increase in dose per monitor unit.

The ‘islanding’ of areas of an electrode can cause very unexpected effects. If a beamsteering servo loop is affected beam symmetry will be distorted, and if switching off theservoing restores normal symmetry, then islanding must be suspected. More confusingly,charge build-up on the isolated area is liable to spark across to the surrounding electrodearea at regular intervals during an exposure. These intervals may be quite short and cantotally confuse the dosimetry system. The same confusion, but on an even larger scale,can occur if a signal lead becomes electrically detached but remains in close proximityto its electrode. Observation of signal levels, at appropriate points in the dosimetrycircuits, with an oscilloscope should allow diagnosis of the cause of the problem. Curewill almost certainly require replacement of the ion chamber.

Ion chambers can be constructed to monitor different parts of the beam. As pointedout by Sutherland (1969) it is more sensible to monitor the central part of the beam sinceotherwise undue weight will be given to the beam periphery and the monitor will beunduly sensitive to changes in beam flatness. In many accelerators the two channels donot monitor the same part of the beam so that a dose channel difference may be anindication of a change in beam profile.

5.5.6 Multileaf collimators

Multileaf collimators (MLCs) are rapidly becoming standard equipment in the UK and itis therefore important to include guidelines on their quality control. However, thetechnology is developing rapidly and it is therefore inappropriate to offer definitiveguidance. The majority of the MLCs in the UK are supplied by Varian Oncology Systemsor Elekta Oncology Systems (formerly Philips). The MLCs provided by these twomanufacturers differ in that the Varian MLC is an add-on to the standard collimatorwhereas the Elekta MLC is fully integrated into the design of the head so that even forstandard fields the MLC is used to shape the field. For the purposes of theserecommendations the Elekta design will be taken as representative of integrated MLCsand the Varian design as representative of add-on systems. Before considering the qualitycontrol aspects it is necessary to include a brief summary of the design features of thecollimators that are necessary to an understanding of what follows. Further specificrecommendations can be found in Klein et al (1995, 1996), Mubata et al (1997) andHounsell and Jordan (1997).

5.5.6.1 Quality control challenges of multileaf collimators

The quality control of multileaf collimators is in two parts: the leaf control systems andthe systems for defining the leaf positions for individual patients. In considering thequality assurance requirements for the leaf control system a number of factors must beborne in mind.


1. The leaves of the collimator must go accurately to the required position. This hasthree aspects:(a) The independent movement of the individual leaves;(b) The movement of the leaves considered as a whole; and(c) The relationship between the leaves and the backup collimators.

2. The leaves of the multileaf collimator must be accurately parallel to the secondarycollimator jaws.

3. Some leakage between the leaves is inevitable. As the collimator wears this leakagemight increase.

4. The weight of the collimator may cause the alignment of the leaves to be different atdifferent gantry angles.

5. The optical field indication may be incorrect. The relationship between the opticalfield and the radiation field may vary with off-axis distance.

The relative importance of these problems will depend on the design of the collimator.Field shapes for the multileaf collimator may be designed and transferred to the

treatment machine in a number of ways:

1. The field may be designed using a BEV (beam’s-eye-view) system on a planningcomputer and then transferred to the treatment machine manually, on a floppy disk orover a network.

2. The field may be based on simulator X-ray films or BEV printouts which are digitisedon a ‘shaper station’.

3. The field may be derived from a series of standard shapes.The approach to quality control will depend on which of these routes is used. As wellas ensuring that the field shape is accurately related to its intended shape there is alsothe need to ensure that the field shape is assigned to the correct patient and to thecorrect beam.

During the early days of MLCs there was a very basic link between the computercontrolling the MLC and the rest of the accelerator. There were therefore potentialdifficulties with ensuring that the MLC was correctly positioned for electron treatments.There were also problems with interfacing the multileaf collimator to the record andverify system of the computer which allowed the possibility of the gantry angles, etc. forone patient being used with the MLC field of another. As better integration is introducedby the manufacturers it should be possible to relax some of the published quality controlprocedures designed to counteract such deficiencies. One of the principle benefits of themultileaf collimator is the potential for reducing the time spent by the patient on thetreatment couch. Those responsible for devising the quality assurance protocol for anindividual hospital will need to balance the need for quality control procedures with thegoal of reducing the time taken to treat a patient.

5.5.6.2 Design of MLC – Elekta design

The Elekta design was described in detail by Jordan and Williams (1994). The jawswhich move parallel to the MLC leaves are of insufficient thickness to attenuate thebeam fully and must therefore be used in conjunction with the MLC leaves to define theedge of a rectangular field. This creates a complication in setting up the relative positionsbecause the apparent position of the leaf edges will be different by a small amount whenthe back-up collimator is present to supplement the leaves and when it is not. Leaf


positions are monitored by a camera mounted above the collimator system which looksat reflectors on the top of the leaves. With this design the leaf positions relative to eachother are likely to be quite stable, but it is possible for all the leaves to become misalignedtogether. To test the leaves and the secondary collimator jaws independently can bedifficult.

5.5.6.3 Design of MLC – Varian design

The Varian design was described in detail by Galvin et al (1992). The collimator isbolted onto the head beneath the standard collimator in the position normally occupiedby the lead tray. When field shaping is not required the leaves may be fully withdrawnfrom the field and leaf positions can be set independently of the secondary collimatorjaws. It is therefore easy to test the secondary collimator jaws independently of the MLC.Confirmation of leaf positions is derived from individual position potentiometers. Theprecision of these may be greater than that of the optical system but it is inherentlyeasier for individual leaves to become misaligned. A rotational misalignment betweenthe secondary collimator jaws and the MLC is also more likely.

5.5.6.4 Definition of MLC fields

It is essential that checks are carried out on every MLC field shape that is created. Apartfrom checking the overall integrity of the design process there is also the need to verifythat:• the data used are for the correct patient;• the field shape is correct; and• the collimator rotation of the shaped field is the same as that used for treatment.

This can be simply done by using a printed template which can be compared with theoptical field on the treatment machine and the simulator film. For these purposes it maybe appropriate to produce templates at different magnifications.

Most field design systems that use a digitiser have some means of calibrating thedigitiser before the field shape is input. If this is not provided as standard a system ofdaily checking of the digitiser would need to be implemented.

Tests carried out on individual patient’s data are not usually intended to be a test ofthe accuracy of the system and for this reason it is appropriate to carry out specificquality control of the design process and data transfer. Routine testing should be morefrequent for digitiser based systems, but verification of computer algorithms should alsobe carried out at regular intervals.

A number of different approaches have been adopted to the positioning of the leavesin relation to the target outline (Zhu et al 1992, Heisig et al 1994, Jordan and Williams1994, Du et al 1995). Each of the algorithms available should be checked for correctoperation. This can be conveniently carried out using a field in which one of the edges isat 45° to the edge of the unblocked field (Figure 5.9).

A simple field shape as shown in Figure 5.9 is convenient for testing the shape designsystem. This has a number of features.

A – a section of the field edge where the angle to the MLC direction of travel is quitesteep so that the alignment algorithm used can be clearly identified.

B – a section of the field edge with the jaw perpendicular to the MLC direction is alignedwith the middle of an MLC leaf.


C – a section where there is a shallow angle to the MLC direction of travel.D – a section of the field edge which is exactly perpendicular to the MLC direction of

travel.E – a section of the field edge where the jaw perpendicular to the MLC direction is

exactly aligned with the edge of an MLC leaf. This should result in the next widerMLC leaf being withdrawn so that the edge of the leaf (which is shaped to helpreduce leakage between the leaves) is not used to define the field edge.

For the Varian MLC the design of the test field could be modified to test whether, if theprotrusion of the MLC leaf into the field is sufficiently great, the trailing edge of the leafis left unshielded. This test should be carried out monthly for digitiser based systems.An appropriate test profile should be designed for BEV based field design systems, buta lower frequency of testing may be appropriate (e.g. three-monthly).

5.5.6.5 Checks of beam transfer and daily set-up

For each field that is transferred via a network or by floppy disk a comparison must bemade between the field design and the field as transferred to the treatment machinebefore the patient is treated. This may be by comparison with a list of MLC leaf positionsor, more conveniently, using a template. Unless the computer systems are extremelywell integrated there will always remain the possibility that there will be errors ofcollimator or gantry rotation or even the wrong patient. With the introduction of fully3-D planning computers whose beam labelling is identical to that of the treatment unitsthe possibility of such errors will be minimised, but a check will always be advisable.

On subsequent days it is common practice to check one field on a rotating basis against

Figure 5.9. Possible test field for MLC field transfer testing.


the template. With the Elekta design the MLC monitor screen shows live video of thecollimator so a transparent template at the size of the screen image would be satisfactoryfor the daily check. The purpose of the initial check is to verify that the transfer processwas carried out correctly. The purpose of the daily check is to verify that the correctfield is being used and that no data corruption has occurred. It is unlikely that a changeof a few mm would occur so a gross check of the overall shape should be adequate witha careful check being carried out weekly. If portal imaging is available checks can becarried out with the imaging device.

5.5.6.6 Alignment of leaf bank to the collimator jaws

The rotation of the leaves with respect to the collimator jaws can be checked with a field(e.g. 260 mm × 260 mm) in which one leaf from each leaf bank protrudes well into thefield. The parallelism between this and the collimator edge is checked by measurementon a film (Jordan and Williams 1994, Mubata et al 1997).

5.5.6.7 Alignment of leaf positions

The approach to the alignment of the leaves will be governed to some extent by thedesign of the MLC.

For the Varian design each leaf is appropriately treated as a separate entity since eachpotentiometer could vary independently. With this design there is no need for thealignment of the main collimator jaws and the MLC leaves to be considered specificallysince full attenuation is achieved with the main collimator jaws on their own. Testingcan therefore be restricted to exposure of a long thin field.

For the integrated MLC the edges of a rectangular field are defined by a combinationof the MLC leaves and the backup collimator jaws. When the MLC leaves alone aredefining the field boundary the boundary position would therefore be approximately 0.5mm further from the field centre, unless the backup jaws are set back so that the beamedge defined by them alone is set to the 55 per cent point (Jordan and Williams 1994).The borders of MLC defined fields will then be in the expected position. The relationshipof the individual leaves to each other will be relatively stable so it is justifiable to userelatively small fields for frequent testing. A check of the individual leaf positions isrequired every six months or if the camera is changed.

5.5.6.8 Alignment of opposing leaves

Because of the uses to which MLCs are likely to be put, the relationship between thepositions of opposite leaf pairs also needs to be considered. This can be convenientlyachieved by a double exposure in which first one leaf bank and then the other is set tothe central axis. A misalignment of 0.5 mm will produce approximately 5 per cent variationin dose across the junction (see Section 5.5.1.2). Experience with MLCs suggests that atolerance of 1 mm is achievable.

5.5.6.9 Field alignment away from the central axis

Conventional collimators are designed so that the jaw face remains parallel to thedivergent beam edge for all positions of the collimator. This involves a rotation of thejaw face as it moves and this is not possible for MLC leaves which have to move linearly.For this reason the leaf ends are shaped as described by Jordan and Williams (1994).This means that potentially the accuracy of field edge positioning may deteriorate at


large off-axis distances. An occasional check of this should be carried out. This may bedone by exposing three parallel strips on one film using the MLC leaves alone. With theintegrated MLC it may be necessary to open one of the leaf pairs slightly more than therest in order that it is the leaves and not the backup jaws that are defining the field edges.A patchwork field along the lines described by Thompson et al (1995) could also beused.

5.5.6.10 Relationship between optical field and radiation field

MLCs allow a very wide range of field shapes and for routine checking the optical fieldis very useful. However, because of radiation penetration of the end of the leaves theoptical field is likely to be about 0.5 mm smaller than the radiation field. Since mostMLC fields are designed using some form of computerised input the optical field displayis somewhat less important than for a conventional collimator. The tests already describedfor conventional collimators are appropriate for this purpose (Sections 5.2.5, 5.2.6).

5.5.6.11 Stability of leaf positions with gantry angle

With a horizontal beam and with the MLC leaves vertical there is a possibility that theleaves will drop down. This is most conveniently checked with a star film (Mubata et al1997) using an approach similar to that described in Section 5.2.9.3, but with rotation ofthe collimator rather than the gantry. The film centre should be placed at the machineisocentre and exposures made with the MLC leaves vertical and collimator rotations of60° either side of this position. This facility is being withdrawn.

5.5.6.12 Library shapes

The Elekta collimator has a library of field shapes that can be set. Those that are usedroutinely should be checked occasionally to ensure that they are functioning as expected.The optical field is adequate for this purpose.

5.5.6.13 Interlocks

Depending on the design of the equipment, interlocks (either hardware or software) willexist to ensure the safety of the patient or of the equipment. An example of the formerwould be an interlock to prevent the use of an electron beam with the MLC leaves in thewrong position. An example of the latter would be the system to prevent collisions betweenleaves. Each system should be assessed and a system put in place to check all suchinterlocks at appropriate intervals. What is appropriate may vary between tests carriedout only at the commissioning of new software to daily checks.

5.5.6.14 Leakage between leaves

Some leakage between the leaves is inevitable and should not present a problem bearingin mind that low-melting-point alloy block thicknesses are often such as to allow 5 percent transmission. Two measurements should be made: the peak transmission betweenthe leaves and the average transmission. Average transmission less than 2 per cent andpeak transmission less than 5 per cent should be achievable (Jordan and Williams 1994).Measurements are best carried out with film. The film density should be corrected todose.


5.5.6.15 Check frequencies for multileaf collimators

A provisional set of check frequencies for multileaf collimator tests is given in Table5.5. In some cases different frequencies of testing are considered appropriate for thedifferent types of multileaf collimator.

Table 5.5. Summary of checks for multileaf collimators.


Each treatmentVerification of correct data 5.5.6.5

Each patient transferredVerification of each beam against template 5.5.6.5

MonthlyDigitiser checks (if used) 5.5.6.4Relationship between light field and radiation field 5.5.6.8Leaf positions for standard field 5.5.6.7

Three-monthlyStability with gantry rotation 5.5.6.11Leakage between leaves 5.5.6.14 5%Average leakage below leaves 5.5.6.14 2%Individual leaf settings (TV monitoring) 5.5.6.7Relationship between leaves and backup jaws 5.5.6.6Electron field interlocks 5.5.6.13

5.5.6.16 Dynamic multileaf collimator

Intensity modulated beams can be produced by changing the MLC leaf positions duringthe course of a beam rather in the manner of dynamic wedges (Section 5.5.4). Suchdevelopments present a particular challenge for quality control procedures.

There are two factors that need to be verified: the geometric accuracy of the jawpositioning and the relationship of movements to the dose delivered. In practice theformer is probably most easily verified with static jaw positions. If a non-uniform dosedistribution is being produced the dose delivered at a point in the field can be a sensitiveindicator of the accuracy of treatment delivery and is in any case the fundamentalconsideration.

Depending on the sort of fields being treated the relative alignment of opposing jawscan be critical and can be usefully tested by two consecutive exposures on a single pieceof film. With computer controlled jaw movements a complicated test pattern can bedelivered automatically – an example is given by Thompson et al (1995). For particularlycomplicated patient treatments the treatment field can be recalculated for a phantom ofthe approximate shape of the site being treated and a dummy run carried out to demonstratethat the dose delivered is as expected. Further information can be found in Chui et al(1996).


5.5.7 Stereotactic radiotherapy

5.5.7.1 Introduction

Stereotactic radiotherapy, including radiosurgery, is the non-invasive use of a linearaccelerator in the conformal treatment of small, usually intracranial, lesions. Steep dosegradients, created by supplementary collimation in conjunction with either multiple non-coplanar arcs or fixed beams, permit high absorbed doses to be delivered to the targetvolume while surrounding tissues are minimally exposed. Exceptionally high precisionand correspondingly effective quality control procedures are essential.

A stereotactic radiotherapy delivery system generally comprises a (relocatable) frame(which forms the baseline for the stereotactic coordinates and assists with patientimmobilisation), image reconstruction and alignment devices and tertiary beamcollimation fitted to a linear accelerator with above average spatial accuracy (bothmechanical and radiation). Although dedicated linear accelerators for stereotactictreatment can be purchased the majority of centres will modify a standard machine toperform stereotactic radiotherapy in parallel with conventional therapy.

The purpose of any quality control programme is to ensure that, with the passage oftime and use, the dose delivery package as a whole remains within pre-prescribed limitsthat are commensurate with the clinical aims. These limits and the ultimate precisionthat can be achieved in stereotactic radiotherapy must be kept in perspective. Identifiedinaccuracies in the planning and treatment chain include:

• relocatability of the stereotactic frame onto the patient;• geometrical precision and resolution of the imaging system and alignment devices

used to determine coordinates;• mechanical and radiation isocentre deviation;• treatment set-up isocentre positioning with respect to the machine isocentre; and• the ability to define clinically the often indistinct three-dimensional target volumes.

In practice the last of these is of the greatest significance.The tolerances necessary for stereotactic radiotherapy are in general tighter than those

applicable to conventional techniques and as discussed in Section 5.1.3, appropriate actionlevels will need to be defined. The routine quality control procedures previously discussedin this chapter will need to be expanded to encompass the additional components and thewider use of the machine’s capabilities such as gantry arcing. The appropriate checkfrequencies for stereotactic radiotherapy are shown in Table 5.6. Tests prior to eachtreatment session can be carried out by the radiographers, but the monthly checks shouldbe carried out by a physicist. Tests that are specific to stereotactic radiotherapy aredescribed below.

5.5.7.2 Relocation of the frame

The precision of relocation depends on the system adopted, on the skill of the operatorand on the individual patient. The aim is to achieve relocation in relation to the targetvolume – verification of which requires a repeat CT scan. However, by taking orthogonalradiographs it is possible to compare the relationship of the frame to bony landmarkssuch as the pituitary fossa. Measurements can also be made from a point on the ears andthis may be the most appropriate method for day to day use (Thomson et al 1990).


Table 5.6. Check frequencies for stereotactic radiotherapy.


Prior to each treatment sessionDisplacement of alignment lasers from meanmechanical isocentre ±0.5 mmAlignment of the optical light field centre withcollimator rotation1 mm diameterAlignment of tertiary collimator 5.5.7.3Verification of predetermined diaphragm setting 5.5.7.3 1 mm diameterVerification of relocation of the stereotactic frameon the patient 5.5.7.2 0.5 – 1 mm

MonthlyCoincidence of the mechanical isocentre and theradiation isocentre 5.5.7.4 1 mm diameterTertiary collimator alignment and movement withrespect to the beam axis with gantry rotation 5.5.7.3 1 mm diameterOperation and calibration of arc mode 5.2.14Couch isocentre rotation 5.2.4.2Mechanical checks 5.5.7.5, 5.5.7.6

Annually or following upgrades or modificationsCoordinate reconstruction 5.5.7.7Treatment planning system and beam data files 5.5.7.8

5.5.7.3 Stereotactic tertiary collimator

Provided that the optical system of the accelerator has been set up accurately theadjustment of the stereotactic collimator can be carried out with the aid of the light field.A radiation check can be carried out using a 20 mm diameter collimator. A film is firsttaken with the collimator at, say, 0° with a lead block covering as nearly half of the fieldas possible. The collimator is then rotated to 180° and the block also moved. A secondexposure is then made. The resulting image should be exactly circular and any departurefrom this indicates a misalignment of the applicator (Tsai et al 1991).

5.5.7.4 Radiation isocentre

The radiation isocentre needs to be smaller than for a conventional accelerator and forstereotactic multiple arc therapy should also include the tertiary circular collimator. Aball-bearing is suspended at the indicated position of the isocentre and films taken at anumber of different gantry angles. The film can be wrapped round a tube that is concentricwith the ball as described by Warrington et al (1994) or supported by a film holderattached to the collimator (Tsai et al 1991).

5.5.7.5 Mechanical checks

Routine checks of mechanical aspects of the machine (see Section 5.2.3) should beextended to include the collimator and frame attachment mechanisms and the collimatorinserts.

5.5.7.6 Safety aspects

The use of multiple non-coplanar beams, often combining gantry arcing for various couch


positions, with rigidly immobilised patients overhanging the couch top and a protrudingtertiary collimator, demands stringent safety measures. These may include anti-collisionmechanisms, couch power inhibit circuits and electronic limit switches, all of whichrequire regular testing.

When used, the inherent record and verify systems and associated assisted set-upfacilities require their tolerance tables to reflect the overall accuracy expected. This mayhave ramifications on the precision of movement readouts such as gantry angle and couchpositions.

5.5.7.7 Coordinate reconstruction

The geometric accuracy of CT and MR image reconstruction devices and of the imagetransfer system must be rigorously tested as described in Chapter 3, Sections 3.2 and3.3. Tests must also be carried out on the neuroangiography system if this is to be tiedinto the treatment data.

5.5.7.8 Treatment planning system

Beam data files need regular monitoring. Comparisons must be made between calculatedand measured dosimetry performed using anthropomorphic phantoms and suitabledosemeters. A specially designed spherical phantom (Warrington et al 1994) is usefulfor this purpose as it permits the use of ionisation chambers which provide greaterprecision.

5.5.7.9 Summary

These generalised areas for quality control must be adapted to encompass any specificstereotactic radiotherapy technique. Many centres (e.g. Tsai et al 1991, Drzymala et al1994, Warrington et al 1994) have published their quality control programmes and havedescribed specially designed test aids, some of which are commercially available.

With this radiotherapy innovation the physicist plays the key role in ensuring precisionby introducing vigilant but workable quality control protocols. Maintaining precisionwill be less arduous if high quality equipment is selected from the beginning. Prerequisitessuch as good beam stability, reliable optics, stable fine line lasers and purpose built toolswill enable stereotactic radiotherapy to be manageably integrated into the RadiotherapyDepartment.

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Galvin JM, Smith AR, Moeller RD, Goodman RL, Powlis WD, Rubenstein J, Solin LJ,Michael B, Needham M, Huntzinger CJ, et al 1992 Evaluation of multileaf collimatordesign for a photon beam Int. J. Radiat. Oncol. Biol. Phys. 23 789–801

Heisig S, Shentall GS, Mirza K and Mayles WPM 1994 Application of the GE targetplanning computer to multi-leaf collimator treatments Proc. XIth Int. Conf. on the Useof Computers in Radiation Therapy, 20–24 March 1994, Manchester, UK ed AR Hounsell,JM Wilkinson and PC Williams (ISBN 0 9523146 0 6) pp. 16–17

Hendry JH, Benzen SM, Dale RG 1996 A modelled comparison of the effects of usingdifferent ways to compensate for missed treatment days in radiotherapy Clin. Oncol. 8297–307

Hounsell AR and Jordan TJ 1997 Quality control aspects of the Philips multileaf collimatorRadiother. Oncol. 45 225–233

IEC (International Electrotechnical Commission) 1989a Medical Electrical Equipment– Medical Electron Accelerators. Functional Performance Characteristics IECPublication 976 (Geneva: IEC)

IEC (International Electrotechnical Commission) 1989b Medical Electrical Equipment– Medical Electron Accelerators in the Range 1 MeV to 50 MeV – Guidelines forFunctional Performance Characteristics IEC Publication 977 (Geneva: IEC)

IPEMB (Institution of Physics and Engineering in Medicine and Biology) 1996 TheIPEMB code of practice for electron dosimetry for radiotherapy beams of initial energyfrom 2 to 50 MeV based on an air kerma calibration Phys. Med. Biol. 41 2557–2604

IPSM (Institute of Physical Sciences in Medicine) 1988 IPSM Report 54 Commissioningand Quality Assurance of Linear Accelerators (York: IPEM)


IPSM (Institute of Physical Sciences in Medicine) 1992 Survey of quality control practicein UK hospitals carried out by the radiotherapy physics topic group. Scope 1 49–61

Jordan TJ and Williams PC 1994 The design and performance characteristics of a multileafcollimator Phys. Med. Biol. 39 231–251

Khan FM, Gerbi BJ and Deibel FC 1986 Dosimetry of asymmetric x-ray collimatorsMed. Phys. 13 936–941

Klein EE, Harms WB, Low DA, Willcut V and Purdy JA 1995 Clinical implementationof a commercial multileaf collimator: dosimetry, networking, simulation and qualityassurance Int. J. Radiat. Oncol. Biol. Phys. 33 1195–1208

Klein EE, Low DA, Maag D and Purdy JA 1996 A quality assurance program for ancillaryhigh technology devices on a dual-energy accelerator Radiother. Oncol. 38 51–60

Majenka I, Rostkowska J, Derezinski M and Paz N 1982 The recombination correctionfor an ionisation chamber exposed to pulsed radiation in a ‘swept beam’ technique. II.Experimental Phys. Med. Biol. 27 213–221

Mubata CD, Childs P and Bidmead AM 1997 A quality assurance procedure for theVarian multi-leaf collimator Phys. Med. Biol. 42 423–431

NRPB (National Radiological Protection Board) 1988 Guidance Notes for the Protectionof Persons Against Ionising Radiations Arising from Medical and Dental Use (Chilton:NRPB)

RCR (Royal College of Radiologists) 1996 Guidelines for the Management of theUnscheduled Interruption or Prolongation of a Radical Course of Radiotherapy (London:RCR)

Sutherland WH 1969 Dose monitoring methods in medical linear accelerators Br. J.Radiol. 42 864

Thompson AV, Lam KL, Batter JM, McShan DL, Martel MK, Weaver TA, Fraass BAand Ten Haken RK 1995 Mechanical Dosimetry and dosimetric quality control forcomputer controlled radiotherapy treatment equipment Med. Phys. 22 563–566.

Thomson ES, Gill SS and Doughty D 1990 Stereotactic multiple arc radiotherapy Br. J.Radiol. 63 745–751

Tsai J-S, Buck BA, Svensson GK, Alexander E, Cheng C-W, Mannarino EG and LoefflerJS 1991 Quality assurance in stereotactic radiosurgery using a standard linear acceleratorInt. J. Radiat. Oncol. Biol. Phys. 21 737–748

Wang X, Spirou S, LoSasso T, Stein J, Chui CS and Mohan R 1996 Dosimetric verificationof intensity-modulated fields Med. Phys. 23 317–327

Warrington AP, Laing RW and Brada M 1994 Quality assurance in fractionated stereotacticradiotherapy Radiother. Oncol. 30 239–246

WHO (World Health Organisation) 1988 Quality Assurance in Radiotherapy (Geneva:WHO) ISBN 92 4 154224 1


Wittgren L, Nilsson M, Knoos T and Ahlgren L 1993 Output factors for asymmetricfields Proceedings of the Second Biennial ESTRO-meeting on Physics in ClinicalRadiotherapy Prague

Woo MK, O’Brien P, Gillies B and Etheridge R, 1992 Mechanical and radiation isocentrecoincidence: an experience in linear accelerator alignment Med. Phys. 19 357–259

Zhu Y, Boyer AL and Desobry GE 1992 Dose distributions of x-ray fields as shaped withmultileaf collimators Phys. Med. Biol. 37 163–173

Additional Reading

Ahnesjo A, Knoos T and Montelius A 1992 Application of the convolution method forcalculation of output factors for therapy photon beams Med. Phys. 19 295–302

Nilsson M and Landberg T 1994 Dose specification with 3-D dose planning (ICRU ‘level3’) Acta. Oncol. 33 471–476

Palta JR, Ayyanga KM and Suntharalingham N 1988 Dosimetric characteristics of a 6MV photon beam from a linear accelerator with asymmetric jaws Int. J. Radiat. Oncol.Biol. Phys. 14 383–387

Thomas SJ and Thomas RL 1990 A beam generation algorithm for linear acceleratorswith independent collimators Phys. Med. Biol. 35 325–332

Woo MK, Fung A and O’Brien P 1992 Treatment planning for asymmetric jaws on acommercial TP system Med. Phys. 19 1273–1275

CHAPTER 6

Cobalt Teletherapy Units

6.1 Introduction

Although cobalt-60 teletherapy units were not the first type of megavoltage unit, in thedecades from 1960 to 1980 cobalt was the mainstay of most radiotherapy centres in thewestern world. Even today, worldwide there are probably as many cobalt units in use aslinear accelerators. There are still approximately 250 cobalt units in operation in theUSA and 30 in the UK.

Cobalt teletherapy units are generally regarded as relatively simple and reliable froma mechanical and operational point of view. The source dimensions and weight of theradiation head shielding are the main factors which have necessitated compromises inthe design with the concomitant trade-off between spatial precision and a practical andeconomic unit. Nevertheless, in spite of the competition from the modern compact andmore precise low megavoltage linear accelerator, many consider that there can still be arole for cobalt in modern radiotherapy and an economic case has also been made (Reaume1987, Mills et al 1991, Van der Giessen 1991).

As already indicated, cobalt units have the advantage of being simple and reliable butherein lies a danger for the unwary radiotherapy physicist in possibly making theassumption that very little needs to be done in the way of quality control proceduresbecause of this. To give one such example from the author’s own experience illustratesthe point. Some variation of radiation output (dose rate) with gantry angle from linearaccelerators has always been accepted as normal; however, to find a 2 per cent variationin dose rate at the isocentre on rotating the gantry of a cobalt unit through 180° initiallymet with incredulity from everyone concerned. Since the treatment was controlled by atimer, this resulted in patients receiving a dose 2 per cent less than prescribed for beamsfiring vertically upwards and 1 per cent less for horizontal beams. The phenomenon waseventually explained by the fact that the source consisted of 1 mm sized pellets whichhad not been backed with sufficient packing discs within the source capsule. How manyphysicists routinely check the output of a new source at more than one gantry angle?

The recommended frequency for some of the checks described in the following sectionsmay seem rather high for a cobalt unit but with ingenuity, simple and quick tests may bedesigned which will check several parameters with the same set-up. These quick testsmay not have the precision required for a full check but they will indicate seriousdeficiencies in the equipment.

6.2 Description of checks

This section contains a description of and comments on some of the more critical qualitycontrol procedures for cobalt teletherapy units. Some of the checks are identical or very

151Cobalt Teletherapy Units

similar to ones already described for linear accelerators (Chapter 5) and therefore arenot repeated in detail. It is essential that the physicist responsible for the unit fullyunderstands its mechanical construction and operation in order to ascertain which checksare applicable to their unit and where the weak points in the design are in order to assigncorrect priorities and frequencies to the checks. This applies especially to having a goodknowledge of the source exposure mechanism and its associated interlocks and safetycircuits. Familiarity with the basic circuit diagrams of the unit is recommended, especiallyif a detailed description of the unit’s operation is not given by the supplier.

6.2.1 Radioactive contamination

Accessible surfaces of the radiation head should be wiped with a swab soaked in a suitabledecontaminant solution such as Decon F5 or alcohol. Usually it is possible to gain accessto the collimator system but the mirror of the beam optical system may prevent wipesbeing taken close to the primary collimator. For some units it is possible to removecovers at the back of the head to enable parts of the source shutter mechanism to bewiped. Such procedures should have prior approval of the Radiation Protection Adviserto ensure that the dose levels to which staff would be exposed in this procedure areacceptably low. Manufacturers of modern units are required to indicate where wipe testsmay be performed in order to comply with IEC standards (IEC 1988, 1993). Furtherguidance on wipe tests has been given by the NRPB (1988).

The swabs should be held in forceps and stored in sealed containers before measuringfor contamination. The principal methods for checking the swabs are liquid scintillationcounting, and scintillation crystal well counting with the latter being the most convenient.Where possible, it is desirable to calibrate the system with a cobalt-60 source of knownactivity which would also need to be of very low activity, no more than a fewkilobecquerel. The source may be considered to be leak free if an activity of less than200 Bq is measured on the swab after allowing for background.

Some cobalt units contain depleted uranium alloy in parts of the head such as thesource shielding, collimator and penumbra trimmers. Depleted uranium has a highpercentage of the U-238 isotope (less than 0.2 per cent U-235) and is mildly radioactivewith alpha particles being the principle radiation although some low energy (<100 keV)gamma rays are present. The high density (approximately 19,000 kg m–3) and atomicnumber of this material gives it superior radiation attenuation properties and it presentslittle radiation hazard to radiotherapy staff as long as it is not handled excessively orinterfered with mechanically. There have been cases reported (DHSS 1978) where themetal has become powdery due to incorrect formulation of the other constituents in thealloy. Users should check for deposits of black powder on face plates and other internalsurfaces when covers are removed. Swabbing and counting for the alpha radiation is notpractical as the equipment required is unlikely to be available in most medical physicsdepartments. The high density of the uranium powder ensures that it does not present aserious airborne dust hazard but the usual handling precautions should be observed andthe supplier notified if such deposits are found. Because of the hazards from depleteduranium, there has been a tendency by manufacturers to revert to using heavy metal(tungsten alloy) shielding. In the UK, it is also necessary to notify the EnvironmentAgency of any depleted uranium on the premises.


6.2.2 Radiation leakage

Leakage from the head should be checked with the source in both the closed and exposedpositions. If both are measured during commissioning of the unit, it is probably sufficientto check only the former subsequently unless there are other changes such as headmodifications. Air kerma rates around the surface of the head should be checked with asensitive detector of small cross sectional area (10 cm2). The most suitable instrumentthat is likely to be available for this purpose is a Geiger tube device, preferably onecalibrated for cobalt-60 radiation in air kerma rate with a full scale range of the order of300 µSv h–1. Allowable leakage is 200 µSv h–1 at 5 cm from the head surface (NRPB1988). A further limit of 20 µSv h–1 at 1.0 m from the source averaged over 100 cm2 isplaced on the shielding requirements. A large volume ionisation chamber is probably themost suitable instrument for this measurement. A check should be made whether build-up on the detector is required at this distance since electronic equilibrium may not beestablished due to the intervening air gap.

While on the topic of radiation safety, reference is made to the annual inspection bythe Radiation Protection Adviser. Particular note should be made of any change inworkload, techniques, accessories, room condition and use of surrounding areas.

6.2.3 Source position

Many cobalt units have a source in a fixed position which is exposed by a mechanicalshutter mechanism. Others, have a system whereby the source physically moves betweenthe off and exposed positions, the latter having a mechanical stop which can be adjustedto align the source with the collimator axis. It may be possible to set this position duringcommissioning so that all that is required subsequently to verify that everything is stillaligned is a simple mechanical check such as measuring a distance between twocomponents or comparing the position of two marks on the relevant mechanism via CCTV.Depending on the collimator design, small adjustments of a millimetre or so in the sourceor shutter position can create an asymmetrical beam with respect to the mechanicalisocentre without significantly affecting the output. Therefore, it is necessary to use filmor a one-dimensional scanning device to look across the beam in the plane of the sourcemovement for any asymmetry if it is not possible to perform the simple mechanicalcheck described above. Since a film is required for optical beam alignment checks,symmetry may be checked at the same time. This test should be performed at the fourmain quadrants of the gantry position.

Any gross incomplete or partial movement of the source or shutter should be detectedby microswitches or other position sensing devices and would most likely result in areduced output.

6.2.4 Isocentre

Checks on the mechanical and radiation isocentre can use similar methods to thosedescribed in Chapter 5, Sections 5.2.4 and 5.2.9. It should be recognised that there willbe a much greater sphere of uncertainty for a cobalt unit isocentre than for linearaccelerators, typically with a diameter of 3–4 mm. To some extent, this can becompensated for with certain standard treatment set-ups but only if the behaviour of the


beam centre is predictable and well documented. There must be clear guidelines aboutwhich positions are chosen for the alignment of devices such as the axis lights (seeChapter 5, Section 5.2.4.1).

A further factor of significance for cobalt units compared to linear accelerators is thegreater likelihood of mechanical sag in the variable collimators. Therefore, the radiationisocentre may be considerably different from the mechanical isocentre defined by thecollimator axis of rotation.

Some cobalt units are not isocentric in which case the checks are reduced to verifyingthat the crosswires and pointers lie on the axis of rotation of the collimator and that thebeam edges are symmetrical about this axis using techniques described in Chapter 5,Sections 5.2.4.3 and 5.2.9.1. Some isocentric cobalt units have the facility to swivel ortilt the treatment head. These movements enable the beam axis to be directed away fromthe isocentre but kept in the vertical plane of rotation (usually called head rotation orswivel) or alternatively, directed closer to or further away from the gantry base but stillpassing through the gantry’s axis of rotation (head pitch or tilt). Of themselves, thesemovements do not require any specific regular quality control checks other than the factof their correct operation and that the scales indicating the angulation of the head are ingood order. However, these movements do have the potential to create a substantialdecrease in the isocentre accuracy if they are not precisely returned to their zero positionfor normal isocentric operation of the unit. The scales for head rotation and pitch areusually not fine enough to give a precision of the order of one tenth of a degree or betterwhich is required to reduce errors from this source to within 1 mm (0.1° is equivalent to1 mm at 60 cm SAD and 1.7 mm at 100 cm SAD). Some units have a pin or othermechanical device to achieve accurate repositioning. If this is not available, it is possibleto achieve satisfactory results with a system of datum lines, one on the gantry to indicateprecisely the position where the beam is firing vertically downwards and another on thefloor below the isocentre to which the crosswires (having been first correctly adjusted)are aligned by adjusting the head rotation and pitch. This should enable radiographers toreset the head angulation after each use without the necessity for lengthy adjustments byphysics staff. Such a system clearly requires accurate establishment at commissioning.Some centres use a spirit level permanently attached to the gantry in order to set its zeroposition instead of using a datum line.

6.2.5 Output calibration and output checks

Apart from the requirement to perform a definitive calibration on new sources, regularoutput checks are required to verify that the shutter mechanism is operating correctlyand that there has been no change to other items which are normally fixed in the beampath, even though the output of the source should be predictable from the cobalt-60decay constant (0.13146 y–1). There has been a case reported (Jackson and Marschke1971) where the measured output decayed at a greater rate than expected due to mixingwithin the source of pellets of different specific activity. Similar procedures are used forcalibration and output checks as for linear accelerators. The national survey (IPSM 1992a)showed that the median minimum frequency for both output calibration and output checkwas monthly. This may be explained by there being some confusion between what ismeant by the two types of test. If the output check is very simple and quick, the case canbe made that this should be daily as for linear accelerators (see Chapter 5, Table 5.2)


since a month is a long time before discovering that a fault has caused an output change.It should certainly be done at least weekly. A physicist should regularly verify that thedecay corrected output chart being used by the radiographers is valid. This can be backedup by using this output chart to calculate the set time for regular calibration and outputchecks. An accurate monthly output calibration is required and consideration could begiven to performing this at different gantry angles each time in rotation rather than havingto make such measurements separately.

6.2.6 Beam quality and output variation with field size

Unless there has been some other major modification to the beam collimation system,the only time when it is necessary to remeasure beam quality and output variation withfield size in detail is following a change of source. A change in the source length (assumingthe diameter remains the same) can give rise to different scattering conditions and henceaffect the photon spectrum with a corresponding change in depth dose and relative outputcharacteristics (BIR 1983).

The variation of output with field size at 5 cm depth should be checked as should therelative dose for a range of field sizes and depths to demonstrate that there has been nosignificant change in beam characteristics. Any changes are most likely to be seen atdepths of less than 5 cm and particularly in the first few millimetres. Such changes mayor may not be of clinical significance depending upon the types of treatment for whichthe unit is used.

A simple check of beam quality may be obtained by taking a measurement with anextra 10 cm of water equivalent material added to the standard calibration set-up at 5 cmdepth. The ratio of readings at these two depths will give a measure of beam quality forannual checks. Likewise, it may be possible to take an output reading for 5 × 5 cm and20 × 20 cm fields relative to a 10 × 10 cm field with the same set-up at 5 cm depth tocheck the field size dependency.

6.2.7 Timer and shutter correction

Modern cobalt units should have dual independent timer systems which comply withinternational standards (IEC 1987). Therefore, any deterioration in the performance ofone timer channel should immediately be apparent. Nevertheless, it is still worth makinga comparison of the timer accuracy against an external stopwatch over a range of typicalvalues at routine intervals. A recent incident involving the failure of a timer unit toterminate an exposure, even though it was designed to meet modern regulations, highlightsthe need for vigilance. The failure occurred on a type of timer used on many cobalt unitsin the UK and appeared to be due to the displayed set time being less than that storedinternally during the setting process. The dual timers should be operated by differentpositions of the source shutter mechanism with the main timer recording only the timeduring which the source is fully exposed whereas the backup timer includes the timetaken to open and close the shutter. The difference between the two timers will be of theorder of a few seconds and should be relatively constant. This difference, which mayvary with gantry angle, should be noted and used as a daily check of the timer operationand the shutter’s performance. It will still be necessary to measure the correction to beapplied to patient treatment times in order to compensate for the fact that the main timer


does not record exactly the exposure time, but the difference between the two timerchannels can be used to reduce the frequency with which this correction need be measuredseparately. A check should also be made that the system for recording exposure time inthe event of power failure is also functioning correctly.

The timer correction can be measured in a number of ways using a dosemeter and set-up such as that in use for output calibration. One method is to make a series ofmeasurements for a range of exposure times. The results are plotted on a graph ofdosemeter reading against set time. This should give a straight line whose intercept onthe abscissa gives the required correction, including direction, i.e. a positive interceptindicates that the correction should be added to the treatment time and vice versa. Aquicker method is to set a time typical of that used for treatment (1 to 2 min) and to maketwo exposures. During the second, the exposure is interrupted by closing the shutter andthen reopening it but without resetting the dosemeter. If the readings obtained during thefirst and second exposures are denoted by R1 and R2, respectively and the time set is t,the correction time is given by:

t (R1 – R2)

2R1 – R2

This formula is also used in Chapter 7, Section 7.2.3.1.

6.2.8 Interlocks, safety devices and procedures

It is difficult to go into detail about the tests to be performed as they need to be selectedin relation to the specific characteristics of the machine’s design and installation. Theobvious tests for entrance interlocks, emergency-off controls, warning lights, deadman’sswitches and anticollision systems will be similar to those already considered for linearaccelerators (Chapter 5, Section 5.2.1). For items peculiar to cobalt units, generalguidelines will be given.

It is recommended that there should be an independent system for detecting incompleteclosure of the source shutter mechanism by having a gamma ray detector suitablypositioned in the treatment room with visual and audible alarms triggered by anappropriate level of radiation. The operation of these alarms should be checked at regularintervals by placing a small radioactive source close to the detector. Depending on theinstallation, the radiographers may also be able to hear or see (via CCTV) the alarmduring normal treatments.

The manual mechanism for returning the source or shutter to the safe position shouldbe examined and tested as far as is practical but it is not recommended that the mechanismis tested with an exposed source. If a tool is required for this purpose, its availabilityshould be checked. Even more important is the need to have a written procedure fordealing with a jammed shutter at the end of a patient’s treatment. It is advised that thisshould be rehearsed at regular intervals by the radiographic staff with guidance from theRPA and the involvement of the RPS. The procedure should also be clearly displayed inthe console area.

The emergency procedure should give guidance on whether staff should concentrateon removing the patient or trying to close the cobalt unit’s shutter. Advice on how best toavoid the main beam should be given, with provision made for obtaining the information


required to estimate any extra dose received by the patient as well as measuring that towhich the staff might be exposed.

A system of calling for help by other staff should be available. The cause of the problemcan then be assessed by physics staff and solutions sought under controlled conditionsonce the patient has been removed.

The emergency procedure should include provision for a reasonably accurateassessment to be obtained of the extra time for which the patient may have been exposed.This can be achieved by the use of a stopwatch. Staff who enter the room should alsowear a direct reading personal dosemeter. The operation and availability of the stopwatchand dosemeters should be checked regularly.

Most cobalt units have a mechanism for returning the source to the safe position inthe event of power or timer failure, usually relying on a spring. The operation of thismechanism is easily tested by turning off the power. Other tests, such as breaking theroom entrance interlock or removing the security key from the control panel during anexposure may also cause this emergency mechanism to be operated. The spring maycause the shutter to be closed in an abrupt and uncontrolled fashion which could causeincreased wear or damage to certain components and therefore the advice of themanufacturer should be sought when determining the frequency of such tests.

6.2.9 Wedges

There are usually several wedges available which are inserted into the treatment headmanually. The greatest hazard is that the wedges become damaged due to mishandling –especially being dropped. They should therefore be examined frequently for signs ofdamage which could affect their performance. It is not unknown for the wedge to havebeen held on its base plate by sticky tape. The mechanism for positioning the wedge inthe beam should also be checked for wear, slackness or damage. A dosimetric check onthe wedge factor should be made at less frequent intervals although this can be donesimply by using the output check device as long as the target value for this is correlatedwith the wedge factor used for routine clinical dosimetry at the time of commissioning.Modern cobalt units are required to have a ‘select and confirm’ system of interlocks(IEC 1987, 1993). It is important that this is foolproof and so it is considered in thissection rather than under the general interlock section. Wedge interlocks should bechecked as in Chapter 5, Section 5.2.1.5.

6.2.10 Mechanical inspection

Apart from the obvious mechanical checks such as those for the isocentre, range finderand so on, it is important to formalise, as far as possible, a general mechanical inspectionand test of all movements. This may involve removing inspection covers to look forsigns of potential faults. All flexible cables should be checked for signs of wear or damage.In view of the weight of the unit, particular attention should be paid to signs of wear ingantry and head bearings. Serious accidents have occurred due to failures in these areas.Such wear should be detected by a deterioration in isocentric accuracy. Unusual noisesor minor erratic movements may also be indicative of a growing problem in this area.


6.2.11 Other checks

Tests for many of the components of cobalt units are identical to those for linearaccelerators (see Chapter 5). The following areas should be included:• radiation field size (Sections 5.2.9.1 and 5.2.9.2);• optical beam defining system (5.2.6);• optical distance indicator (5.2.4.4);• front and back pointers (5.2.4.5);• other accessories (5.2.1 – 5.2.3);• treatment couch (5.2.3.1 and 5.2.4.2);• electrical safety (5.2.3.2); and• documentation (5.2.15.1).

6.2.12 Arc therapy

Checks will be similar to those described for linear accelerators (Chapter 5, Section5.2.14). The operation of this mode with a cobalt unit will probably be less automaticthan for a linear accelerator and therefore careful attention should be given to the variousdevices and controls that determine the arcing limits, the rotation speed and the sourcecontrol. Since the output will be constant, dose delivery will depend solely upon thespeed of rotation and its constancy. If multiple arcs are used the accuracy and the actualspeed are not critical but its constancy should not vary by more than 4 per cent.

6.3 Test equipment required

The following test equipment is required for quality control of cobalt units:• field dosemeter, thermometer, barometer;• water phantom + solid phantom;• stopwatch/independent timer;• check dosemeter – single or multiple detector system;• therapy verification film (envelope wrapped) or long persistence phosphor device;• build-up sheet (5 mm thick);• tape measure/metre rule;• graph paper;• jig for isocentre checks;• low activity sealed source for gamma alarm test;• swabs, decontamination fluid, sealed containers, sensitive counting system for

radioactive leakage checks;• Geiger survey meter (300 µSv h–1 full scale);• large volume ionisation chamber survey meter (30 µSv h–1 full scale);• electrical safety tester;• CCTV system; and• scanning densitometer.


6.4 Statutory requirements and other guidance

Particular attention is drawn to the following documents:• Ionising Radiations Regulations 1985 (HSC 1985a)• Approved Code of Practice (HSC 1985b)• Guidance Notes for the Use of Ionising Radiation in Medical and Dental Practice

(NRPB 1988)• Ionising Radiations Regulations 1988 (HSC 1988)• IEC 601-2-11 (1987) Medical Electrical Equipment – Particular Requirements for the

Safety of Gamma Beam Therapy Equipment. Also amendments 1 (1988) and 2 (1993)• BS 5724: Section 2.11 (1989) including supplements 1 (1991) and 2 (1993) (duplicate

of the IEC document)• Quality Assurance in Radiotherapy (WHO 1988)• IPSM 54 (Supplement) (IPSM 1990)• IPSM Procedures for Definitive Calibrations (IPSM 1992b)• BS 5288 (1976) Specification: Sealed Radioactive Sources• NCRP Report 102 (1989) Medical X-ray, Electron Beam and Gamma Ray Protection

for Energies up to 50 MeV• Accuracy requirements and quality assurance of external beam therapy with photons

and electrons (Brahme et al 1988)• AAPM Task Group 40: Comprehensive QA for radiation oncology (AAPM 1994)

There are not many statutory requirements or guidance documents given specifically forcobalt teletherapy units, the majority being given for linear accelerators, which are ofcourse often applicable to the former. The main standard that relates to gamma beamequipment is that produced by the IEC (IEC 1987, 1988, 1993) and its British Standardequivalent (BSI 1989, 1991, 1993). However, these standards say very little about whatroutine quality control tests should be done nor do they give specific tolerances on manyof the equipment parameters. The main emphasis is placed on the supplier to providedetails of what tests should be performed by the user and the frequency of such tests.Having said this, Amendment 2 (IEC 1993) does give details of how to perform tests toshow that equipment complies with the standard. These tests fall into two main categories,Type B which can be done without modification to the equipment and Type C, whichrequires modification to generate a fault condition or another means of simulating thefault. Type C tests will usually require the cooperation of the supplier. Type A testsrelate to design analysis or inspection of the accompanying documents and therefore areonly relevant for acceptance procedures.

The IEC standard concentrates on a few areas which are principally related to staffand patient safety. These are: control of the source exposure mechanism (transit times,treatment timers, emergency operation, indication of beam state and interlocks) beammodifiers (flattening filters and wedges), absorbed dose at 0.5 mm depth, radiation leakagefrom the head and wipe tests for radioactive contamination.

The Guidance Notes (NRPB 1988) to the Ionising Radiations Regulations 1985 (HSC1985a) state that all gamma apparatus should be checked by a qualified and experiencedphysicist at least once every month (or before use if only used infrequently orintermittently) for correct operation and applicability of the output data in use. This isusually interpreted that a basic set of quality control checks should be performed monthly


including an output calibration. The action level of 5 per cent for a discrepancy betweenexpected and measured absorbed dose rate is unnecessarily and unacceptably high,especially for cobalt. Guidance is also given about the importance of checking interlocksand other radiation safety features and procedures to be followed during source changesand emergencies involving failure of the operation of the exposure mechanism.

IPSM published a Supplement to its Report 54, Commissioning and Quality Assuranceof Linear Accelerators (IPSM 1990). The Supplement includes a section oncommissioning and quality assurance of gamma beam units. Brief comments are given,specifically where procedures for gamma beam units might differ from linear accelerators.A table of routine checks and their frequency is given which does not differ significantlyfrom Table 6.1. No tolerances specific to cobalt teletherapy are given. The Supplementindicates that wipe tests for leakage of radioactivity from the source should be performedat six-monthly intervals as does NCRP Report 102 (NCRP 1989). The survey showedthat most centres perform this check annually.

The WHO publication, Quality Assurance in Radiotherapy (WHO 1988), also givessome guidance on routine tests for cobalt units although for the mechanical tolerancesthere is no distinction made between linear accelerators and cobalt which many units ofthe latter type would have difficulty in meeting. On the other hand, some tests wouldappear to be rather lax in their application such as yearly for wedge transmission and±3% for symmetry. Tables of tolerance levels and frequency are given for checking manyparameters.

A comprehensive report on ‘Accuracy requirements and quality assurance of externalbeam therapy with photons and electrons’ (Brahme et al 1988) has been produced by aworking group in 1988 but it only has a minimal reference to the requirements for gammabeam equipment, making a brief reference to emergency closure of the source mechanismand the need to have a transition time for beam exposure of less than 2 seconds.

The report of AAPM Task Group 40 (AAPM 1994) on ‘Quality assurance for radiationoncology’ includes a table of recommended routine tests with tolerances and frequenciesfor cobalt units. There are a few minor differences to the tests proposed in Section 6.5 ofthis chapter, including no recommendation by the AAPM for a daily output check. Thereport does not attempt to describe how any of the tests should be performed.

6.5 Recommended frequencies for tests

Recommended check frequencies for cobalt units are contained in Table 6.1. Where morethan one time period is given against a check, it indicates that two or more levels oftesting are required with the most rigorous and time consuming checks being performedless frequently. All of the checks will need to be carried out during commissioning andon source replacement.

It has already been stated that some of the tolerances for cobalt units are generallygreater than for linear accelerators but that allowances can often be made for predictabledeviations. For example, if the collimators sag due to gravity with the beam horizontalsuch that the beam centre is 2 mm below the collimator rotation axis at the isocentre, thelateral laser axis lights could be set up to allow for this. It would also be unnecessarilygenerous to increase the allowable tolerance for this measurement by 2 mm for all


situations. Therefore, the tolerances given in the table are, in general, meant to indicatedeviations from the originally measured or predictable values.

Table 6.1. Minimum check frequencies for cobalt units.


DailyInterlocks and safety devices 6.2.8 FunctionalOptical alignment, size and crosswires 6.2.11 2 mmAccessories 6.2.11 FunctionalAxis lights 6.2.4 2 mmDistance indicators 6.2.11 3 mmTimer check 6.2.7 1 s

WeeklyOutput check 6.2.5 ±4%Emergency stops 6.2.8 FunctionalAxis lights 6.2.4 2 mmDistance indicators 6.2.11 3 mm

MonthlySource position/beam symmetry 6.2.3 2%Radiation size 6.2.11 2 mmMechanical isocentre 6.2.4 2 mmOptical alignment, size and crosswires 6.2.11 2 mmAccessories 6.2.11 FunctionalMechanical condition/operation 6.2.10 FunctionalOutput calibration (NRPB 1988) 6.2.5 ±3%Timer accuracy 6.2.7 ±1%Shutter correction 6.2.7 < 2 sWedge factor/inspection 6.2.9 ±2%Arc therapy (if used) 6.2.12 FunctionalGamma alarm/personnel monitors 6.2.8 FunctionalElectrical safety 6.2.11 BS 5724Documentation 6.2.11 Functional

YearlyWipe test (NRPB 1988) 6.2.1 <200BqHead leakage 6.2.2 BS 5724Radiation survey 6.2.2 IRR 1985Electrical safety 6.2.11 BS 5724Output versus field size 6.2.6 ±1%Output versus gantry angle 6.1, 6.2.5 ±1%Quality (depth dose) 6.2.6 ±1%

References



Brahme A, Chavaudra J, Landberg T, McCullough E, Nüsslin F, Rawlinson JE, SvenssonG and Svensson H 1988 Accuracy requirements and quality assurance of external beamtherapy with photons and electrons Acta Oncol. 27 (Suppl 1)

BIR (British Institute of Radiology) 1983 Central axis depth dose data for use inradiotherapy. Br. J. Radiol. Suppl 17

BSI (British Standards Institute) 1976 BS 5288 Specification: Sealed Radioactive Sources(London: BSI)

BSI (British Standards Institute) 1989 BS 5724: 2.11 Medical Electrical Equipment,Part 2: Particular requirements for safety, Section 2.11 Specification for Gamma BeamTherapy Equipment (London: BSI) [Identical to IEC 1987]

BSI (British Standards Institute) 1991 BS 5724: 2.11 Medical Electrical Equipment,Part 2: Particular Requirements for Safety, Section 2.11 Specification for Gamma BeamTherapy Equipment. Supplement 1. Revised and Additional Text (London: BSI) [Identicalto IEC 1988]

BSI (British Standards Institute) 1993 BS 5724: 2.11 Medical Electrical Equipment,Part 2: Particular requirements for Safety, Section 2.11 Specification for Gamma BeamTherapy Equipment. Supplement 2. Methods of Test for Radiation Safety (London: BSI)[Identical to IEC 1993]

DHSS (Department of Health and Social Security) 1978 Health Notice HN (Hazard)(78) 26 (London: Department of Health)

HSC (Health and Safety Commission) 1985a Statutory Instrument No. 1333 The IonisingRadiations Regulations 1985 (London, HMSO)

HSC (Health and Safety Commission) 1985b Approved Code of Practice. The Protectionof Persons Against Ionising Radiation Arising from Any Work Activity (London: HMSO)

HSC (Health and Safety Commission) 1988 Statutory Instrument No. 778 The IonisingRadiations (Protection of Persons Undergoing Medical Examination or Treatment)Regulations 1988 (London: HMSO)

IEC (International Electrotechnical Commission) 1987 Medical Electrical Equipment,Part 2: Particular Requirements for the Safety of Gamma Beam Therapy Equipment(Geneva: IEC Publication 601-2-11)

IEC (International Electrotechnical Commission) 1988 Medical Electrical Equipment,Part 2: Particular Requirements for the Safety of Gamma Beam Therapy Equipment.Amendment 1 (Geneva: IEC Publication 601-2-11 Amdt 1)


IPSM (Institute of Physical Sciences in Medicine) 1990 Supplement to Report 54Commissioning and Quality Assurance of Linear Accelerators (York: IPEM)

IPSM (Institute of Physical Sciences in Medicine) 1992a Survey of quality control practicein UK hospitals carried out by the radiotherapy physics topic group. Scope 1 49-61[Summary in Appendix B]


IPSM (Institute of Physical Sciences in Medicine) 1992b Procedures for the definitivecalibration of radiotherapy equipment Scope 1 No 1 [Reproduced in Appendix A]

Jackson HL and Marschke CH 1971 Abnormal decay characteristics of a replacementcobalt-60 teletherapy source Health Phys. 20 353–357

Mills JA, Martin-Smith P, Grieve RJ and McIntosh JA 1991 Options considered forreplacement of a cobalt unit at Coventry Proceedings of European Symposium on CobaltTherapy (London: CIS (UK) Ltd)

NCRP (National Council on Radiation Protection and Measurements) 1989 Report 102Medical X-ray, Electron Beam and Gamma Ray Protection for Energies up to 50 MeV(Bethesda MD: NCRP)

NRPB (National Radiological Protection Board) 1988 Guidance Notes for the Protectionof Persons Against Ionising Radiations Arising from Medical and Dental Use (London:HMSO)

Reaume L 1987 Rethinking cobalt therapy Administrative Radiology Journal (California:Glendale Publishing)

Van der Giessen PH 1991 A comparison of maintenance costs of cobalt machines andlinear accelerators Radiother. Oncol. 20 64–65

WHO (World Health Organisation) 1988 Quality Assurance in Radiotherapy (Geneva:WHO) ISBN 92 4 154224 1

CHAPTER 7

Kilovoltage X-ray Units

7.1 Introduction

Recommendations for quality assurance in radiotherapy have tended to concentrate onmegavoltage and brachytherapy equipment and techniques (NCRP 1981, AAPM 1984,WHO 1988, AAPM 1994). However, the continued successful use of kilovoltage X-rayunits for superficial therapy and their different, albeit simpler, design compared withlinear accelerators, indicates that specific advice for this type of equipment is required.In addition to this chapter, useful guidance on quality control of kilovoltage units isgiven by Klevenhagen and Thwaites (1993) and Nikolic and Van Dyk (1993).

7.2 Description of tests

7.2.1 Output measurements and output constancy checks

Two kinds of measurement of X-ray tube output are recommended.

1. Output measurementAn output measurement is a determination of the absorbed dose to water at a referencepoint in the X-ray beam for a chosen field size and tube voltage-filter combination.The recommended technique depends on the beam quality and is given in Sections7.2.1.1, 7.2.1.2 and 7.2.1.3 for medium, low and very low energies respectively. Outputmeasurements are defined in terms of a single field or applicator size. The relationshipbetween tube output measurements at different field or applicator sizes (i.e. applicatorfactors) should be determined at commissioning and checked, using the proceduresrecommended below, at least annually or following repair. This could convenientlybe arranged on a cyclical basis. Notice that, since, for low energy X-rays, calculationof the output depends explicitly on the backscatter factor (Section 7.2.1.2), care mustbe taken to include the appropriate backscatter factors when determining applicatorfactors. Output measurements should be made at least monthly, or following repair.Apparent changes in output which exceed ±3% should be investigated further, andshould lead to a definitive calibration being carried out (Appendix A).

For some X-ray units, it may be found on commissioning that output is a functionof tube orientation. Where differences are observed, the output measurements describedbelow in Sections 7.2.1.1, 7.2.1.2 and 7.2.1.3 should be repeated at least annually at+90°, -90° and 180° to the conventional vertical beam direction (0°).

The use of a commercial megavoltage output constancy meter for kilovoltageequipment has been described by Nikolic and Van Dyk (1993).

2. Output constancy checkAn output constancy check is not an output measurement as defined above, but a


daily instrument reading (corrected only for temperature and pressure) taken underreproducible geometrical conditions for each tube-voltage and filter combination usedand designed to check that the output measurements in clinical use are not grossly inerror. Characteristically, constancy checks may be performed more quickly thancalibrations and, because of their function, it is reasonable to assign a wider toleranceband (±5%) than would be appropriate for a calibration. Constancy checks arecommonly based on ionisation chamber readings either in-air or in-phantom, but theycould be based on diode dosemeter readings. Constancy checks should be comparedwith output measurements at least annually (a constancy-check system calibration),conveniently at the time of definitive calibration of the treatment unit or of thecalibration dosemeters, or following repair. However, a drift of constancy-check resultsbeyond tolerance might be an indication for recalibration of the constancy-checksystem. Constancy checks should be made daily.

Guidance on dosimetry in this quality range is given in the ‘Code of Practice fordetermination of absorbed dose for X-rays below 300 kV generating potential (0.035mm Al – 4 mm Cu HVL, 10–300 kV generating potential)’ (IPEMB 1996, andhenceforth referred to as the ‘Code of Practice’), from which the followingrecommendations are summarised.

7.2.1.1 Medium energy X-rays

The term ‘medium energy X-rays’ refers to X-rays generated at tube voltages in therange 160–300 kV, covering approximately the range of first half-value layers 9–20 mmAl, or equivalently 0.5–4.0 mm Cu.

1. Measurement of X-ray tube outputIt is recommended that absorbed dose to water be measured on the central axis of thebeam at a depth of 2 cm in a water (or water-equivalent) phantom, using the formalismand correction factors given in the Code of Practice.A calibrated field instrument (see recommendations in Chapter 8, Section 8.2 and inAppendix B of the Code of Practice) should be used to measure the output of the X-ray tube for each tube-voltage and filter combination available and for a single fieldsize or applicator, as follows:(a) Position the chamber of the field instrument on the central axis of the X-ray beam

with its centre at the reference depth of 2 cm in a water phantom. (A ‘solid water’phantom may be used, subject to the recommendations of the Code of Practice.)

(b) Use a full-scatter phantom which extends outside the beam edges for the largestfield or applicator used and is at least 10 cm deep. The surface of the phantomshould be positioned normal to the central axis and at the SSD appropriate to theapplicator or field.

(c) Correct the instrument reading to a chamber temperature of 20°C, an ambient airpressure of 1013.25 mbar and for losses caused by recombination, if therecombination correction is not already incorporated into the calibration factorof the field instrument.

(d) Use the equation given in the Code of Practice to calculate the absorbed dose towater for the appropriate radiation quality.

(e) From a knowledge of depth dose data for the appropriate quality and field size,the absorbed dose may be calculated at any other point chosen by the user (e.g.the phantom surface).

165Kilovoltage X-ray Units

2. Constancy checksThe design of a jig for constancy checks is left to the discretion of the user, but thefollowing features of a constancy check system should be borne in mind:(a) Day-to-day geometrical reproducibility must be assured. The jig should be robust

and straightforward to set up. For this quality range, it may be convenient to basea constancy check on a measurement at 2 cm in a water-equivalent phantom.

(b) The same measuring instrument (ionisation chamber or diode dosemeter) shouldbe used for each daily measurement.

(c) The ratio of the constancy-check reading to a defined output measurement mustbe determined for each tube-voltage and filter combination. This ratio must beremeasured following changes or additions to the tube-voltage and filtercombinations, or when a different measuring instrument is used.

7.2.1.2 Low energy X-rays

The term ‘low energy X-rays’ refers to X-rays generated at tube voltages in the range50–160 kV, covering approximately the range of half-value layers 1.0 to 8 mm Al. Therecommended reference measurement position for low energy X-rays is in-air at the endof an applicator.

1. Measurement of X-ray tube outputIt is recommended that absorbed dose to water be measured on the central axis of thebeam, in air, at the end of the applicator, using the formalism and correction factorsgiven in the Code of Practice.A calibrated field instrument (see recommendations in Chapter 8, Section 8.2 and inAppendix B of the Code of Practice) should be used to measure the output of the X-ray tube for each tube voltage-filter combination available and for a single selectedapplicator, as follows:(a) Position the centre of the chamber of the field instrument on the central axis of

the X-ray beam at the SSD appropriate to the field size or close to the face of theapplicator. The long axis of the chamber should be normal to the central axis ofthe beam. When an applicator is used, an offset is usually necessary for practicalreasons. This should be recorded and an inverse-square-law correction applied.The scatter from the applicator and filter may result in deviations from the inverse-square-law (Klevenhagen 1979). This effect should be investigated for theapplicator/X-ray tube combination.

(b) Correct the instrument reading to a chamber temperature of 20°C, an ambient airpressure of 1013.25 mbar and for losses caused by recombination, if therecombination correction is not already incorporated into the calibration factorof the field instrument.

(c) Use the equation given in the Code of Practice to calculate the absorbed dose towater at the surface of a semi-infinite phantom for the appropriate radiation quality.Since the output measurements are made in air, the calculation of absorbed doseto water must include the backscatter factor appropriate to the applicator as amultiplying factor.

(d) From a knowledge of depth-dose data for the appropriate quality and field size,the absorbed dose may be calculated at any other point chosen by the user.


2. Constancy ChecksThe same general guidance as given in Section 7.2.1.1 is valid except that the checkmay conveniently be made in-air rather than in-phantom.

7.2.1.3 Very low energy X-rays

The term ‘very low energy X-rays’ refers to X-rays generated at tube voltages in therange 10–50 kV, covering approximately the range of half-value layers 0.035 to 1.0 mmAl. The recommended reference measurement position for very low energy X-rays is atthe surface of a water or water-equivalent phantom.

1. Measurement of X-ray tube outputPlane-parallel ionisation chambers should be used in this energy range. These areusually mounted in a block of material which provides substantial scatter. For thisreason, it is recommended that output measurements be made at the surface of aphantom providing full-scatter conditions, using the formalism and correction factorsgiven in the Code of Practice.

A calibrated field instrument (see recommendations in Chapter 8, Section 8.2 andin Appendix B of the Code of Practice) may be used to measure the output of the X-ray tube for each tube-voltage and filter combination available and for a single selectedapplicator, as follows:(a) Position the centre of the chamber of the field instrument on the central axis of

the X-ray beam with its entrance window at the surface of a water phantom. (A‘solid water’ phantom may be used, subject to the recommendations of the Codeof Practice.)

(b) Use a full-scatter phantom which extends outside the beam edges for the largestfield or applicator used and is at least 10 cm thick. The surface of the phantomshould be positioned normal to the central axis and at the SSD appropriate to theapplicator or field.

(c) Correct the instrument reading to a chamber temperature of 20°C, an ambient airpressure of 1013.25 mbar and for losses caused by recombination, if therecombination correction is not already incorporated into the calibration factorof the field instrument.

(d) Use the equation given in the Code of Practice to calculate the absorbed dose towater for the appropriate radiation quality.

(e) From a knowledge of depth-dose data for the appropriate quality and field size,the absorbed dose may be calculated at any other point chosen by the user.

2. Constancy checksThe same general guidance as given in Section 7.2.1.1 is valid except that the checkmay conveniently be made using a plane-parallel chamber at the surface of a waterphantom.

7.2.2 Filter interlocks

Kilovoltage X-ray units will usually be used with several tube-voltage and filtercombinations. It is essential to ensure that operation at each value of tube voltage involvesthe correct filter, otherwise large under- or over-doses may result. The electromechanicalinterlock system should be tested as follows:


1. Check that the filters are not damaged and that they fit easily into the treatment head.2. For each filter in turn, fit the filter into the treatment head and check that an exposure

may be made with the correct tube voltage selected.3. For each filter, check that an exposure is not possible with all other available tube

voltages.4. For each filter, check that exposure is not possible until the filter is inserted fully and

correctly into the treatment head.These checks should be carried out daily.

7.2.3 Timers and monitor chambers

7.2.3.1 Timers

On older X-ray units, the treatment dose is determined by the time set on the console, sothat it is essential to demonstrate correct operation of the timer system. On modern units,dose is commonly determined by setting monitor units which are calibrated against theoutput from a monitor chamber in the treatment beam. In both systems, backup is normallyprovided by an additional timer.

1. Timer end-error(a) Set up an ionisation chamber and phantom as described for a measurement of

tube output (Section 7.2.1). Make four exposures each of 1 min and comparereadings for consistency. All four readings should be within ±1%.

(b) Set the timer to 2 min, expose, and record the ionisation reading (R1).(c) Make four consecutive 0.5 min exposures, allowing full automatic termination in

each case, but without resetting the dosemeter. Record the ionisation reading forthis interrupted, 2 min exposure (R2).

The timer end-error (α) is given by:

This formula is given in general form in NCRP Report 69 (1981, p. 55) and isessentially the same as that given in Chapter 6, Section 6.2.7.

Negative values of timer end-error imply that the effective exposure time isless than that set on the timer, in which case |α| min should be added to thecalculated treatment times.

(d) Repeat (c) for each tube-voltage and filter combination.These checks should be repeated monthly, or following repair. Timer end-errorshould be consistent to within ±0.01 min.

2. Other timer checks(a) For one tube-voltage and filter combination, check the timer against a stopwatch

for 1.00 min during a longer exposure, e.g. from 0.5 to 1.5 min. Repeat for a 3min exposure (e.g. from 2.0 to 5.0 min). The timer should read to within ±2% ofthe stopwatch.

(b) Ensure that the timer terminates the exposure correctly when the set time expiresand that the backup timer is within ±0.02 min of the primary timer.

(c) With no time set, check that no exposure can be made for any tube-voltage andfilter combination.

α =2(R2 – R1)

4R1 – R2


(d) Set timer to 2 min, expose and, during exposure, simulate a mains power failure.Check that the elapsed time is retained.

These checks should be repeated monthly, or following repair.

7.2.3.2 Monitor chambers

Linearity of the monitor chamber system should be checked as follows:1. Run the machine for 100 monitor units and note the reading of an ionisation chamber

placed in the beam.2. Chamber readings for other monitor unit settings should be predictable to within ±2%.

A convenient approach to calibrating the monitor chamber for each tube-voltage andfilter combination is to set the sensitivity of the monitor chamber to read 100 for eachGy of absorbed dose which would be delivered on the surface of a water phantom by theapplicator used to measure the output for that particular radiation quality.

These checks should be carried out monthly, or following repair.

7.2.4 Beam quality

The following guidance is a summary of that given in the Code of Practice.

7.2.4.1 Quality parameters

For X-ray beams covered by this report, the currently accepted practice is to specifyquality in terms of the first half-value layer (HVL1), i.e. the thickness of a specifiedabsorber which reduces the air kerma rate in a narrow beam to one half its unattenuatedvalue. For most clinical purposes at least a statement of peak tube potential and HVL1 isrecommended (Greening 1963, Harrison 1981).

The HVL1 is obtained by measuring narrow beam attenuation in the specified materialand determining the thickness corresponding to 50 per cent attenuation.

The usual materials for specifying HVL1 are aluminium for low energy X-ray beams(10–160 kV) and copper for medium energy beams (160–300 kV).

1. HVL measurement techniqueA determination of HVL involves the measurement of air kerma at a point in a narrowbeam, as increasing thicknesses of the attenuating material are placed in the beam.

A monitor chamber may be useful to permit correction for variations in air kermarate, provided that it is placed so that its response is independent of the thickness ofattenuating material placed in the beam and provided it does not perturb the narrowbeam measurements by adding to the scatter component.

The attenuating material should be placed approximately midway between thesource and the chamber (Trout et al 1960). A source-chamber distance of at least 50cm, and preferably 80 to 100 cm, should be used. The attenuating sheets, for lowerenergy beams particularly, should be of high purity and accurately known thickness(HPA 1977, 1980, Wagner et al 1990). SIA grade (99.8 per cent purity) is suitable forthis energy range, although for soft X-rays below 50 kV generating potential, SI grade(99.9 per cent purity) is recommended.

The ionisation chamber used for the attenuation measurements should be selectedto have a minimum – and known – quality dependence over the energy range concerned.


Corrections to ionisation chamber readings may be necessary for photon energiesbelow 100 keV.

Guidance on experimental technique for HVL determination is given by Trout etal (1960), HPA (1977), HPA (1980), Cranley et al (1991) and Klevenhagen andThwaites (1993). A scatter-free and narrow-beam arrangement should be employed,with the attenuators placed midway between X-ray focus and detector, although itshould be noted that several conflicting recommendations on HVL measurementtechnique have been published (Trout et al 1960, IAEA 1987, Carlsson 1993).

Half value layer measurements should be carried out at least annually, or followingrepair, and it should be noted that WHO (1988) recommends twice yearlymeasurements. Variations of HVL

1 greater than ±10% should be investigated.

2. HVL constancy checksFor more frequent monthly checks of beam quality, a simplified HVL constancy checkis recommended by analogy with the output constancy check (Section 7.2.1). Thedesign of this check is left to the discretion of the user. It should aim to measure aratio of the ionisation reading in the normally filtered beam (for a defined tube-voltageand filter combination) to the reading at the same position in the beam but with anadded thickness of any convenient material approximately equal to one HVL placedbetween the X-ray focus and the chamber. The jig should guarantee reproduciblegeometry. (A single jig may be designed to accommodate both output-constancy andHVL-constancy checks.) The relationship of this ratio to the HVL measurement shouldbe determined at commissioning and checked annually.

The constancy check provides an indication of relative beam quality changes.Changes greater than ±10% should be investigated.

An interesting method for a simplified HVL constancy check has been given byNikolic and Van Dyk (1993), who used a commercial megavoltage constancy meterand related a ratio of readings with and without a copper filter to HVL

1.

7.2.5 Field uniformity

7.2.5.1 Applicators

The following checks are designed to demonstrate non-uniformities in the dose profilewithin the X-ray field:

1. Place a verification film (e.g. Kodak X-Omat V) on top of a water equivalent slab ofthickness greater than 5 cm.

2. Position the applicator so that it just touches the film packet. Mark the position of theapplicator and the anode–cathode axis on the film.

3. Make an exposure to produce an optical density of approximately unity and processthe film.

4. Visually confirm that the penumbra of the applicator appears to be sharp and uniform.5. Check that the diameter of the radiation field corresponds to the applicator diameter

(to within ±2 mm), or in the case of square applicators, that the edges of the radiationfield are coincident with the applicator edges to the same tolerance.

6. If the applicator can be rotated, repeat steps (1) to (5) inclusive for at least twoorientations which bracket the range of rotation.

7. Repeat steps (1) to (6) inclusive for all applicators in use.


8. For the largest applicator, use a film densitometer to measure the optical density atthe centre of the X-ray field and at +5 cm and –5 cm from the centre along the anode–cathode axis. Using the characteristic curve of the film, calculate the correspondingabsorbed dose to water at these points and the percentage differences (D

1, D

2) between

the doses at these points and the dose at the field centre. D1 and D

2 should both be

within ±5 percentage points of the values accepted at commissioning.Field uniformity checks should be carried out at least annually, or following repair.

7.2.5.2 Units fitted with light beam diaphragms

The following checks are designed to demonstrate non-uniformities in the dose profilewithin the X-ray field:

1. Place a verification film (e.g. Kodak X-Omat V) on top of a water equivalent slab ofthickness greater than 5 cm so that the phantom surface is at the SSD.

2. Mark the position of the light field and the anode–cathode axis on the film envelopeusing radioopaque markers or as recommended for similar tests for linear acceleratorsin Chapter 5, Section 5.2.9.

3. Make an exposure to produce an optical density of approximately unity and processthe film.

4. Check that the edges of the radiation field are coincident with the light field edges.5. Repeat steps (1) – (4) inclusive for at least three field sizes which bracket the range

used.6. For the largest field, use a film densitometer to measure the optical density at the

centre of the X-ray field and at +5 cm and –5 cm from the centre along the anode–cathode axis. Using the characteristic curve of the film, calculate the absorbed doseto water at these points and the percentage differences (D

1, D

2) between the doses at

these points and the dose at the field centre. D1 and D

2 should both be within ± 5

percentage points of the values accepted at commissioning.Field uniformity checks should be carried out at least monthly, or after repair.

7.2.6 Focal spot measurement

Changes in beam uniformity or output may be linked with changes in either the appearanceof the focal spot or its position (for example, because of rotation of the insert within thehead of the unit). While regular checks of the focal spot are not deemed necessary,examination of the focal spot may help to diagnose faults observed during routine checks.A suggested procedure is as follows:

1. Position the pinhole assembly (typically a 0.1 mm diameter pinhole in a 2 mm thicklead disk) at or near the position where the applicator engages the treatment head,with the pinhole on the central axis of the beam (a special insert may need to bedesigned to do this). Attach a large field applicator to the tube head in the normalway.

2. Place a sheet of verification film (e.g. Kodak X-Omat V) at the end of the applicatorand mark the applicator axis and the film orientation on the film.

3. Make a pinhole image of the focal spot. The geometry should be such as to give amagnification greater than 2.

4. Note any displacement of the focal spot image from the applicator axis.


5. Note any change in appearance of the focal spot image compared with previous images.Changes may indicate deterioration of the X-ray tube or movement of the applicatorhousing relative to the tube.

A discussion of pinhole camera techniques can be found in Arnold et al (1973).

7.2.7 Interlocks and warnings

1. While an exposure is underway, check that the exposure is terminated when the dooris opened and does not restart on closing the door.

2. If a ‘close door’ prompt is shown on the control console, check that this remains onuntil door is closed.

3. Check that the stop button terminates an exposure when pressed.4. Check that warning lights and audible warnings function correctly.5. Check that the emergency stop terminates an exposure correctly and that subsequent

switch-on is normal and elapsed time is retained.These checks should be carried out daily.

7.2.8 Mechanical fixtures

1. FiltersSee Section 7.2.2.

2. DoorsSee Section 7.2.7.

3. Applicators(a) Place each of the applicators in turn into the treatment head. Check that they

engage correctly.(b) Check that any PMMA component of an applicator is not damaged or cracked.

Check that no part of the applicator is loose and that the apertures at both endsare clean and symmetric.These checks should be performed daily.

4. Machine(a) Check the HT cable, water hoses and water cooler for signs of wear. These checks

should be performed at least monthly, or following repair.(b) Check vertical, longitudinal and tube head rotation movement and locks. These

checks should be performed daily.

5. CouchCheck movements and locks, including limit switches, if fitted.These checks should be performed at least monthly, or following repair.

6. AccessoriesCheck condition of accessories, such as eye shields, for damage.These checks should be performed daily.


7.3 Dosimetry equipment

This information is a summary of that given in the Code of Practice.For medium and low energy X rays, the recommended field instrument is a thimble

chamber with a volume of less than 1.0 cm3 which is approximately air or waterequivalent, connected to any suitable electrometer. No materials with an atomic numbergreater than 13 (aluminium) should be used in the vicinity of the cavity. The chambermust be vented to the atmosphere.

The preferred chambers are: NE 2505/3A; NE 2571; NE 2581; PTW TW30002; orany chambers of the ‘Farmer’ type constructed with either a carbon wall and aluminiumelectrode or completely air equivalent plastic. Chambers with graphite coated nylon orPMMA walls (e.g. the NE 2505/3B or PTW TW30001) are more vulnerable to suddenchanges of energy response and should be used only with appropriate precautions.

For very low energy X-rays, the recommended field instruments are plane parallelionisation chambers. There are no preferred instruments in this range, but the desirablefeatures of an ionisation chamber of this type are as follows:

1. The depth of the sensitive volume should not exceed 2 mm and the internal diametershould not exceed 10 mm.

2. The entrance window thickness should be greater than the range of secondary electronsto ensure that the secondary electrons generated in the clinical filter and applicatorwalls do not enter the chamber (Klevenhagen et al 1991).

3. The polarising voltage should be high enough so that ion recombination is negligiblein the chamber.

4. The variation in response of the chamber in air should not exceed 5 per cent over therange of X-ray energies used.

7.4 Recommended frequencies

In recommending testing frequencies, consideration has been given to the results of anational survey of quality control (Appendix B).

Table 7.1 gives recommended minimum frequencies for quality control tests, forequipment which is in regular use (i.e. used at least four days per week).


References

AAPM (American Association of Physicists in Medicine) 1984 Physical Aspects ofQuality Assurance in Radiation Therapy AAPM Report 13


Arnold BA, Bjärngard BE and Klopping JC 1973 A modified pinhole camera method forinvestigation of x-ray tube focal spots Phys. Med. Biol. 18 540–549

Carlsson CA 1993 Differences in reported backscatter factors for low energy x-rays: aliterature study Phys. Med. Biol. 38 521–531

Cranley K, Gilmore BJ and Fogarty GWA 1991 Data for estimating x-ray tube totalfiltration (York: IPEM, PO Box 303)

Table 7.1. Recommended frequencies for quality control checks.


DailyOutput constancy check 7.2.1 ±5% (Note 2)Interlocks and warnings 7.2.7 Note 1Mechanical fixtures 7.2.8 (3,4b,5,6) Note 1Filter interlock 7.2.2 (1) Note 1

Weekly (or following repair)Filter interlocks 7.2.2 (2–4) Note 1

Monthly (or following repair)Output measurement 7.2.1 ±3%Timer end-error 7.2.3.1 (1) ±0.01 minTimer accuracy 7.2.3.1 (2a) ±2%

7.2.3.1 (2b) ±0.02 minOther timer checks 7.2.3.1 (2c,2d) Note 1Filter interlocks 7.2.2 Note 1Mechanical fixtures 7.2.8 (4a) Note 1Monitor chamber linearity 7.2.3.2 ±2%Light beam – X-ray beam coincidence 7.2.5.2 (4) ±5 mm

7.2.5.2 (6) ±10%HVL constancy 7.2.4.1 (2) ±10%

Annually (or following repair)Field uniformity 7.2.5.1 (5), 7.2.5.2 (4) ±2 mm

7.2.5.1 (8), 7.2.5.2 (6) ±5%Half-value layer 7.2.4.1 (1) ±10%

Applicator factors 7.2.1 ±3%

1 Where no quantitative action level is indicated, the assessment is subjective or based on a ‘yes/no’ decision.2 If the daily output constancy check varies by more than ±5% from the previous monthly output calibration, an

investigation should be performed. This should include at least a measurement of HVL.


Greening JR 1963 The derivation of approximate x-ray spectral distributions and ananalysis of x-ray ‘quality’ specifications Br. J. Radiol. 36 363–371

Harrison RM 1981 Central axis depth dose data for diagnostic radiology Phys. Med.Biol. 26 657–670

HPA (Hospital Physicists’ Association) 1977 The Physics of Radiodiagnosis HPAScientific Report Series 6 (York: IPEM)

HPA (Hospital Physicists’ Association) 1980 Measurement of the PerformanceCharacteristics of Diagnostic X-ray Systems Used in Medicine: Part 1 X-ray Tubes andGenerators TGR-32 Part 1 (York: IPEM)

IAEA (International Atomic Energy Agency) 1987 Absorbed Dose Determination inPhoton and Electron Beams. An International Code of Practice IAEA Technical Report277

IPEMB (Institution of Physics and Engineering in Medicine and Biology) 1996 TheIPEMB code of practice for the determination of absorbed dose for x-rays below 300 kVgenerating potential (0.035 mm Al – 4 mm Cu HVL; 10–300 kV generating potential)Phys. Med. Biol. 41 2605–2626

Klevenhagen SC 1979 Physical aspects and application of the RT 305 hard-ray therapymachine Medica Mundi 24 (3) 127–135

Klevenhagen SC and Thwaites D 1993 Kilovoltage X-rays. in Radiotherapy Physics. edWilliams and Thwaites (Oxford: Oxford University Press)

Klevenhagen SC, D’Souza D and Bonfoux I 1991 Complications in low energy x-raydosimetry caused by electron contamination Phys. Med. Biol. 36 1111–1116

NCRP (National Council on Radiation Protection and Measurements) 1981 (Thirdreprinting 1993) Report 69 Dosimetry of X-ray and Gamma-ray Beams for RadiationTherapy in the Energy Range 10 keV to 50 MEV (Bethesda MD: NCRP)

Nikolic M and Van Dyk J 1993 Use of a constancy meter for orthovoltage qualityassurance Med. Phys. 20 1747–1750

Trout ED, Kelley JP and Lucas AC 1960 Determination of Half Value Layer Am. J.Roentgenol. 84 729

Wagner LK, Archer BR and Cerra F 1990 On the measurement of half-value layer infilm screen mammography Med. Phys. 17 989–997

WHO (World Health Organisation) 1988 Quality Assurance in Radiotherapy(Geneva:WHO)

CHAPTER 8

Dosimetry Equipment

8.1 Introduction

This chapter will consider the quality control of the dosimetry equipment used inradiotherapy for acceptance and commissioning, output calibration, routine quality checksand patient dosimetry systems. The items of equipment used for these tasks must first beestablished as being suitable and accurate for the type of measurement being made. Thedosimetry equipment must itself be commissioned and calibrated before using it tocommission and calibrate other radiotherapy equipment. The uncertainty in themeasurements made with the equipment should be assessed at each stage to ensure thatit does not compromise the overall accuracy required of the clinical dosimetry. ICRU 14(ICRU 1969) has analysed the uncertainties in using their dosimetry protocol and foundthem to be about ±2.3% at the calibration quality (2 MV), increasing to about ±3.3% at30 MV X-ray energies).

Development of a consistent dosimetry technique requires careful documentation ofthe procedure and clear tabulation of both the results and the calculations made to achievethe final answer. Staff responsible for commissioning, calibration or dosimetry shouldhave appropriate experience (IPSM 1992, HSE 1998).

To minimise the possibility of a significant error occurring, a second check isrecommended for all measurements of absolute dose or dose rate. This can range from adosimetry check by an independent expert with a second, independent measuring system,to a mailed dosimeter intercomparison.

In the following sections the main categories of dosimetry equipment used inradiotherapy and the tests required will be investigated, with recommendations, wherepossible, of minimum test frequencies and tolerances. In order to cover all the equipmentcategories, exact experimental techniques will not be discussed, but assumed to beaccepted procedures in accordance with current codes of practice.

8.2 Ionisation chambers

8.2.1 Introduction

Ionisation chambers and associated electrometers are used extensively in radiotherapyto measure both relative and absolute doses or dose rates. They are delicate instrumentswhich may be kept in one department or transported carefully further afield betweenseveral departments or hospitals. The frequency and extent of quality checks performedon these instruments should be tailored to their use. Choice of a particular ionisationchamber for a specific task should take account of such factors as modality, energy, doserate and angular dependence as well as the chamber size relative to the dose gradientbeing measured. The ionisation chambers which will be used for absolute dose/dose rate


measurement must be cross calibrated against a national standard instrument understandard conditions, according to national or international protocols. The importance ofexact procedures and second checks cannot be emphasised too strongly in this stage ofthe determination of dose delivered to the patient. Ionisation chambers intended formeasurement of relative doses and dose rates need not be calibrated against a nationalstandard.

It is essential that, before use, the chambers, check sources and any phantoms inwhich they are to be used should be allowed to come to thermal equilibrium with theirsurroundings. The manufacturer’s recommendations should be followed regarding thewarm up time allowed after switching on before the system is used. The reading fromthe initial exposure of the ionisation chamber after switching on is often different by asmuch as several percent from subsequent readings, particularly with high energy photons.The reason for this is not entirely clear but may be associated with surface charge on theinsulators in the ionisation chamber. The first one or two readings may therefore need tobe disregarded. Useful publications regarding the use of ionisation chambers are IEC731 (IEC 1986) and NCRP 69 (NCRP 1981).

Table 8.1. General initial and ongoing quality control tests for ionisation chambers.


Each useLeakage current 8.2.2.1 Negligible

Three-monthly and commissioningRadionuclide stability check (e.g. Sr-90) 8.2.2.8 ≤1%

Annually and commissioningCross calibration against secondary standard * 8.2.2.9, 8.2.5 Measure

Three-yearlyCross calibration against primary standard * 8.2.2.9, 8.2.4 Measure

CommissioningRecombination 8.2.2.2 Measure and correctPolarity 8.2.2.3 Measure and correctStem effect 8.2.2.4 Measure and correctLinearity with dose/dose rate 8.2.2.5 Measure and correctRange check 8.2.2.6 Measure and correctAngular dependence 8.2.2.7 Measure and correctSecond system check 8.2.2.10 <1%

* Required for absolute dose determination.

8.2.2 Initial commissioning

The checks required on a new ionisation chamber to establish reference characteristicsare discussed in this section. The parameters discussed below and listed in Table 8.1 arespecific to the chamber or chamber/electrometer combination in use and should beevaluated initially, in order to calculate correction factors to apply to subsequent readingsunder calibration or clinical conditions.

177Dosimetry Equipment

8.2.2.1 Leakage current

Leakage current (rate) depends on the integrity of connectors and cables and the correctfunctioning of the system. The leakage level should comply with the manufacturer’sspecification and be negligible over the expected time of the measurement (e.g. <0.1%).Leakage currents should be checked with the electrometer in the measurement modeboth before and after an exposure has been made (i.e. with zero and finite charge).

8.2.2.2 Ion recombination

Ion recombination is usually small for continuous radiation sources, but can besignificantly higher for the pulsed radiation emitted from a linear accelerator and shouldbe measured. The effect is instrument specific and depends on the instantaneous doserate. The correction for ion recombination may be derived using the ‘half voltagetechnique’ (Boag and Currant 1980). If the chamber reading decreases by X per centwhen the polarising voltage V is reduced to V/2, then the correction to be applied to thechamber reading with the normal polarising voltage is +X per cent. An alternative methodis to use the ‘two voltage technique’ (Weinhous and Meli 1984). The ion recombinationcorrection is typically less than 0.5 per cent. This correction is only valid if the responseof the ionisation chamber is close to linear with applied voltage. There is some evidencethat this may not be the case with some parallel plate chambers used at polarising voltagesin excess of 100 V. Care should therefore be taken when choosing a polarising voltage,in order to avoid errors in the ion recombination of up to 3 per cent. The linearity can bechecked by plotting a graph of dose recorded against polarising voltage (IPSM 1994).

In the complete absence of polarising voltage, the chamber may appear to functioncorrectly, albeit with reduced sensitivity. An additional check for the presence of thepolarising voltage is to compare readings with those from a standard instrument at twodifferent pulse rates or for a continuous and a pulsed beam.

8.2.2.3 Polarity

Ionisation chambers should be designed so that the effect on the response of reversingthe polarity of the voltage applied between the electrodes is negligible. The polarityeffect depends on the measuring depth of the chamber in the phantom and the length ofcable in the field and may even change sign between small and large depths. The polarityeffect generally increases with decreasing energy. The correction required should be<0.2 per cent, although may be as high as 1 per cent for the worst case of 5 MeV electronsor low energy photons (IAEA 1987). In the latter case, use of a different chamber isrecommended to reduce the correction required.

8.2.2.4 Stem effect

Irradiation of the stem of the chamber will generate extra scatter which is notrepresentative of the dose received by the patient. This effect should be small (typically<0.5 per cent), but can be evaluated using a long rectangular field and placing theionisation chamber along the two major field axes in the plane perpendicular to thebeam axis, in order to investigate the effect of irradiating different lengths of the stem(NCRP Report 69 1981). The effect should be evaluated for several representativesituations of intended use of the ionisation chamber, and rechecked if changes are madeto either the stem or the cable. Any correction for stem effect may be normalised to thelength of stem irradiated with the calibration field size.


8.2.2.5 Linearity

Linearity of response of the measuring system to dose or dose rate is difficult to isolatefrom the linearity of the system producing the readings, unless a constant current sourceis available. The manufacturer’s specification will give some guidance to the expectedlinearity of response, but can also be measured in the following way. A series of readingscan be taken with a fixed number of monitor units e.g. 100 MU, resetting the electrometerbetween readings, to establish the stability of the linear accelerator output. Then furtherexposures of 100 MU can be made, resetting the electrometer after, for example, 3, 5and 10 exposures, to reveal any non-linearity of the measuring system. Use of very smallexposures (e.g. <10 MU) for this test should be avoided as they may be influenced bythe stability of the accelerator. Low electrometer readings may be obtained using largermonitor unit settings with the chamber at depth in a phantom or at an extended distancefrom the source. Any non-linearity correction can be normalised to the monitor unitsetting used for the calibration.

8.2.2.6 Range check

Switching from one measuring range to another may be done after comparing readingsat the new range with those obtained on the range used for calibration, and incorporatinga correction factor where necessary.

8.2.2.7 Angular dependence

The manufacturer may be able to provide reference data on this characteristic of thechamber and reduce the need to test angular dependence if the chamber is used in astandard geometry.

If there are differences in reading with orientation (usually ≤1 per cent), the chamberstem can be marked so as always to use the same orientation. Otherwise, particularlywith a new chamber design, the response of the chamber when positioned in a range oforientations relative to the radiation source should be investigated, in order to establishthe optimum measurement orientation.

8.2.2.8 Radionuclide stability check

One of the most important of the initial measurements to be made with a chamber requiredfor absolute dose measurements is a reference reading with a long half life radionuclidesource (e.g. strontium-90 or cobalt-60) (Barish and Lerch 1992). Using a carefullydocumented procedure, the time taken to obtain a temperature and pressure correctedreading for a particular chamber–electrometer combination should be measured and usedas a reference against which the long-term stability of the system can be checked. It isimportant to align the chamber with the radionuclide source using, for example, linesmarked on each component, to ensure reproducibility of results. Change in response willreveal any change in the chamber volume. Changes may occur when the graphite capbecomes dislodged or breaks, or there is a malfunction in the electrometer. A radionuclidecheck should show results in agreement with a reference measurement to within 1 percent (IPSM 1992).

A radionuclide check may not reveal all the potential problems. For example, thechamber may become contaminated with higher atomic number dust particles and thesecould affect sensitivity, particularly in the superficial voltage range. If there is any doubtas to the correct functioning of the chamber, a measurement with a second system can beused to narrow down the source of the problem.


8.2.2.9 Cross calibration against standard

Ionisation chambers to be used for measurement of absolute dose should be crosscalibrated against a standard instrument at each energy required before use (see Sections8.2.4 and 8.2.5). The recommended frequency of calibration is set to ensure consistencyin the calibration factor but also depends on the expected drift in the measurement systemand frequency of equipment malfunction.

8.2.2.10 Second system check

Any definitive calibration of radiotherapy equipment must be checked independently,ideally by a second person with a different measurement system (IPSM 1992).Independent measurements should be performed for at least one energy and thecalculations should be checked by a second physicist at all energies. Independent checkscould also be done with thermoluminescent dosimeters mailed from another centre whichwill usually have an independent calibration traceable to national standards.

8.2.3 Ongoing quality control

Table 8.1 also shows the checks that should be made on a regular basis after the ionisationchambers are brought into service. Full records should be kept of results of calibrations,modifications, repairs, stability checks, etc. If modifications or repairs are made, repeatmeasurements of some of the parameters listed in Table 8.1 will be required to maintaingood dosimetric accuracy (depending on the nature of the modification or repair).

Each time an ionisation chamber is used, the leakage should be checked and a visualinspection of the chamber cap, stem, cable and connectors will save valuable time indetecting gross fault conditions. When an ionisation chamber malfunction is suspected aradionuclide check should be performed, and the chamber and electrometer checkedseparately, for example by substitution of a second system.

The system’s response to a radionuclide source should be compared with the referenceresponse obtained at the initial calibration. This check should be made both before andbetween cross calibrations, correcting for decay of the isotope in the period since thereference measurement. The frequency of this check will depend on the frequency of useof the instruments. For chambers in daily or weekly use it would be appropriate to checkthem every three months, but if used infrequently they should be checked before beingused.

Ionisation chambers use high polarising voltages applied between the graphite capand the metal chamber stem. They must not be used for in vivo patient dosimetry.

To reduce further the possibility of undetected calibration errors, the availabledosimetry systems (chamber + electrometer) in regular use can be circulated round thetreatment machines in a department. Regular cross-checking of this kind can reduce thefrequency of quality control checks required.

8.2.4 Secondary standard instruments

A limited number of ionisation chambers of a specified design are calibrated directlyagainst a national primary standard to a high degree of accuracy and are set aside assecondary standard instruments. The laboratory performing the calibration will issue a


calibration certificate and will usually recommend a frequency of recalibration (e.g. every3 years). Before sending the secondary standard instrument to the standards laboratoryfor recalibration, and also on its return, the chamber stability should be checked (seeSection 8.2.2.8). This procedure should also be carried out before and after each time ofuse and if the secondary standard is to be taken to another hospital, by both the host andrecipient centres. The secondary standard should be kept solely for the purpose of crosscalibration of field instruments. It will therefore typically be used once a year for crosscalibration and for recalibration of field instruments after their malfunction and repair.A secondary standard ionisation chamber should have its own radionuclide check sourceand calibrated analogue thermometer, which are sent with it to the standards laboratoryat the time of calibration. If the stability check shows the calibration factor to differ by>0.5 per cent from the calibration value then the instrument should not be used and islikely to require recalibration.

8.2.5 Field instruments

These are ionisation chambers in regular use to measure absolute and relative doses andmay be of parallel plate or cylindrical design, depending on their intended use. Chambersused for relative or absolute dose measurement need to be checked for leakage beforeuse and inspected for obvious signs of damage. In addition to these checks, the chambersto be used for absolute dosimetry should be calibrated regularly, usually at least annually,against the secondary standard instrument, following a current code of practice andguidance notes (NRPB 1988, IPSM 1990b, IPSM 1992, IPEMB 1996a,b, HSE 1998).The stability of the chamber should be checked regularly (see Section 8.2.2.8) and alsoafter malfunction and repair. If necessary, a recalibration should be done. Calibrationfactors should be measured for particular combinations of chamber and electrometer ateach energy to be used.

8.2.6 Effective point of measurement of ionisation chambers

During the preparation of this book it has become evident that practice throughout theUK in respect of correction for the effective point of measurement is not consistent (seealso Section 4.2.1.1). The recommended practice for photons is as follows:

1. When making photon measurements at the reference depth using the IPSM (1990b)protocol or the IPEMB (1996b) protocol the effective point of measurement shouldbe taken as the geometric centre of the chamber.

2. When calculating the doses at other points in the phantom the depth dose or TMRdata normally in use in the centre must be used and the depth dose at 5 cm (or 7 cm)should be used as shown on these charts.

3. When measurements of relative depth doses are being made, the effective point ofmeasurement of an ion chamber should be taken as a point 0.6 of the distance betweenthe centre of the chamber and the inside surface of the cavity (for details see Chapter4, Section 4.2.1.1). For diode detectors the effective point of measurement is the surfaceof the detector.

Since some hospitals have not followed the third recommendation in preparing theirdata the question of what difference this makes to their data needs to be addressed.


When the reference measurement is made the effective point of measurement of thechamber is not in fact the centre of the chamber, but the calibration carried out by theNPL is such as to give the actual dose at the centre of the chamber provided that it is atapproximately 5 cm deep in a water phantom. Thus they have effectively applied acorrection factor (about 0.9 per cent) to the reading. This correction factor is appropriatefor all depths in the phantom at which the rate of change of dose with depth is the sameas that at the reference depth. This is the case at every point except in the build-up zoneor at the peak. Thus except at the peak the dose calculated from depth dose data that arenot corrected for the effective point of measurement will be correct. Finally it must bestressed that it is essential that only one set of depth dose data is used and that the valueused when making the reference measurement is the value shown for 5 cm (or 7 cm)deep in that data set.

For electrons the effective point of measurement correction should always be madefollowing the IPEMB (1996a) protocol. It is, of course, also necessary to correct for thechange in stopping power ratio and perturbation factor with depth.

8.3 TLD and diodes

8.3.1 Introduction

The following discussion is a brief summary of the main factors affecting the calibrationand use of these types of dosimeter. It is intended as a guide. It is not a detailed instructionmanual, neither is it a comprehensive review of the literature. For either of these thereader is referred to one of the more advanced texts (e.g. for thermoluminescent dosimetry:McKinlay (1981) or for semiconductors: Rikner (1983)).

8.3.2 Uses and limitations

The main areas of use are summarised in Table 8.2.

Table 8.2. Summary of main areas of use of thermoluminescent dosimeters and diodes.

Dosimeter type Main uses Reference

Thermoluminescent dosimeters Dosimetry intercomparisons 8.3.2.1In vivo measurements 8.3.2.3Total body irradiation 8.3.2.3

Semiconductors Beam profiles 8.3.2.2Flatness and beam alignment checks 8.3.2.2Electron depth doses 8.3.2.2Electron energy and relative output 8.3.2.2Photon and electron output checks 8.3.2.2In vivo measurements 8.3.2.3Total body irradiation 8.3.2.3


8.3.2.1 Thermoluminescent dosimeters

In quality control, thermoluminescent dosimeters (TLDs) are particularly suited todosimetry intercomparisons between different centres because of the ease of exposure atsites remote from the initiating centre. However, the ways in which they are used makethem too slow for beam flatness and alignment measurements in normal circumstances.TLDs are vulnerable to large random sensitivity variations and so are unsuitable foroutput calibrations or checks. The sensitivity of lithium fluoride TLDs (Cameron et al1961, Fowler and Attix 1966) varies significantly with X-ray energy in the kilovoltagerange and this type is therefore not suitable for depth dose, backscatter or output factormeasurements in this energy range. However, lithium borate TLDs have an energyresponse close to that of water (Christensen 1967, Jayachandran 1970) and may be usedfor measurement of these parameters at kilovoltage energies instead of ionisationchambers where the water equivalent scattering properties and good resolution of thematerial outweigh its poor sensitivity and large random errors.

8.3.2.2 Semiconductors

Semiconductors exhibit an energy dependence in photon beams of all energies, but theirfine resolution (Dixon and Ekstrand 1982) makes them particularly suitable for beamprofile, flatness or alignment measurements. In these cases the relatively small variationin energy across the main beam and the large dose gradient at the beam edge render theeffects of sensitivity variations with energy insignificant.

The sensitivity of semiconductors varies significantly with photon energy in thekilovoltage range (Dixon and Ekstrand 1982). Therefore, like LiF TLDs, they shouldnot be used for measurement of depth-dose, backscatter or output factor measurementsin this energy range.

Even for megavoltage X-rays, a substantial proportion of the photons within thespectrum will be of a lower energy and this will change with depth and field size (Rikner1985b). Semiconductors should not, therefore, be used for backscatter or output factormeasurements in this energy range. They should only be used for depth-dosemeasurements with megavoltage X-rays after careful comparison with an ionisationchamber at the energy concerned over the full range of depths.

However, the sensitivity of semiconductors varies little with electron energy (Rikner1985a) and so may be used for the measurement of depth-dose, energy and relative outputmeasurements for electron beams.

Semiconductor sensitivity changes with dose history (Rikner and Grussel 1983) makingthem unsuitable for output calibration. However they may be used, with caution, fordaily output checks with both electron and megavoltage X-rays provided they arefrequently recalibrated. In these circumstances, regular output measurements must alsobe made with an ionisation chamber (see Section 8.5). Semiconductor measurementsalone must never be used as a basis for adjustments to the machine dose monitor oroutput charts.

8.3.2.3 In vivo and total body irradiation dosimetry

Both semiconductors and TLDs are suitable for measurements in vivo and for total bodyirradiation. In all cases, thermoluminescent dosimeters are easily affected by experimentaltechnique (Cox et al 1976) and the sensitivity of semiconductors is affected by extraneousfactors, such as temperature (Grussel and Rikner 1986) and radiation age. Great care is


therefore always necessary with such measurements. This will now be discussed in greaterdetail.

8.3.3 Dosimeter commissioning


In order to obtain reliable results, a TLD system needs to be carefully set up from thebeginning. The correct material must be chosen: either lithium fluoride, where highsensitivity or minimal random errors are required; or lithium borate, where a smallvariation in sensitivity with photon energy or water equivalent scattering is essential.Next the physical form should be chosen. Solid crystals or sintered chips are moreconvenient for most purposes. Only virgin powder (associated with careful weighing)gives sufficiently small random errors for dosimetry intercomparisons (Dutreix et al1993), although sintered chips have sometimes been used (with less accuracy).

PTFE discs embedded with thermoluminescent powders are not suitable for use inradiotherapy (unlike radiation protection) since they cannot be heated to a sufficientlyhigh temperature to anneal out the effects of radiation history on sensitivity (Wilson etal 1966).

The choice of build-up cap is also important. At megavoltage X-ray energies, andeven for 60Co and 137Cs, the thickness of a TLD chip is small compared with the electronrange and it will therefore behave as a Bragg-Gray cavity. Thus the primary electronsproduced in the build-up cap will deposit energy in the TLD and the atomic number ofthe cap will affect sensitivity. Metal will have a significant effect on the energy responseand should not normally be used. If its use is unavoidable, account must be taken ofpossible variations of sensitivity with depth and field size. Even differences betweenone plastic and another significantly affect sensitivity (Miller and McLaughlin 1982).Therefore a single material should be chosen and used consistently throughout the system.For the same reason, where a TLD chip is to be used for measurements in a phantom itshould be calibrated in a phantom of the same material unless it is used with a build-upcap. A build-up cap should always be used for in vivo measurements with TLD atmegavoltage X-ray energies although it may be reduced in thickness in somecircumstances. For kilovoltage X-rays the size of a TLD chip is large compared with theelectron range and it should therefore be used without a build-up cap (although a thin,protective covering may be necessary in some circumstances). When measurements aremade directly in water, care must be taken to ensure that the build-up cap or protectivecovering is leakproof. Otherwise the TLD material may dissolve.

Where a new TLD material, physical form or TLD reader is used for the first time aglow curve should be measured. The TLD reader should then be adjusted to ensure thatthe low temperature peaks are adequately read out before the integration period starts(McKinlay 1981) and that the integration does not stop until the main glow peak hasbeen fully read out and the light output has fallen back to an insignificant fraction of itspeak. Failure to do this will result in fading or large random errors respectively. A copyof the glow curve should be kept as a reference for future comparisons.

An adequate annealing temperature and time should be established either frompublished data or from measurements. A furnace will usually be necessary for annealing,as the annealing cycle provided on TLD readers, while sufficient for radiation protection,is rarely adequate for radiotherapy doses (Wilson et al 1966).


TLD chips should be handled with vacuum tweezers in an environment free fromobvious loose dust. The grease and acids on skin will chemically affect the luminescenceon the surface of the chips and forceps will scratch them (Cox et al 1976). Any remainingchemiluminescence can be reduced by using a nitrogen flow through the reader to ensurethat the TLD chip is heated in oxygen free conditions.

If necessary, TLD chips may be cleaned in methanol (Spanne 1979) but an ultrasonicbath should not be used (Mason et al 1975). The materials should also be stored awayfrom ultraviolet radiation and high ambient temperatures. Provided that this is done andthat the heating cycle is carefully chosen, fading and ultraviolet induced luminescencewill not be significant in the measurement of doses of more than 10 mGy (McKinlay1981). Greater precautions are necessary for high precision or low dose measurements.


Only semiconductors specifically designed for radiation dose measurements should beused. Other types will, almost certainly, not have suitable characteristics. For a fulldiscussion of this and of the advantages and disadvantages of different types ofsemiconductor, the reader is referred to the literature.

Some new semiconductor dosimeters should first be aged (Rikner and Grussel 1983),i.e. they should be exposed to a large dose in order to reduce the effects of dose historyon sensitivity, which are worst in their initial stages. This may have already been doneby the manufacturer. However this will also increase the variation of sensitivity withtemperature (Grussel and Rikner 1986). With low energy photons, the rate of change ofcalibration is slower, making ageing less important. A balanced decision will thereforehave to be made, based on the particular application, the characteristics of the particularsemiconductor type and the manufacturer’s recommendations. Semiconductor detectorsfor measurements with megavoltage X-rays are generally provided with integral build-up, sometimes including a thickness of metal. Since semiconductors already have a largevariation of sensitivity with photon energy (Wright and Gager 1977, Johansson 1982)the presence of metal will not significantly worsen this response (unlike TLDs) and maysometimes improve it (Gager et al 1977, Rikner 1985b).

The variation of dark current with temperature and of sensitivity with temperature,dose rate and direction should then be measured (Rikner and Grussel 1987). The use ofreliable manufacturers’ type-testing data will reduce the number of measurements thatneed to be made and the number of dosimeters that have to be rejected. The conditionschosen should reflect those likely to be used. For example, if the dosimeter is to be usedfor in vivo measurements it should be in contact with a thin part of the wall of a largewater tank and the temperature of the water varied slowly over a range from the maximumbody temperature to the minimum ambient temperature.

8.3.4 Methods of calibration


TLD chips must first be batched. The whole of the batch should be exposed to the samedose within ±2 per cent. This is most easily achieved in the central portion of a fieldfrom a cobalt-60 unit. If a linear accelerator has to be used then the position of thedosimeters should be chosen carefully to minimise flatness variations and if necessarythey should be rotated by 180° halfway through the exposure to average out asymmetry.


The TLD chips should then be read out and the procedure repeated a number of times,interchanging the positions of individual dosimeters.

The mean response of each chip and its standard deviation should then be calculatedand hence the relative responses of the chips. The chips should then either be allocatedan individual sensitivity or divided into groups of the same sensitivity. The range ofsensitivities within a group will depend on the accuracy required for the intendedapplication, although ±1 per cent is suitable for most purposes.

Chips with large standard deviations (i.e. those which would result in an overall randomerror greater than that required for the intended application) should be rejected. Herecompromise is sometimes necessary and standard deviations of up to 4 per cent may beaccepted and compensated for by increasing the number of measurements. Great careneeds to be taken to ensure that the chips do not become mixed. Chips with individualsensitivities can be marked on their underneath with a fine waterproof marker pen. Thosewhich are grouped should be stored in separate marked containers and groups neverused together unless they can be easily identified, e.g. by using marked build-up caps.TLD powder, on the other hand, should be well mixed before use.

For entrance and exit surface dose measurements, TLD materials should be calibratedwith build-up caps on the surface of a tissue equivalent phantom. This is discussed furtherin Chapter 10, Sections 10.1.3.1 and 10.1.3.2. For other measurements TLDs should beexposed at depth in a tissue equivalent phantom. The build-up caps, or any protectivecovering used, should be identical to those used for the measurements. (If none are usedfor the measurements, then none should be used for the calibration). As discussed earlier,at megavoltage energies the thickness of a TLD chip is small compared with the electronrange and will therefore behave as a Bragg–Gray cavity. Even a large capsule of TLDpowder will behave as a partial Bragg–Gray cavity. Primary electrons from the phantomare thus contributing significantly to the energy deposited and good tissue equivalenceis required. Neither PMMA nor polystyrene are suitable as phantom materials forkilovoltage measurements unless the subsequent measurements are to be made in aphantom of similar construction. For kilovoltage X-rays good tissue equivalence isrequired for a different reason. The difference between the X-ray absorption and scatteringproperties of PMMA or polystyrene and tissue will significantly affect X-ray spectraand scatter factors. A suitable general calibration phantom is shown in Figure 8.1.Following normal good practice, at least 10 cm depth beyond the TLDs and at least 5 cmmargin round the edge of the field should be allowed to ensure full scatter conditions.For measurements near the exit surface the calibration phantom should be reversed, sothat there is at least 10 cm depth above the TLDs and adequate build-down betweenthem and the exit surface. When possible, at least 5 cm margin should be allowed betweenthe TLDs and the edge of the field to ensure good dose homogeneity. If build-up caps arenot used, the TLDs should be placed sufficiently far apart that each is surrounded by abuild-up region composed only of the phantom material, beyond the range of electronsfrom other detectors.

During calibration for general measurements, several dosimeters should be exposedsimultaneously with an ionisation chamber at the same depth, as shown in Figure 8.1. Inthe case of entrance surface dose measurements for in vivo dosimetry, the TLDs shouldbe placed in build-up caps on the surface of the phantom with the ionisation chamber atthe depth of full build-up. Similarly, for exit dose measurements, the TLDs should beplaced in build-up caps on the exit surface of the phantom, with the ionisation chamberat the depth of full build-down from this surface. This is discussed further in Chapter 10,


Section 10.1.3.2. The number of dosimeters chosen should reflect the final accuracyrequired and if necessary the exposure should be split and the positions of the dosimetersinterchanged in order to reduce systematic errors due to beam non-uniformity. This is

Figure 8.1. Calibration phantom for TLDs and semiconductors.


particularly necessary for calibration with linear accelerators.For general measurements with megavoltage X-rays and 60Co, beyond the depth of

maximum build-up the effects of depth on TLD sensitivity are not significant (Aukett1991) and any convenient depth may be used. However the effects of changes in sensitivitybetween one megavoltage beam energy and another are significant as they are for electronsand kilovoltage X-rays (Cameron et al 1961, Fowler and Attix 1966). The calibrationshould therefore always be done at the same energy as that which would be used forsubsequent measurements. A different machine may be used only if the change insensitivity has been measured and found to be insignificant. For kilovoltage X-rayschanges in spectra (e.g. with depth) will significantly affect the sensitivity of LiF becauseof the variation of the response with photon energy. Lithium borate should therefore beused where measurements are to be made at different depths or under different scatteringconditions at these energies.

Where the doses to be measured are above 1 Gy, supralinearity may be significant(Suntharalingam and Cameron 1969). It will therefore be necessary to expose the TLDsto a number of different doses and to construct a graph of response versus dose.

The measurement of the background light output of a number of unexposed TLDs,under the same conditions of nitrogen flow, will always be required. The backgroundshould then be subtracted from the readings.

Each time a dosimeter is annealed sensitivity, background and supralinearity will allchange (McKinlay 1981). It is therefore necessary to anneal all the TLDs from one batchtogether under closely controlled conditions and to follow the whole of the calibrationprocedure after each annealing.


The calibration phantom for semiconductors is similar to that used for TLDs and is shownin Figure 8.1. They should be exposed simultaneously with an ionisation chamber underthe same calibration conditions as for TLDs, except for the following differences. Likethermoluminescent dosimeters, at megavoltage energies beyond the depth of maximumbuild-up the variation in the sensitivity of semiconductors with depth will not normallybe significant (Aukett 1991). However their sensitivity may vary from one energy toanother (Wright and Gager 1977, Johansson 1982, Rikner 1985b).

In the case of electron beams these variations may be small (Rikner 1985a) and afteran initial calibration at all energies subsequent calibrations at a single energy may besufficient. The sensitivity of semiconductors varies too much with photon energy (Dixonand Ekstrand 1982) for them to be used with kilovoltage X-rays for anything other thanprofile measurements. For use with megavoltage X-rays, semiconductor detectors aregenerally provided with integral build-up of a thickness at least equal to the approximateelectron range. The tissue equivalence of the surrounding phantom is therefore not critical.

Several semiconductors can be calibrated at the same time and, where necessary, theirpositions and that of the ionisation chamber should be interchanged to reduce systematicerrors due to beam non-uniformity. Unlike TLDs, the sensitivity of semiconductors doesnot vary with the calibration dose and any suitable value may therefore be used. Howeverthere may be a variation of sensitivity with dose rate and this may vary with dose history(Grussel and Rikner 1984).

In the case of pulsed dose rates, such as those from a linear accelerator, the criticalparameter is the instantaneous dose rate during the pulse, not the pulse repetitionfrequency. Therefore each calibration should be carried out at a range of instantaneous


dose rates covering those which are to be used in subsequent measurements. Whererelative measurements are required using several dosimeters then the ionisation chambermay be omitted. Where relative measurements with a single dosimeter are required thenit is only necessary to check the variation of sensitivity with dose rate at less frequentintervals. If depth doses are to be measured then the most appropriate method ofcalibration is to compare a depth-dose curve measured with the semiconductor with onemeasured immediately beforehand on the same machine and under the same conditionsusing an ionisation chamber, taking into account the positions of the effective measuringpoints within the dosimeters.

The dark current should be checked and adjusted to zero before each measurement. Insome instruments this is achieved by setting the input offset voltage to zero.

Where it is not set by this means, the amplifier input offset voltage must also be set tozero. Otherwise the dark current will increase significantly with temperature. In theory,with no offset voltage there can be no dark current. Since this can never be perfectlyachieved, the dark current may still vary slightly with temperature (Grussel and Rikner1986) and it will be necessary to wait until the temperature of the dosimeter and phantomhave stabilised. Where the sensitivity of the semiconductor was found at commissioningto vary with temperature then it will also be necessary to calibrate at a temperaturesufficiently close to that which is to be used for subsequent measurements. The calibrationof semiconductors for the measurement of entrance and exit surface doses is discussedin Chapter 10, Section 10.1.3.1.

8.3.5 Frequency of calibration


Calibration, and all the associated checks, should be performed after each annealing.This and the other check frequencies are summarised in Table 8.3.

Table 8.3. Summary of thermoluminescent dosimeter checks.

Frequency Check Reference

Commissioning Glow curve 8.3.3.1Annealing cycle 8.3.3.1

After each annealing Calibration 8.3.4.1Variation with dose 8.3.4.1Variation with machine and energy 8.3.4.1

Annual (or as required) Relative sensitivity within batch 8.3.4.1Random sensitivity variationsof all individual dosimeters 8.3.4.1

Re-batching should be carried out as often as is necessary to find any chips whichhave changed their relative sensitivity or whose sensitivity has become unstable.

This will vary according to the frequency with which the dosimeters are exposed, thetype of measurements undertaken and the care with which they are handled. A commoncourse of action is to re-batch annually and to use three exposures for this. A better


system if suitable records are kept, is to review the standard deviation of the TLD readingsregularly and to re-batch when these become unacceptable. TLD readers should beserviced regularly (at least annually). In order to check that the readout cycle has notchanged significantly, the glow curve should be measured immediately after servicing.


The frequency of recalibration required depends on the dose history of the semiconductorsand the characteristics of their particular type. Unless the semiconductors accumulatedose in a very regular manner, dose intervals should be used rather than time intervals. Afrequency should be chosen which is sufficient to anticipate changes in sensitivity of 2per cent or less. Until this has been established calibration should be carried outapproximately every 10 Gy (Rikner and Grussel 1983). Alternatively, the initial (but notthe final) frequency may be decided on the basis of manufacturers’ testing and theexperience of other centres with semiconductors of the same type. This and the othercheck frequencies are summarised in Table 8.4.

Table 8.4. Summary of semiconductor checks.

Frequency Check Reference

Commissioning Radiation ageing (if required) 8.3.3.2Variation with dose rate 8.3.4.2Variation with machine and beam energy 8.3.4.2Variation with direction 8.3.3.2Variation with temperature 8.3.3.2Calibration 8.3.5.2

Every 10 Gy or as required Recalibration 8.3.5.2Calibration variation with machine and energy 8.3.4.2Changes in variation with dose rate 8.3.4.2Changes in variation with temperature 8.3.3.2

At each use Set zero 8.3.4.2Ambient temperature stabilisation 8.3.4.2

8.4 Plotting tanks

8.4.1 Introduction

This section deals with the control tests needed to ensure that a plotting tank system,comprising a water tank, positioning system, ionisation chambers and electrometers, iscapable of performing satisfactorily (Mellenberg et al 1990). The use of such a systemto collect beam data is discussed in Section 4.2.1.2.

Modern plotting tanks possess a variety of features and can be used for a wide rangeof measurements; the advice offered here can be adapted to meet local needs. The readershould consider carefully the task being undertaken and the accuracy required beforefinalising a quality control programme. Table 8.5 summarises the checks to be madewith the recommended frequencies and tolerances. Because of the key role of beam data


acquisition, the plotting system quality control should be included in the departmentalquality system and all test results, software updates, etc. must be documented.

8.4.2 General safety

The water in a plotting tank can weigh up to 180 kg and therefore the loading capabilityof supporting structures should be considered and their safety checked regularly. Plottingtanks should not be placed directly on the top of treatment machine couches unless theweight is determined to be within the couch loading tolerance. Hydraulic lifts used toraise tanks may fall gradually or stepwise under the weight of the tank. Regular checksof the source to water surface distance (SSD) should be made during a series ofmeasurements to ensure that the SSD does not deviate by more than 2 mm, which wouldresult in a 0.4 per cent change in dose at 100 cm SSD.

The proximity of water to the high voltage power supplies for ion chambers and mainsvoltage for ancillary equipment has potentially fatal consequences.

The electrical safety of the equipment must be checked at regular intervals by aqualified person. In particular all metal parts must be correctly earthed.

8.4.3 Regular checks on the tank and the accuracy of movements

The plastic tank should be inspected for signs of damage, distortion and water leakageespecially at thin X-ray windows and along the tank seams.

To minimise tank distortion, water should not be left in the tank any longer thannecessary. Metal structures should be free from corrosion. Some manufacturersrecommend demineralised water with an algae inhibitor to limit corrosion and keep thewater clear. Limit switches must prevent collision of any part of the movement mechanismor radiation probe with the tank sides. Extra care is particularly necessary when usingionisation chambers which may protrude a variable distance from the holder.

The positional accuracy (e.g. 0.5 mm) and repeatability (e.g. 0.1 mm) in any onedimension will usually be specified by the manufacturer but should be verified by theuser. Ideally, measurements should be made from a fixed point in space to the probe, butsince most medical physics departments will not have a three-dimensional measuringdevice capable of this task, the following checks constitute a practical test regime. Physicalinspection of the drive arm parts should indicate no loose components. Probe holdersshould keep the probe in a stable position, well away from metal structures. Visualobservation of each drive should indicate smooth operation in both directions of travel.

The orthogonality of movements should be checked, for which a reference frame isneeded. The sides of the tank may be a sufficiently accurate reference or in a practicalcontext it may be adequate to prove that the probe can move parallel and perpendicularto the water surface. An error of 0.3° will result in the probe moving 2 mm away fromthe true position over a distance of 400 mm, which may be acceptable for the axesperpendicular to the beam axis. However, in order to maintain acceptable symmetry inprofiles across the beam at depth, an accuracy of ≤0.1° may be required.

The accuracy of positional readouts can be checked using a calibrated ruler or verniercalliper on each drive mechanism and should be within 0.5 mm. Backlash in the drivemechanism can be checked by taking measurements in both directions of movement,and should be less than 0.2 mm.


The effect of the speed of the movements should be assessed. Too low a speed willresult in inefficient use of treatment machine time, while too high a speed may haveseveral consequences. A high speed close to the water surface may produce waves whichwould affect the accuracy of cross plots (plots across the beam at a constant depth)particularly. Too short a time for data collection may result in unacceptable signal noise.In older systems, which measure dose rate, hysteresis effects can result from the lagbetween the dose rate signal and the indication of position. This can be observed byanalysis of plots made in different directions of travel, from which an appropriate scanspeed can be chosen to give a positional accuracy of ≤0.2 mm.

The repeatability of positional readouts can be checked most thoroughly by performingrepeated dose plots in a stable radiation beam.

For cross plots the beam edges (defined as the points in the penumbra at which 50 percent, or sometimes 80 per cent of the central axis dose occurs) act as reference points.The variation in the calculated positions of the beam edges and beam width are a measureof the repeatability of the whole plotting system, which should be within 0.2 mm.Similarly, repeated plots can be analysed in terms of the depths of selected dose levels tocheck the readouts in the depth direction. The influence of the direction of travel andspeed of travel on the accuracy of positional measurements should also be checked withthis method.

8.4.4 Checks on radiation detectors and electrometers

In general, it is preferable to use only detectors which have undergone thorough typetesting, by which the magnitude of factors such as ion recombination, stem scatter, cableleakage and polarity effect are thoroughly investigated. Detectors with thin walls maysuffer from distortion due to the varying hydrostatic pressure in the water tank. Diodesmay produce results significantly different from ion chamber measurements and detectorsshould be suitable for the intended task.

Visual checks should be made to ensure that detectors, cables and connectors are ingood condition. The integrity of the waterproof sheath should be checked regularly.

The polarising voltage required for ion chambers should be checked using a highimpedance meter with the detector disconnected from the electrometer. To prove that thevoltage is actually present at the probe and is of adequate magnitude, ion recombinationlosses should be measured (see Section 8.2.2.2).

Leakage currents in the detector/electrometer combination should be checked asoutlined in Section 8.2.2.1.

The effect of radio frequency fields generated near linear accelerators on the radiationdetectors and electrometers attached to the water tank should be checked. This can bedone easily if the linear accelerator can be run in a test mode with the electron gunswitched off.

Most plotting systems use a reference detector to compensate for drifts and changesin the dose rate produced by linear accelerators, or measure the detector signal for astated number of pulses generated by the treatment unit dosimetry system. Whichevermethod is used, the stability of the compensated signal should be 100±0.2 per cent whenthe sample probe is placed at the normalisation point. This degree of stability should bemaintained over periods needed to complete plotting sequences. The compensated samplelevel can be visually observed and with some systems it is possible to calculate theaverage reading and its standard error.


Table 8.5. Checks on plotting tanks, frequencies and tolerances.


Before first useSoftware check 8.4.5 Functional

Before commissioning a treatment unitElectrical and mechanical checks:

Earth tests 8.4.2 FunctionalGeneral inspection 8.4.2 FunctionalSafety of supporting structures 8.4.2 FunctionalHydraulic lifts 8.4.2 2 mmPlastic tank 8.4.3 FunctionalDrive arms 8.4.3 FunctionalProbe holders 8.4.3 FunctionalSmooth movements 8.4.3 FunctionalOrthogonality of drives 8.4.3 0.3°

Position indicator accuracy 8.4.3 0.5 mmBacklash in position readout 8.4.3 0.2 mmEffect of speed of movement 8.4.3 0.2 mmRepeatability of position readout 8.4.3 0.2 mmCorrect choice of radiation detectors 8.4.4Visual checks of radiation detectors 8.4.4 Functional

Radiation measurements:Polarising voltage 8.4.4 PresentIon recombination losses 8.2.2.2, 8.4.4 0.5%Leakage current 8.2.2.1, 8.4.4 0.1%Effect of RF fields 8.4.4 0.1%Stability of compensated signal 8.4.4 0.2%Standard percentage depth dose plot 8.4.4 0.5%Tests of programmed movements 8.4.5

During collection of dataStability of compensated signal 8.4.4 0.2%Leakage current 8.4.4 0.1%Constancy of standard percentage depth dose plot 8.4.4 0.5%Standard profile plots: Flatness 8.4.4 ±3%Standard profile plots: Field size 8.4.4 2 mm

The absolute accuracy of the radiation measurements must be independently assessed.Several percentage depth dose plots should be made with the water tank system understandard conditions. The average values of percentage depth dose and their standarddeviations should be calculated. It is very important to check measurements with anindependent system, for example, using an ionisation chamber and electrometer in anintegrating dose mode at fixed depths in a water tank. The two sets of data should agreeto within 0.5 per cent, expressed in terms of the percentage of the dose at the dosemaximum, or 1 mm in terms of displacement, whichever represents the largerdisagreement. It should be emphasised that a 0.5 per cent difference is equivalent to a2.5 per cent error in local dose at the 20 per cent level.

During commissioning of a treatment unit, the overall constancy of performance ofboth the plotting tank and the treatment unit should be checked at regular intervals understandard conditions. This can be done as follows: (i) repeated depth dose measurements


should agree with the values obtained in the absolute accuracy determination within 0.4per cent; and (ii) profile plots should indicate stable beam flatness and field size towithin ±1 per cent and 1 mm respectively.

8.4.5 Checks on software by the manufacturer and the user

Software packages can be very complex and are rarely free from errors. It is ofconsiderable importance that each feature of a program is tested before it is used tocollect or analyse data for clinical use. A new software version should be treated likenew software.

Movements, programmed for specific tasks, should be tested to ensure that the detectorcannot move into collision situations.

It is preferable that purchasers obtain plotting tank systems from manufacturers whocan demonstrate adherence to a recognised quality management system and who will beactive in solving software and hardware problems.

8.5 Constancy instruments

These ‘black boxes’, encasing ionisation chambers or diodes, are often used instead of afield instrument, for daily checks on, for example, output, energy or flatness ofmegavoltage photon and electron beams. They are usually more robust than the ionchambers used as field instruments and are simple to set up and use. As their namesuggests, they are only designed for checking the stability or constancy of a particularparameter, rather than its absolute value.

Readings with a constancy instrument (CI) should be established by direct substitutionwith a calibrated ionisation chamber in water or water substitute material for output,energy and flatness measurements. The readings on the CI which correspond to thetolerance limits of the measured parameter should be established (e.g. ±2 per centtolerance on output measured with an ionisation chamber may not be ±2 per cent changein reading on the CI).

Confirmatory measurements of the machine’s output calibration must be made atregular intervals not greater than 1 week for linear accelerators and 1 month for cobalt-60 units (NRPB 1988, IPSM 1992). This guidance must be followed; the CI must beevaluated carefully if results from it are to be relied upon to confirm the output calibration.The commissioning of a CI should include evaluation of the rate of drift of the readingsby performing a regular cross-check against a calibrated field instrument. Once thestability of the calibration of the CI has been established, a less frequent check of the CIcalibration against a calibrated ionisation chamber (e.g. three-monthly) should besufficient. If a CI reading is found to change, the first step should be to insert an ionisationchamber into the beam to establish whether the CI reading or the machine output haschanged. The user should be aware that the device may respond differently from anionisation chamber, e.g. in energy dependence, linearity, etc. but this is not usually ofconcern if only the stability of a parameter is under investigation. Constancy devicescan be used for energy constancy checks, using extra build-up.


As with ionisation chambers, a leakage check and a quick visual inspection to checkfor obvious problems should be performed each time the instrument is used.

8.6 Thermometers and barometers

Corrections for variation in air density are necessary for unsealed ionisation chambers,correcting to standard temperature (293.15 Kelvin) and standard atmospheric pressure(101.325 kPa or 1013.25 mbar), in order to be able to use the calibration data from thestandards laboratory. It is important to include these corrections for measurements ofabsolute absorbed dose and therefore to have the means of measuring temperature andpressure correctly.

Quality control of thermometers and barometers is just as important as that forionisation chambers. Both thermometers and barometers should have a calibrationtraceable to national standards. A limited number of instruments may be kept as secondarystandards within a hospital, group of hospitals or, more usually, in an accredited laboratory(e.g. National Measurement Accreditation Service (NAMAS) in the UK), against whichinstruments in routine use may be calibrated.

The frequency of calibration of the routine instruments against the secondary standardsdepends on the frequency of use of the instrument and the liability of the instrument todrift. More frequent calibrations should be performed until the stability of the system isestablished, particularly for digital devices. Both thermometers and barometers shouldbe calibrated at the extremes of temperature and pressure found in the clinicalenvironment. It is also good practice to check readings from the routine instrument againsta second routine instrument at more frequent intervals between cross calibrations.

8.6.1 Thermometers

The most common types are mercury or spirit in glass, with an analogue display, or ofthe resistor, thermocouple or thermistor type with an electronic digital display. Thesecondary standard instrument should have an accuracy of ±0.5˚C and the routineinstrument of ±1.0˚C. The mercury and spirit in glass types are inherently more stablethan the electronic types and will require initial calibration only. The electronic digitaldisplay thermometers are usually more robust and portable and are therefore morepractical for regular use but will, however, need more frequent calibration.

8.6.2 Barometers

The secondary standard and routine instruments should have an accuracy of ±1.0 mbaror ±1 mm mercury. The secondary standard barometers are commonly of the Fortin ormore accurate analogue aneroid types, and the routine instruments of the digital aneroidtype, the latter needing more frequent calibration checks. Fortin barometers are calibratedat 0°C and 0° latitude. The reading from a Fortin barometer needs to be corrected for thethermal expansion of the brass scale and for the change in acceleration due to gravity atother degrees of latitude (Kaye and Laby 1973).


A source of a second, independent, check on ambient pressure is a reading from ameteorological office in close proximity to where the measurement is being made.Meteorological offices usually have barometric instruments with calibrations traceableto national standards and take pressure readings at fixed times during the day. Thepressures quoted by them will only be valid at the time of the reading and apply at meansea level.

Corrections for altitude will need to be made if their readings are to be used to calculatecorrection factors, but these readings can be used to give a second, independent check ofambient pressure.

8.6.3 Hygrometers

Corrections for ambient humidity, as measured with a hygrometer, are required bycalibration procedures, but ionisation chambers are designed to give a stable readingover a wide range of humidity, e.g. 20–80 per cent. This results in humidity correctionsrarely having to be made.

8.7 Ancillary equipment

There are numerous extra devices used in quality control programmes, including rulersand graph paper for checking field size, distances, etc. The accuracy of such devicesshould never be assumed, but needs to be checked upon initial receipt of the goodsbefore their integration into quality control programmes. Uncalibrated devices shouldbe marked ‘for indication only’.

8.8 HVL filters

Half value layer (HVL) is measured during the commissioning and routine quality controlof kilovoltage treatment units, and quoted in terms of thickness of the material in whichit is measured, usually aluminium or copper.

It is important that the purity of the material is high and of a known value in order forthe HVL measurements to be meaningful and comparable between centres. It isrecommended that purity be at least 99.2 per cent for copper and at least 99.9 per centfor aluminium (IAEA 1994).

The thickness of material in each individual sheet should be measured and marked onthe sheet.


8.9 Solid phantoms

Ideally, absolute and relative dose measurements to establish and check the characteristicsof radiotherapy treatment machines should be made in a water phantom, with the radiationentering through the surface of the water or through a thin window in the side of thetank. However, it is time consuming to set up a water tank at each quality control session,and blocks of water substitute materials (ICRU 1992) or sealed boxes containing waterare in common use. A range of checks which should be carried out initially and at regularintervals thereafter are detailed in Table 8.6.

The phantom material may show ageing from the doses of radiation received andshould be inspected regularly. The frequency of checks on the composition of the watersubstitute material, brittleness, etc., will depend on the doses received and guidanceshould be sought from the manufacturers of the material. Variation in composition betweennominally the same material can occur during manufacture. It is recommended to checka sample of each new batch for water equivalence.

The composition of solid water substitute materials will need to be tailored to theintended use, since the composition which is water equivalent for megavoltage photonenergies will not be water equivalent for low energy electrons.

Due to the permeability of PMMA (Perspex or Lucite) to water, boxes containingwater will warp and so will need to be topped up or emptied after use. The outerdimensions of water-filled phantoms should be checked annually.

Table 8.6. Quality control of solid phantoms.

Frequency Check

Commissioning Volume, mass and linear thickness for each solid water substitute sheetDensity calculation and comparison with density of water for each sheet,at each energyDepth of chamber insert below phantom surface

Each time used Visual check for chipped edges, cracks, leaks, etc.Temperature equilibration to ambient conditions

Annually Depth of chamber insert below phantom surface for water filled phantoms

Every three years Ageing of phantom material

8.10 Film dosimetry

Photographic film has been in use for many years for both relative and absolute dosimetry,and its uses and limitations are generally known and understood. Its most common usesin quality control in radiotherapy are for the size and agreement of light and radiationfields, and for the flatness and symmetry of the radiation field. It is also used for themeasurement of percentage depth dose and isodose information (Williamson et al 1981),particularly in electron beam dosimetry (Shiu et al 1989). Quality control of the film


dosimetry process should include all items in the system: the film itself, any cassetteused, the phantom, the film processor and the optical densitometer.

Use of film relies on being able to relate the optical density (OD) to the dose received.The relationship between the two varies with the type of film and the film batch, andcareful calibration of the film over the range of expected doses is required. Potentiallythe most variable parameter in the film dosimetry process is the film processor in whichchanges in the chemicals may produce significantly different ODs for the same givendose and film type. If the ratio of film densities to dose is to be relied upon, carefulquality control of the processor system is necessary. For the most exacting requirements,it will be necessary to calibrate the system at the time that the measurements are made.When using film to measure the radiation field size, a simple approach is to make anexposure on the same film at 50 per cent of the dose (or 80 per cent if the 80 per centfield size definition is in use), although this would not be necessary if the film processoris sufficiently stable. If a standard dose is given, the resulting OD can be used to assessprocessor stability.

When analysing percentage depth dose or isodose information, a range of calibrationdoses will be required to correspond to the range of doses on film.

The OD of the film is measured using a densitometer, with a small light beam anddetector. The stability of this system should not be assumed but checked to ensureconstancy over the time needed to analyse a set of films with interdependent results.

Film used for dose measurement should be used with the utmost caution due to thedifficulties already mentioned and the fact that its energy response differs from that ofionisation chambers, diodes or TLD. When irradiating a film ‘end on’, the beam shouldbe angled a few degrees in order to avoid the effects of the lack of tissue equivalence ofthe film on the primary beam. Independent verification of measurements made with filmshould always be made.

Phantoms and cassettes used for film dosimetry should produce tissue equivalent scatterand should be checked regularly to ensure that any markers, etc. are correctly positioned.

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IPEMB (Institution of Physics and Engineering in Medicine and Biology) 1996b TheIPEMB code of practice for the determination of absorbed dose for x-rays below 300 kVgenerating potential (0.035 mm Al – 4 mm Cu HVL; 10–300 kV generating potential)Phys. Med. Biol. 41 2605–2626


IPSM (Institute of Physical Sciences in Medicine) 1990a Guidance on assessment criteriafor medical physicists in different grades prepared by the Board of Assessors of theInstitute of Physical Sciences in Medicine. HPA Bulletin December 1990 (York: IPEM)(Available separately as a Policy Statement 1994)

IPSM (Institute of Physical Sciences in Medicine) 1990b Code of practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service.Phys. Med. Biol. 35 1355–1360

IPSM (Institute of Physical Sciences in Medicine) 1992 Procedures for the definitivecalibration of radiotherapy equipment Scope 1 No 1 [Reproduced in Appendix A]

IPSM (Institute of Physical Sciences in Medicine) 1994 Radiotherapy News Sheet No.10 July 1994 (York: IPEM)

Jayachandran CA 1970 The response of thermoluminescent lithium borate equivalent toair, water and soft tissue and of LiF TLD-100 to low energy X-rays Phys. Med. Biol. 15325–334

Johansson KA 1982 Studies of different methods of absorbed dose determination and adosimetric intercomparison at the Nordic radiotherapy centres. Thesis, University ofGothenburg.

Kaye GWC and Laby TH 1973 Tables of Physical and Chemical Constants and SomeMathematical Functions 14th Edition (London: Longman)

Mason EW, McKinlay AF, Clark I and Saunders D 1975 Thermoluminescence SensitivityVariations in LiF:PTFE Dosemeters Incurred by Improper Handling Procedures NationalRadiological Protection Board Report 37 (Harwell: National Radiological ProtectionBoard)

McKinlay AF 1981 Medical Physics Handbooks 5, Thermoluminescence Dosimetry(Bristol: Adam Hilger)

Mellenberg DE, Dahl RA and Blackwell CR 1990 Acceptance testing of an automatedscanning water phantom Med. Phys. 17 311–314

Miller A and McLaughlin WL 1982 Calculation of the energy dependence of dosimeterresponse to ionising photons Int. J. Appl. Radiat. Isotopes 33 1299–1310

NAMAS Directory of Accredited Laboratories NAMAS Executive, National PhysicalLaboratory, Teddington, Middlesex TW11 0LW

NCRP (National Council on Radiation Protection and Measurements) 1981 (Thirdreprinting 1993) Report 69 Dosimetry of X-ray and Gamma-ray Beams for RadiationTherapy in the Energy Range 10 keV to 50 MeV (Bethesda MD: NCRP)


Rikner G 1983 Silicon diodes as detectors in relative dosimetry of photon, electron andproton radiation fields. Thesis, Uppsala University.


Rikner G 1985a Characteristics of a p-Si detector in high energy electron fields. ActaRadiol. Oncol. 24 65–71

Rikner G 1985b Characteristics of a selectively shielded p-Si detector in 60Co and 8 and16 MV roentgen radiation Acta Radiol. Oncol. 24 205–208

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Rikner G and Grussel E 1987 Patient dose measurement in photon fields by means ofsilicon semiconductor detectors Med. Phys. 14 870–873

Shiu AS, Otte VA and Hogstrom KR 1989 Measurements of dose distribution using filmin therapeutic electron beams Med. Phys. 16 911–915

Spanne P 1979 Thermoluminescence dosimetry in the mGy range – Theoretical andexperimental investigations of the optimum performance of a LiF-TLD system ActaRadiol. Suppl. 360

Suntharalingam N and Cameron JR 1969 Thermoluminescent response of lithium fluorideto radiations with different LET Phys. Med. Biol. 14 397–410

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CHAPTER 9

Quality Control in Brachytherapy

9.1 Introduction

The principal objective in brachytherapy is to deliver safely a prescribed radiation doseto a specified area or volume of tissue by means of encapsulated radioactive sourcesimplanted or placed in close proximity to the irradiated tissues. In any system, qualitycan only be measured if the need has been defined. In brachytherapy, the objectivesmight be considered under two headings:

1. Delivery of the requisite dose to the tissue volume.2. Control of dose outside the tissue volume being irradiated.

In other words, one should consider those aspects of the system which are directed to thedelivery of the prescribed dose within acceptable limits and also how best to ensure thatthe treatment can be achieved in accordance with the ALARA principle.

A quality assurance system will give assurance that specific objectives are being met.Like any other quality system, quality assurance in brachytherapy is addressed best bybreaking down the system into its component parts and answering three questions:

1. What is the objective of this procedure? (Policy)2. How is this objective going to be achieved? (Procedure)3. In practice is the objective being achieved? (Conformance)

In this chapter, these three elements are considered in the context of remote afterloadingbrachytherapy.

9.2 Remote afterloading

The use of remote afterloading machines permits sources of increased strength to beutilised in order that treatment times can be reduced. This has led to the concept of low,medium and high dose-rate (respectively LDR, MDR and HDR) brachytherapy. The ICRU(1985) Report No. 38 advocates high dose-rate as exceeding 0.2 Gy per minute, and lowdose-rates between 0.4 and 2.0 Gy per hour. These definitions are somewhat controversialand are not accepted universally, but they at least allow some form of separation relatedto the associated radiobiological effect. More recently, pulsed dose-rate (PDR) techniqueshave been developed in which the source activity is 18–37 GBq. The length of pulse canbe varied to simulate a continuous low dose-rate treatment, and the dose rate used can beselected according to optimal dosimetric requirements.


Table 9.1. Properties of principal brachytherapy nuclides used in remote afterloading.

Isotope Half-life Principal Air kerma rate Transmission Transmissionγ energy constant through lead through concrete

MeV µGym2 Gbq–1 h–1 HVT TVT TVTmm mm mm

60Co 5.27 y 1.17, 1.33 309 12 40 206137Cs 30 y 0.66 78 6.5 21 157192Ir 74 d 0.30–0.60 113 4.5 15 113

9.2.1 Room design

The treatment of brachytherapy patients in open wards, even when low strength sourcesare used, should be discouraged. In the case of LDR, MDR, HDR and PDR remoteafterloading equipment, it is always best to provide single room accommodation or atleast a two bedded room with inter-bed shielding. The layout of the room will dependupon local circumstances and the intended use of the room, but it will be necessary toensure that the room is adequately protected, suitably located, large enough to allowaccess for patients on beds, has enough space for the afterloading equipment and sufficientaccess for emergencies to be dealt with safely. Adequate space should also be allowedfor servicing. Experience has shown that it is also useful to have a small room adjacentto the treatment room for storing applicators, calibration equipment, test devices, radiationwarning notices and so on. Consideration should be given to the implications of qualitycontrol early in the planning of a brachytherapy facility – short cuts and compromises inthe design of the treatment room are rarely cost effective and often restrict the overallusefulness of the room. Even though non-brachytherapy patients might occupy the roomfrom time to time, the facility should be designed and equipped to allow routine qualitycontrol to be carried out regularly and efficiently.

Wall thicknesses will depend upon the activity and energy of the nuclide and willrange from 200 mm concrete for LDR caesium to 600 mm or more for the HDR cobaltsources (Table 9.1). If a protected room door is used then a thickness of more than 6 mmlead is likely to make the door inconveniently heavy to use manually. It is important tocheck on the need or otherwise for floor and ceiling protection, always bearing in mindthat the use of the areas above, below and adjacent to the brachytherapy room might bechanged during the course of time. During the planning stage, prior to installation andmachine delivery, consideration should be given to the following items which have qualitycontrol implications:

• facilities for service and source changes;• emergency stop control;• radiation warning lights and fireproof signs;• time delay interlock control;• door switches;• designation and posting of Controlled Areas;• independently wired Geiger–Müller (GM) tube in room (for HDR machines there

should be an audible indication that the source is exposed);• availability of a hand held GM monitor;

203Quality Control in Brachytherapy

• CCTV for patient viewing;• patient call system;• emergency storage safe; and• dosimeter cables to be plumbed in.

In the case of pneumatically controlled machines, the compressor should be housed awayfrom the patient area to reduce noise and allow access for servicing. All equipment powerpoints should be clearly labelled and all equipment containing a radioactive source shouldbe marked with a radiation trefoil and source description.

For HDR machines used in treatment rooms or specially designed operating theatres,it is also appropriate to consider the installation of a suitable X-ray machine for localisingfilms and screening. This might involve additional wiring for radiation warning lightsand Controlled Area signs.

9.2.2 Commissioning of equipment

Before equipment is loaded with the source or sources, it is appropriate to ensure that itfunctions within its technical specification. Not all tests can be carried out with dummysources or dummy source trains. However, with the aid of a CCTV camera, source transittimes, dwell times and source positions can be checked with dummy sources when thisis considered to be appropriate. More importantly, all interlocks and emergency stopsshould be checked: loss of power should be simulated, and loss of air pressure in thosesystems dependent upon pneumatic transfer. Source control instruments such as primaryand backup timers should be checked, as should printouts and display instrumentation.Radiation warning lights and Controlled Area lights, if fitted, should be checked notonly for functionality but also for ease of viewing – they are usually sited best at eyelevel. Applicator fitments, transit cables and the simulation of dummy treatments can allbe undertaken at this stage without the added concern of unnecessary radiation. Theinput of pseudo-source and patient data provides the user with machine familiarisationand allows checks to be made on the software.

9.2.2.1 Source integrity

The commissioning procedure will depend upon the type of equipment. Table 9.2summarises the source types that are available for use in automatic remote afterloading

Table 9.2 Automatic remote afterloading machine data

Nuclide Activity No. of Type of Source Dose rate Transfersources source diameter mechanism

60Co 4–20 GBq 20 Variable Pellet 2.5 mm HDR Pneumatic cable

137Cs 0.5–1.5 GBq 48 Pellet 2.5 mm LDR/MDR Pneumatic

137Cs 0.75 GBq Variable Capsules 1.7 mm LDR Cable

137Cs 30–250 MBq cm–1 15 Ribbon <2.3 mm LDR/MDR Pneumatic cable

192Ir 400 GBq 1 Cylinder 5 × 1.1 mm HDR Cable

192Ir 20–40 GBq 1 Cylinder 5 × 1.1 mm PDR Cable

192Ir 10 MBq cm–1 15–20 Wire, ribbon 0.5/2.3 mm LDR/MDR Pneumatic cable


systems. The radioactive sources should be sealed sources and should conform with BS5288. Sources will have been tested by the manufacturer who will also provide informationabout source strength and, in the case of multiple sources or source trains, will givesome indication of the activity variance. Whenever possible, the user should makeindependent measurements to confirm or improve upon the supplied data. It is not possibleto carry out rigorous leakage tests and therefore it is usually necessary to rely on sourcewipe tests. In the case of high activity sources and in equipment where the source isinaccessible, it might be necessary to check the applicator or catheter through which thesource has been driven for signs of radioactive contamination. For such tests, the safetylevel is usually taken as 200 Bq (5 nCi) (NRPB 1988 (9.24), AAPM 1993). In the eventof contamination being found, the nature of the nuclide should be identified so that thepossibility of external contamination of the source capsule in the manufacturing processcan be eliminated. A suspect source should not be used and should be returned to thesupplier in a sealed container.

9.2.2.2 Radiation protection

Once the source (or sources) has been loaded into the machine, radiation protectionmeasurements should be carried out. These will include the following.

1. Dose rate measurements over the surface of the equipment: the dose rate at any positionreadily accessible to staff when all sources are within the source storage containermust not exceed 20 µGy per hour (NRPB 1988 (8.4)). (Note that IEC 1989 recommendsa 10 µGy/hr limit at 50mm from the surface of the container.) In some systems thesources are sealed in flexible trains and these are stored in a separate storage container.The storage safe must be designed to reduce dose rates to an acceptable level, belockable and be identifiable as a container of radioactive sources.

2. Dose rate measurements outside the treatment room: some machines are quite mobileand consideration must be given to the location of the machine during clinical use.Measurements must be made in all areas surrounding the treatment room – includingany areas immediately above or below the room.

3. It is also good practice to acquire dose-rate measurements within the treatment roomso that should an emergency arise and staff must enter the room with the sourcesexposed, the best path of entry will be known.

4. Some estimation of transit dose to the patient should be made if this is not to beincluded in the prescribed dose (see Section 9.2.2.4).

It will be necessary to identify Controlled Areas, and possibly Supervised Areas, inaccordance with NRPB (1988). These should be clearly marked, either by means ofilluminated signs, or by sign posting.

9.2.2.3 Autoradiography and radiography of sources and applicators

These techniques can be used separately, or conjointly, to provide information about thedistribution of radioactive material within its container, and positional information aboutindividual sealed sources in radioactive source trains.

The autoradiographic technique is simplified by having a PMMA or wax support whichhas a recess with the same dimensions as the source (Figure 9.1). Autoradiographs areuseful for checking the uniformity of radioactivity throughout the source, especiallywhen the source is made up of more than one capsule, or when it is in the form of a wire.


Visual inspection of the autoradiograph might indicate lack of uniformity at the 10 percent level, but for more reliable assessment, densitometric scanning is to be preferred.

In the case of positional studies, it is useful to incorporate lead markers into the support:these cause secondary electron emission which, in effect, results in an autoradiograph ofthe markers and provides the required positional data (Jones 1988). This is particularlyuseful for checking the position of sources loaded into applicators or catheters. Themethod allows precise comparisons to be made of different applicators and provides arecord of the location of the radioactive sources inside loaded applicators (Figure 9.2).(Envelope-wrapped Kodak X-Omat Verification film is suitable for these studies.)Uniform pressure should be maintained over the film and source to keep both in closecontact. The maximum difference in source position within a set of applicators shouldbe preferably less than 1 mm. Radiographic markers which are used in applicators fordosimetry purposes should be imaged by X-rays to provide data to correlate thegeometrical markers with source positions. The variation in marker positions should beless than 1 mm in applicators of the same dimensions.

For high activity sources, such as those used in HDR equipment, film exposure takesonly a fraction of a second and is therefore less than optimal because the transit time isof a comparable duration. GafChromic Dosimetry Media has shown itself to be a usefulalternative: it is a thin radiation film which is colourless, grainless and offers high spatialresolution (1200 line pairs/mm). When exposed to high doses of radiation, typically200 Gy, a dye in the film turns blue, the density of which depends upon the absorbeddose. Exposures of the order of 20 seconds are therefore required for autoradiography,which results in better control of image quality. The film is not light sensitive and produceshigh quality images. Figure 9.3 illustrates multiple exposure of an HDR 192Ir source ina test jig designed for the measurement of source position.

Detex paper is a cheap alternative to GafChromic. This paper is used in the printingindustry and also changes colour when exposed to high radiation dose. Before exposure,

Figure 9.1. Wax discs for holding applicators for autoradiography; the lead foil acts as reference markers.


Figure 9.2. Radiograph of a dummy source train and autoradiograph of loaded sources on which the foil markershave been imaged and outlined.

Figure 9.3. Part of a test jig for autoradiography (A) and the resultant annotated image recorded on GafChromicdosimetry media (B) showing the position of the HDR Ir192 source with respect to the marker grid.


the paper is yellow, but under irradiation, hydrochloric acid is released from the ink andthe yellow azo dye turns red. The shade of red does not indicate the actual dose received,but rather indicates that a certain level of dose has been reached. Detex labels, for example,are used to indicate whether a product has received a sterilising dose of radiation, i.e.within the dose range 1 kGy to 100 kGy.

In summary, autoradiography forms an important part of commissioning and may beused to:

1. examine the distribution of radioactivity in the source;2. record the position of the source with respect to the end of the transfer cable;3. record the relative positions of individual sources in source trains; and4. check the reproducibility of source position when inserted into catheters, needles or

applicators. Every such device that is to be used clinically should be tested and theautoradiographic test data should be used as a baseline for subsequent quality controlinvestigations.

There is also a place for X-ray radiography in quality control. For example, it is necessaryto check the spatial distribution of sources in the patient for dosimetry purposes. Thismight require the use of radiographic markers. In the course of commissioning newequipment, radiographic checks should be made to ensure that these markers arereproducible and that their position within a set of applicators is clearly defined in relationto the radioactive sources.

9.2.2.4 Calibration of brachytherapy sources

The method of calibration will be determined by the source activity, the physicaldimensions of the source and whether it is a single source or one of several mounted ina source train. It is usual for the manufacturer to supply a calibration certificate with thesource, but generally the accuracy of this measurement is less than that desired for clinicaluse and might only be within 10 per cent of the true activity. It is therefore necessary tocarry out an independent calibration at commissioning or at any subsequent time whenthe source is changed. The calibration of some typical, remote afterloading sources willbe considered:

1. Single high activity sources such as those used in HDR equipment.2. Multiple high activity sources of approximately the same activity.3. Multiple low or medium activity sources of approximately the same activity.4. Sources encapsulated in source trains in which the activity of individual sources might

vary and the separation between sources might vary.5. Linear source trains such as those used in conventional interstitial brachytherapy.

Before considering these specific cases it is necessary to comment on the units of sourcestrength. For the purposes of source specification, the SI unit of air kerma rate is eithermGy per hour for sources used in low dose-rate brachytherapy, or mGy per minute andmGy per second for material used in higher dose-rate applications. The specificationquantity is called reference air kerma rate (RAKR) which is the name used by ICRU(1985) and is defined as the kerma rate to air, in vacuo, at the reference point, which is 1m from the centre of the source. For small rigid sources, the direction from the sourcecentre to the reference point should be along a direction at right angles to the long axisof the source.


For linear sources it is appropriate to work in terms of the RAKR per unit length. Theincremental unit of length should be 1 mm and the specification quantity is defined asthe kerma rate to air ‘in vacuo’ at the reference distance of 1 mm length of the source inisolation. The unit for the RAKR per unit length is mGy per hour per mm.

Although the practice of specifying a source in terms of its radioactivity content shouldbe discontinued, it is recognised that in the short term, its use might be unavoidable,especially where commercially supplied software requires source information in theseterms. An equivalent activity may be determined from the RAKR by using the followingexpression:

Aeq = RAKR × 10–6 × Ft × dr2 / Γδ

where Aeq is the equivalent activity in Bq, 10–6 is the µGy to Gy conversion factor, Ft isa constant and equal to 1/3600, 1/60, or 1 (depending on whether the RAKR is in µGyper hour, µGy per minute or µGy per second respectively), dr is the distance at which theRAKR is defined (i.e. 1 m), and Γδ is the air kerma rate constant for the isotope at 1 m,measured in m2Gy per Bq per second (BIR 1993).

For individual low-activity sources, calibration can be achieved by use of a reentrantionisation chamber (i.e. an isotope calibrator), the calibration of which is traceable to aNational Standards Laboratory. For this purpose, it is appropriate for the user to haveavailable one or more long-lived calibration sources so that the chamber calibration canbe checked periodically. The reentrant ionisation chamber must respond linearlythroughout its measuring range: its energy response must be known and care must betaken to ensure that, when measuring high activities, there is no drop in sensitivity. Beforeuse, the dependence of the chamber sensitivity on the radionuclide, the position of thesource within the chamber and the length of the source should be investigated. Theresponse of the chamber will also be dependent upon the source filtration andencapsulation. All of these factors apply equally to high activity sources, but usuallyisotope calibrators are too sensitive. Specially designed chambers for 370 GBq sourcesare available: one of these instruments, the HDR 1000, is a well-type ionisation chamberdesigned specifically for the calibration of 192Ir HDR sources, although it could be usedfor other nuclides providing that it is equipped with a suitable insert and is calibratedappropriately.

1. Single high activity source calibrationFor high activity sources such as those used in high dose-rate afterloading, smallvolume ionisation chambers may be used for in-air calibrations at distances of about100 mm. The traceability route for such measurements is via an external beamsecondary standard. The Joint Working Party Report of the BIR and the IPSM onBrachytherapy Dosimetry (BIR 1993) recommended that, for a source whose RAKRis nominally greater than 1 µGys-1, in-air measurements are made with a 0.6 cc Farmer-type ionisation chamber, at a source-centre to chamber-centre distance of 100 mm. Atthis distance, the correction required for the finite size of the ionisation chamber is0.4 per cent and, providing a suitable measurement jig is used, it is possible to restrictthe positional uncertainties to less than 0.5 per cent.

To reduce scatter to negligible levels, it is necessary to mount both source anddetector at a good distance above floor level and also away from the walls or otherlarge structures. The effect of room scatter on measurements made in air may beevaluated experimentally by making a series of exposures at different source–detector


distances. If it is assumed that the room scatter is a constant and independent of source-detector distance, the magnitude of scatter can be determined. Typically, for an Ir192

source, this is approximately 1 per cent for a source–detector distance of 300 mm.The influence of the inverse-square law reduces this to 0.1 per cent at a source–detectordistance of 100 mm. Larger source–detector distances may introduce larger scatterdoses, whereas for shorter distances, the positioning of the source becomes morecritical and a greater factor will be required to correct for finite chamber size. As anextended time period might be required to collect sufficient charge for acceptableprecision, it is recommended that the leakage for the electrometer system is measuredand an appropriate correction made.

The two radionuclides that are used in high dose-rate brachytherapy are 60Co and192Ir. For 60Co, it is recommended that the 2 MV calibration factor currently providedby the NPL is used and that the intercomparison of the 0.6 cc field instrument with aFarmer secondary standard is made either in a 60Co or a low megavoltage X-ray beam.A 2 MV build-up cap is required for these measurements. Some 60Co afterloadingmachines have multiple sources and calibration of these devices is described in Section9.2.2.4 (2).

The emission spectrum of 192Ir is more complex. The emission-weighted averageenergy is 397 keV, but a small proportion of photons of energy up to 800 keV areemitted. It has been suggested by Goetsch et al (1991) that it might be necessary touse a build-up cap to increase the thickness of the Farmer-type ionisation chamber toensure adequate charged-particle equilibrium. More important, however, is the findingby the same author that for in-air measurements at short distances, it is necessary toexclude high energy photo-electrons emitted from the source capsule. It isrecommended that these secondary electrons are absorbed either by use of a 2 MVbuild-up cap on the ion chamber or by using a PMMA sheath around the source capsule.In some systems, the sheath is built into the calibration jig and improves the rigidityof the source catheter. If it is assumed that the calibration factor will be for theionisation chamber without build-up cap, then a correction will be required for thepresence of the build-up cap or the sheath. The recommended value for the 2 MVbuild-up cap is 1.017 and 1.004 for a 1 mm thick PMMA sheath around the source.More generally, for small thicknesses of low atomic number material, the recommendedcorrection is 3% per g cm-2.

The weighted average of 192Ir falls in between 280 kV, the highest energy at whichthe NPL Secondary Standard is calibrated without a build-up cap, and 2 MV, theenergy at which a build-up cap is used. Until a primary calibration at a more appropriateenergy becomes available, it is recommended that the calibration factor for use with192Ir should be that for heavily filtered 280 kV X-rays.

In practice, some sort of device is required to hold the ionisation chamber at afixed distance from the radioactive source. Typical calibration jigs for in-aircalibrations are shown in Figure 9.4. It is recommended that new sources are calibratedindependently by two physicists, preferably using different equipment. There is anadvantage in a definitive calibration being made by one physicist whose measurementsare then used to programme the treatment machine. The second physicist calculatesthe exposure time required to deliver a prescribed dose at the calibration distance,sets the timer accordingly and measures the dose delivered. The measured dose shouldagree with that prescribed. This procedure circumvents errors which might occur in


the calibration and also those which could occur in programming the treatmentmachine.

Depending upon the electrometer which is used for calibration purposes, it maybe necessary to make a correction for the dose delivered to the chamber during transitof the source from the safe to the measurement position. This may be achieved bymaking a series of different length exposures and plotting dose against time. The totaltransit dose (to and from the programmed position) can be determined from where thegraph crosses the abscissa. Although transit dose corrections might be applicable whencalibration measurements are made, it is not usual to include such corrections intreatment calculations.

The principal disadvantage of this procedure is that in-air measurements are timeconsuming. For this reason, the periodic use of a reentrant ion chamber is useful.Several commercial chambers are available. Goetsch et al (1993) have compared threechambers and found excellent agreement between the systems tested. Figure 9.5 showsthe Standard Imaging HDR 1000 reentrant chamber that was designed specificallyfor calibrating 192Ir sources. Chamber calibration is specified in terms of pA MBq–1,and it is therefore necessary to employ a high quality electrometer to measure theionisation current.

Although it might be advisable to calibrate new sources according to the BIR/IPSM protocol, a reentrant ion chamber can be used to make a measurement with an192Ir source of known air kerma rate, just prior to changing the source for a new higheractivity source. In this way, it is possible to reduce the amount of work involved inthe calibration procedure, and at the same time, maintain the principle of using twodifferent measuring systems (Jones 1995).

Mention should also be made here of the use of tissue-equivalent phantoms whichhave been used as calibration phantoms. Such devices provide means by which theion chamber can be positioned accurately, although corrections must be applied toaccount for attenuation and scatter.

Figure 9.4. Two jigs for in-air calibration of HDR Ir192 sources: (A) Royal Marsden Hospital device; and (B)Nucletron jig.


2. Calibration of multiple high activity sources of approximately the same activityBefore clinical use, the variation in activity between individual sources should beascertained. This can be achieved either by means of a reentrant ionisation chambersimilar to that described in (1) above or by measuring the air kerma rate of individualsources at the same distance. Reentrant chamber measurements provide rapid meansof obtaining relative activities but for a definitive source calibration, in-airmeasurements of one or more sources should be made. For example, in the case of theNucletron 60Co HDR Selectron there might be up to 20 sources (500 mCi per source).Usually a batch of sources is within 2.5 per cent of the mean RAKR, which is typically6 µGy/h. In-air measurements with a Farmer type 0.6 cc ionisation chamber would besimilar to those for an HDR 192Ir source but would require longer exposure times toachieve the same accuracy of measurement. It is therefore necessary to allow for anyelectrometer leakage that might occur during exposure.

For patient dosimetry it is not possible to identify individual sources and a weightedmean of the RAKR is used in calculations.

3. Calibration of multiple low or medium activity sources of approximately the sameactivityAlthough well-type (reentrant) ionisation chambers are suitable for field and secondarystandard measurement, calibration factors for the NPL calibrator are not currently

Figure 9.5 The Standard Imaging reentrant ion chamber with a specially designed insert


available for all types of LDR/MDR afterloading sources. Caesium-137 sources suchas those used in the LDR/MDR Selectron can be compared so that the variation inactivity between individual sources can be ascertained, but until such time as a standardsource of the same construction as those used in the commercial equipment becomesavailable, it will be necessary to make some form of alternative measurement todetermine the RAKR for one or more sources. One such method has been describedby Meertens (1984).

4. Sources encapsulated in source trains for which the activity of individual sources andthe separation between them might varyThese source trains must be calibrated individually. It is very important to obtain asmuch detailed information as possible from the manufacturers about the activity ofeach source prior to source train construction. The position of individual sources ineach train may be determined with autoradiographs, and dose rates may be verifiedusing TLD or some form of scanning device.

5. Calibration of linear sourcesIn the case of multiple linear sources such as those used in remote afterloadinginterstitial machines for low and medium dose-rate treatments, it is necessary to checkthe strength of each source and also the uniformity of activity throughout the source.This can be obtained by means of a reentrant ion chamber and autoradiography. Analternative method is to use a scanning device which records the activity per unitlength.

9.2.3 Routine quality control

Many of the checks made at commissioning form the basis of routine daily, monthly andless frequent measurements. These may be considered under the following headings:

• facility testing;• machine function;• source and applicator checks;• radiation safety; and• dosimetry.

9.2.3.1 Facility testing

It is important that checks are definitive and to this end some form of check-list whichcan be reviewed at the beginning of each treatment session is helpful. The detail to beincluded will depend upon the type of equipment being used and on the localcircumstances, but inclusion of all, or many, of the following items might be appropriate:

• CCTV camera function: at console and/or at nurse station;• patient intercom;• radiation warning lights: inside room, at door entrance, at control console;• function of independent audible GM alarm;• door interlock checks;• controlled area signs: posted and/or illuminated;• function of audible time delay interlock;• availability of shielded safe for storing source(s) in the event of an emergency;


• applicator clamp (as in the case of HDR equipment);• emergency trolley with posting of emergency procedure;• emergency stop;• air compressor function;• check of cables, transfer tubes, etc. for damage;• positioning of treatment support frame; and• X-ray unit for localising films: check warning lights and availability of necessary

protection.

9.2.3.2 Machine function

1. Test button functions by programming and carrying out a simulated treatment.2. Verify that all displays are correct.3. Verify data on printout agrees with programmed data; check printout of date, time,

source activity, etc.4. Test backup storage batteries by periodically (once a month) simulating loss of power.5. Test emergency stop.6. Compare machine timers with a stopwatch.7. Check source safety lock when fitted.8. Check programme card data input when appropriate.

9.2.3.3 Source and applicator checks

1. Check applicator interlocks; simulate interlock error.2. Establish source positioning relative to applicator surfaces either by autoradiography

or by one of the methods described above.3. Compare autoradiographs with radiographs of applicators and dummy sources.4. Document and check positioning of internal tungsten shields in colpostats and

applicators.5. Inspect applicators, colpostats, needles, catheters and ovoids before use and also check

screws, nuts, etc.6. Monitor applicators, colpostats, needles, catheters for radioactivity after use.7. Inspect applicators, colpostats, needles, catheters, ovoids for damage after use.

9.2.3.4 Radiation safety

1. Radiation surveys should indicate expected dose levels but change of source or layoutof treatment room should lead to additional surveys.

2. Personnel dosimetry should be used to monitor personnel doses especially in situationsrequiring source transfer from safe to remote afterloading machine.

3. Simulated emergency procedures should be carried out so that operating staff arewell versed in the requisite actions.

4. Dose-rate measurements inside the treatment room should be available so that optimalpath to patient is known before room entry.

5. Personnel dosimeter available for emergency use.

9.2.3.5 Dosimetry

Quality control in dosimetry covers a wide range of subjects and this section deals onlywith those pertinent to afterloading brachytherapy. Aids to dosimetric planning includeclassical methods such as those of Manchester and Paris. The accuracy of source


strength(s) will clearly influence the accuracy of the calculated dose so source strength(RAKR) must be measured with acceptable accuracy before sources are used clinically.Source strengths should be checked at regular intervals. The following additional pointsare considered to be important:

1. The precision with which a particular dosimetry system predicts a dose distributiondepends upon compliance with the underlying rules and principles upon which thesystem is based. It is not always possible to fulfil these requirements and the dosedelivered must be calculated on the basis of the actual source arrangements.

2. To calculate the dose distributions, radiographic localisation of the implanted (inserted)sources is necessary: the precision of the method used should be measured for eachtype of technique. The accuracy of the reconstruction program must be checked. Aphantom can be used for such an evaluation and it is useful for the program to haveprovision for internal checks such as comparison of calculated length versus actuallength and longitudinal position calculated from the AP view versus the same positionscalculated from the lateral view. The accuracy of source localisation reconstructiontechniques has been evaluated by Slessinger and Grigsby. Methods which useorthogonal radiographs were found to be accurate within 2 mm.

3. In computer calculations the algorithm used should be evaluated. Some algorithmsprovide accurate data in regions near to the source and are less precise at greaterdistances. The characteristics of the algorithm should be investigated at variousdistances (typically 20, 50 and 80 mm) either by independent calculation or byexperimental measurement. Dose algorithms should calculate doses to within a fewpercent of the true dose at distances up to 50 mm from the source. The algorithmsused should allow for scatter and absorption in tissue, absorption in the source, sourceencapsulation, absorption in the applicator or catheter, and any special shieldingdevices.

Dosimetry techniques should be evaluated before being used clinically and checkedafter software changes or machine fault rectification. Particular care should be takento ensure consistency between the physical coefficients used in the dosimetry programand those used to specify source strength. The user should confirm that the programmeddecay corrections are accurate.

4. In estimating the overall accuracy of a particular technique, some estimation shouldbe made of the likelihood of source displacement during the course of treatment.

In practice the positions of removable applicators should be ascertained as soon aspossible after their placement and checked during treatment. Any occurrence ofapplicator movement must be reported to the clinician in charge of the patient so thatsuitable corrections can be made.

5. It is good practice for all treatment dosimetry computations to be checkedindependently by a second person.

9.3 Manual afterloading

A significant number of intracavitary and interstitial treatments are carried out using amanual afterloading technique. Applicators or tubing are inserted into the patient under


general anaesthetic and localisation films are taken. Once the patient has returned to theward, radioactive sources or source trains are manually inserted into the localisingapplicators or tubing, thus ensuring that the radiation exposure to staff members is keptto a minimum. Treatment techniques include gynaecological intracavitary treatmentswhere source trains can be evenly or differentially loaded, and 192Ir wire implants whichcan be used to treat the chest wall, for example. 192Ir hairpins are manually afterloadedinto introducers, but, as this is generally carried out in theatres, the reduction in radiationexposure to staff is not as significant.

9.3.1 Commissioning sources and source trains

9.3.1.1. Checking sources on receipt

The checks required on new sources may be summarised as follows:

1. Verification of source documentation.2. Confirmation of source strength.3. Distribution of activity within the source.4. Leakage of radioactive material from the source.

All new sources are received from the manufacturer with a delivery note and a testcertificate. These documents must be compared with the original order to check that thecorrect sources have been delivered. The label on the source container must correspondto the delivery note. If the documentation is satisfactory, the sources may be unpackedin a suitable handling facility to verify that the numbers and types of sources receivedare as per the original order and the delivery note.

The source strength of each of the sources should then be measured in a suitablecalibrated measuring system such as a radionuclide calibrator. Any measuring systemwhich is used must have a calibration for the source type being measured. The result ofthe measurement, corrected for radioactive decay, must be compared with the value quotedon the test certificate and discrepancies of 5 per cent or more must be investigated. Thesource details must then be entered into the sealed sources register.

1. Additional checks for tube and needle sourcesIt is recommended that line sources should be autoradiographed to check that thedistribution of radioactivity within the source is as expected. This may be carried outby placing the source in good contact with a slow film, e.g. Kodak X-Omat V andusing a suitable exposure time, typically a few minutes for most sources.

Leakage and contamination tests should also be performed. Typically, these maybe done by wiping the source with a swab moistened with water or alcohol (BS 5288).The source manufacturer may give advice on a suitable moistening agent.

2 Additional procedures for 192Ir wires192Ir wire is a source which can be cut into discrete lengths for subsequent use. Theflexibility of the source is both an advantage in use and a disadvantage in qualitycontrol measurements. The wire is obtainable in 0.6 mm, 0.5 mm and 0.3 mm diameters.The 0.3 mm diameter wire is available in the form of loops of 140 mm or 500 mmcoils. The other diameters are available in loop, hairpin or single pin form. Differentgeometries are used by different manufacturers. It is advisable to make source strengthmeasurements for the whole delivered source prior to cutting it for clinical use.


However, this will not verify that the source strength per unit length is in agreementwith the test certificate, since it is possible that the source length is incorrectly supplied,or even that the iridium core has been omitted from part of the wire. The sourcelength can be verified by autoradiography. This method is feasible for loops and wiresbut is impractical for coils. As an alternative, for wires cut from coils, the sourcestrengths for the cut wires should be measured prior to clinical use and the measuredsource strengths compared with those expected from the quoted source strength perunit length. Any discrepancy of greater than 5 per cent must be investigated. Ideally,the cut wire should also be autoradiographed. (NB: if sources are to be cut to size, itis important to use a cutting edge which is clean.) 192Ir does not meet the definition ofa sealed source, and therefore the leakage test described previously may be omitted,if the user has assured himself or herself that there is no contamination risk.

9.3.1.2 Calibration of sources

The reference air kerma of a brachytherapy source is strongly dependent upon theradionuclide, the source and wall material and the source geometry. Measurements ofsource strength must be made with a measuring system which is calibrated for sourcesof the particular types which are to be measured, i.e. for sources of the appropriateradionuclide and the particular construction to be measured.

There are three methods by which a satisfactory calibration can be obtained:

1. The purchase of calibrated sources of the types for which measurements are to bemade.

2. The use of a calibrator which has been type tested and which has a calibration traceableto the national standard.

3. Calibration using an ionisation chamber – either a secondary standard instrument, ora field instrument which has been calibrated in a manner analogous to the calibrationroute used for external beam treatment machine calibrations.

These methods are discussed below:

1. Use of calibrated sourcesThis method is straightforward and simply requires the measurement of the calibratedsources in the calibrator. The purchase of several calibration sources can be expensive.

2. Use of a type-tested calibratorThe National Physics Laboratory calibrator, (Bicron-NE’s Isocal IV) has been producedas a secondary standard instrument.The calibration is checked by NPL and the calibration is traceable to the nationalstandard. At present, this instrument has calibration factors for 0.6 mm diameter and0.3 mm diameter 192Ir wire. The factors for commonly used 137Cs sources will beavailable in the near future. This instrument has been type tested and is supplied witha 137Cs source for the purpose of checking the instrument’s calibration (Sephton et al1993).

3. Comparison with a secondary standard instrumentA field instrument may be compared with a secondary standard instrument byperforming a comparison of the two systems for sources of the types for which thefield instrument is to be used. It is important that the sources used for calibration areof similar construction to those which will be routinely measured in the field instrument


and that the measurements are carried out using the same positioning jigs which willbe routinely used, since the source construction and the source position within thecalibrator may both significantly affect the instrument’s readings.

4. In-air measurements with a small ionisation chamberIn some circumstances it is possible to make in-air measurements using a Farmertype chamber (Deshpande and Wilkinson 1994). The positioning of the ionisationchamber with respect to the source is critical in these measurements, since shortdistances are required to achieve a sufficiently high dose rate for accuratemeasurements. If possible, the use of multiple sources of similar construction mayalso be used to increase the dose rate. Correction must be made for the finite size ofthe chamber. In general the detail given in the section on calibration of high activitysources will apply.

9.3.1.3 Calibration of radionuclide calibrators

Low strength sources are most conveniently measured in a radionuclide calibrator of thetype commonly used for the routine measurements performed in nuclear medicinedepartments.

The most convenient method for most centres will be to establish regional referenceinstruments. These may be secondary standard instruments which have been calibratedagainst a primary standard instrument. Alternatively, the reference instrument may be atertiary standard which has been calibrated against a secondary standard instrument.Field instruments which are used routinely to measure source strengths administered topatients should then be checked against a reference instrument. These checks need bemade only for those radionuclides assayed on the field instrument: typically 137Cs, 192Irand possibly 198Au.

The accuracy with which this calibration can be performed will depend upon a numberof factors. In particular, the geometry of the source will affect the sensitivity of the ionchamber. Ideally, standard sources with the same geometry as the sources for use inpatients should be used to calibrate the calibrators. The position of the source in thecalibrator will also affect the reading. A jig that always positions the source in the sameplace is essential for this operation. This subject is dealt with more fully in ‘Protocol forestablishing and maintaining the calibration of medical radionuclide calibrators, andtheir quality control’ (IPSM 1992), which, although written primarily for unsealed sources,contains some useful information for sealed sources. In particular, there is a section onthe quality control of the calibrator which details the routine checks which should bemade on these instruments.

9.3.1.4 Additional notes concerning source calibrations

1. Non-standard source trainsThere may be some difficulty in obtaining a satisfactory calibration of a radionuclidecalibrator for the measurement of source trains which may be of non-standardconstruction. In these circumstances every effort should be made to investigate theresponse of the radionuclide calibrator to variations in source position, and use thisinformation in conjunction with the calibration data for sources containing the sameradionuclide which are as close as possible in construction. It may be more satisfactoryin these circumstances to make measurements in air as described by Deshpande andWilkinson (1994) for long needle sources.


The measurement of non-standard source trains presents a difficult situation andthe above procedures, which are far from ideal, should not be used if a calibratedsource can be obtained.

2. Measurement of iridium-192 coilsThe calibration factors supplied for the NPL secondary standard are not directlyapplicable to the coils of wire in their delivery geometry nor is it possible to obtain acalibrated source in this geometry. A calibration factor may be established for thecoiled wire by first measuring the wire, as delivered, in a convenient jig in thecalibrator. This procedure should be carried out by two physicists workingindependently. The wire is then cut into pieces which can be used with the NPL suppliedfactors in the secondary standard calibrator, or compared directly with the calibrationsource. The comparison of the total source strengths for the cut wires with the originalmeasurement allows a calibration factor for the coil in its delivery geometry to befound.

9.3.2 Commissioning an iridium-192 loader

Patients who are receiving treatment with 192Ir wire are normally implanted with nylontubing of approximately 1.25 mm diameter in the operating theatre, and then returned toa protected room on the ward for afterloading with active wire. The active wire is usuallyencapsulated in nylon tubing of approximately 0.75 mm diameter. An iridium wire loaderis used to cut wire of the required lengths and load it into the narrow diameter nylontubing. A typical loader is shown in Figure 9.6.

Figure 9.6. Loader for 192Ir wire (photograph courtesy of Amersham International).


9.3.2.1 Checking the operation of the loader

On receipt of the loader, visually inspect the machine and check that all parts appear tofunction. If all is satisfactory, cut several pieces of inactive wire (preferably inactiveiridium but any similar wire, such as copper, may be used) and check the cut piecesagainst a standard ruler to confirm lengths. Also verify that there is no zero error. Examinethe ends of the cut wire, using a microscope or magnifying glass, to check that the end ofthe wire has been cut cleanly, without any hooking or other damage which might impedethe passage of the wire into the plastic tubing.

Make several seals using the sealing mechanism and then subject the sealed plastictubing to some stress to check the integrity of the seal. The user must be satisfied thatthere is no possibility that the wire can escape from its plastic tube.

Check that the seals do not impede the passage of the inner plastic tubes through theouter tubing. In particular, attempt to do this for outer tubing which is bent into a tightcurve such as might be used to treat a mandible or tongue.

9.3.2.2 Radiation protection measurements

Check that the pot in which 192Ir is stored prior to the loading procedure is satisfactoryand can be conveniently positioned.

It may be appropriate to measure the finger doses received during the first few uses ofthe machine.

After the first occasion of use with active wire, check that there is no significantcontamination of the loader.

All the checks described should be recorded.

9.3.2.3 Instructions to the operators

A set of operating instructions should be provided for the user and users should be advisedto practice with inactive wire before using the machine for the first time.

9.4 Commissioning a treatment planning system forbrachytherapy purposes

Commissioning procedures may be divided into four parts:

1. Examination of the delivered system to ensure that all required hardware, softwareand manuals have been received.

2. Entry of source data into the system.3. Verification of the dose distributions calculated by the system for various test implants.4. Establishing quality control procedures which may be carried out routinely to ensure

the integrity of the calculation system and training staff in use of the system.

As with all critical procedures, the commissioning should be planned in advance andcarefully recorded. Any part of the procedure which will affect the dose a patient mayreceive must be independently checked by a second physicist and the entirecommissioning procedure should be independently reviewed by a second physicist toensure that there are no omissions or errors of procedure.


9.4.1.1 Examination of the delivered system

This is usually a straightforward matter of checking the order against the equipment asdelivered. Arrangements should be made for any training offered by the manufacturersto be carried out. The manuals should be read with care before starting the commissioningprocess. Particular attention should be paid to the method the system uses for specifyingsource strength and conversions of source strength from one system of units to anothershould be carried out with great care. For further advice on this subject consult the BIR/IPSM report (BIR 1993).

9.4.1.2 Entry of source data into the treatment planning system

Data are usually supplied to treatment planning systems in one of two basic forms:

1. Source parameters (e.g. source strength, thickness of source wall, effective attenuationcoefficients of the wall and source material, source dimensions, etc.) are entered intothe program and from these parameters the dose distribution around the source iscalculated.

2. The complete dose distribution around a source may be entered for a standard sourcestrength.

Method (1) is usually the most convenient method of entering basic data, but method (2)may be more suitable for non-standard sources. In both cases the commissioningconsiderations are much the same. The actual data entered must be checked by a secondperson to verify that it has been done correctly. The system should then be used in clinicalmode to produce dose distributions around single standard sources and the resultscompared with published data for those sources or with calculations of dose performedindependently using a widely accepted algorithm. Check calculations may be made eitherby hand or with a second computer system. Agreement should be better than 2 per centfor all points of clinical interest. One aspect of the computer dose calculation whichshould be checked at this stage is that the calculated dose matrix is on a sufficiently finegrid to enable accurate interpolation at points of clinical interest. A matrix spacing of 2mm may be sufficiently accurate for calculation of gynaecological doses specified at 20mm from a source but could be inadequate for an implementation of the Paris system for192Ir wires with a source separation of 12 mm or less where accurate knowledge of thedose 6 mm from the radioactive source is needed.

9.4.1.3 Verification of multi-source dose distributions

At this stage of the commissioning, it has been accepted that the dose distributions aroundindividual sources are calculated accurately. It is now necessary to ascertain that themethods of entering source positions, the addition of dose distributions for a number ofsources and the subsequent output of dose distributions take place correctly. The precisetechniques used for this will depend to some extent on the facilities offered by thedosimetry system, but some guidelines are offered below:

Data entry methods are usually taken from the following list:

• isocentric films (i.e. two films taken at an angle other than 90° to each other);• orthogonal films (i.e. two films taken at 90° to each other);• keyboard entry of source coordinates;• entry from tomographic data.


The ideal method of verifying the data entry is to construct a phantom of dummy sourceswhose position coordinates are known. Films of this phantom are taken using all themethods available to the planning system and the film data are digitised into the planningsystem (Figure 9.2). A selection of planes through the implant may then be reconstructed,viewed on the planning system’s graphics display and printed out. The source positionsare then compared with the known positions of the dummy sources in the phantom.Source positions made on the screen or from the hardcopy should be within 2 mm ofthose expected. This method allows one to examine the accuracy with which sourcepositions may be reconstructed from film and the accuracy of the system’s input andoutput devices.

If it is not possible to manufacture a phantom of dummy sources, dummy ‘films’giving two projections of an imaginary implant of known source positions may beconstructed instead but this does not test the accuracy of the imaging method.

For each data entry method available, at least one sample set of data should be enteredfor wires whose coordinates are accurately known.

Some systems have a facility to calculate the dose distributions from curved wire aswell as straight sources. If this facility is available then separate tests must be carriedout to verify that the curved sources are accurately reconstructed. Suitable test sourcesfor this purpose may be in the form of an arc of a circle or two straight sections joinedtogether at angles between 45° and 90°. The planning system usually regards curvedwires as a number of straight wires and the reconstruction of the implant should bechecked to see that not only are the coordinates of each straight wire accurate, but thatthe approximation of the curved wire by the straight sections is acceptable.

Assuming the reconstruction of the sources is acceptable, the accuracy of the graphicsfeatures should be checked. Implants should be translated and rotated though a varietyof angles and distances and the images reviewed to see that they are as expected. Asimple method of checking the ‘rotate’ facility is to take four views of the dummy sourcephantom. If orthogonal films are used, then 0°, +45°, –45° and 90° angles are convenientto use. The 0o and 90o views are used to enter the wire data and then the planning systemrotate facility is used to rotate the reconstructed implant through +45° and –45°. Theimages obtained can usually be printed out at the same magnification as the films tomake comparison simple. Again the source positions should be accurate within 2 mm.

Other features which require testing are parameters influenced by the editing of sourcestrength (e.g. dose-rate dependence on source strength, calculation of total dose andtreatment time).

9.4.1.4 Quality control procedures

The frequency of quality control testing depends to some extent on the number ofbrachytherapy calculations the department carries out each week. The check frequencyspecified below assumes the department makes several calculations per week. If therecommended frequency of checking exceeds the frequency of use, then the checks needonly be performed prior to use. It will be noted that the frequencies here are not the sameas those in Table 4.6 (see Chapter 4). Table 9.3 is designed for planning computers thatare only used for brachytherapy. If the computer is used for external beam therapy aswell, problems with the input and output devices should be spotted when treatment plansare checked.

In addition to the checks listed in Table 9.3, brachytherapy treatment planning


Table 9.4. Recommended frequencies for radionuclide calibrator QC tests.


Before each useConstancy check 9.3.1.3 ±5%

MonthlyEnergy response 9.3.1.3 ±3%

AnnuallyCalibration 9.3.1.3 ±2%

Table 9.3. Check frequencies for brachytherapy treatment planning.

Minimum frequency Check

Compare source data file against a standard copy heldseparately to detect corruptions and unauthorisedalterations to data (bit by bit comparison of two files isa facility offered by most operating systems)

Test geometric accuracy of input and output devices

Calculate standard plan and compare result with thesame plan calculated at the time of commissioning

Repeat of commissioning work or parts ofcommissioning work

Daily

Weekly – can be omitted if these devices are regularlyused for external beam planning and these tests areperformed as part of external beam quality control

Monthly

On introduction of new software releases or receipt ofnew sources

calculations, which determine the dose individual patients receive, must be checked bya second person.

The commissioning process is completed by training staff in the use of the systemand teaching them how to carry out the necessary quality control procedures.

9.4.2 Summary of test frequencies

Test frequencies for brachytherapy hardware are contained in Table 9.4 – Table 9.6.


Table 9.5. Recommended frequencies of QC tests for radioactive sources.


On receipt (or if source identity is in doubt)Documentation check, etc. 9.3.1.1Source strength 9.3.1.2 ±5%Autoradiography 9.3.1.1(1)

Three-monthlySource inventory 9.6.1 All sources present

AnnuallyAudit of source control records 9.6.1

Every two yearsAutoradiography of sources 9.3.1.1(1)

Table 9.6. Recommended frequencies for brachytherapy equipment QC tests.


Before each useVisual inspection of applicators 9.2.3.3Protective shields and live-loading equipment

AnnuallyApplicator dimensions/damage 9.2.3.3192Ir loader operation 9.3.2.1Contamination of loader cutter 9.3.2.2 None detectableCalibration of radiation monitors Note 1

Before repair or maintenanceContamination of loader cutter 9.3.2.2 None detectable

Note 1 The calibration of radiation monitors should be carried out by an approved dosimetry service and isoutside the scope of this book.

9.5 Live loading

There are occasions when it is necessary to load radioactive sources directly into thepatient rather than use an afterloading technique. The disadvantages of this techniqueare well known, but on some occasions, the extra hazard to staff and the public can beoutweighed by the benefit to the patient in using the technique. An example of this is inthe use of iridium-192 hairpins in the treatment of the tongue.

If live loading is undertaken, then there are a number of precautions which must betaken:

1. The theatre will need to be designated a controlled area as required by the IonisingRadiations Regulations 1985.

2. Appropriate local rules must be available.


3. The theatre must be equipped with shields, long-handled forceps and any other loadingequipment designed to minimise radiation doses to staff. This equipment must bechecked regularly to ensure that it is in good working condition.

4. A calibrated radiation survey meter must be available to monitor the theatre when theimplant has been completed and the patient has returned to the ward. A record of thesurvey must be kept.

The custody of the radioactive sources must be passed from the sealed source custodianto the theatre staff, from the theatre staff to the ward staff and from the ward staff to thesource custodian when the implant has been removed. All these movements andtransactions must be carefully recorded.

9.6 Control of sources

9.6.1 Storage of sources

The employer should appoint a custodian of radioactive sources to be responsible for thesecurity during storage of all radioactive substances and for all necessary records.Radioactive sources issued from a store should be in the care of responsible individualsat all times until their return. The local rules should state the responsibilities of allindividuals, including patients, for the care of radioactive sources.

Sources should be stored in the preparation area in a shielded safe with a thickness of50–100 mm of lead or its equivalent, depending upon the amount and type of radioactivitystored. The safe should be compartmentalised to permit easy access and control overindividual sources. A detailed inventory of sources is necessary. Each source should beidentified individually with the following details: isotope, type of source, strength,calibration date, date of purchase, and serial number. The inventory should include arecord of its acceptance and leakage tests.

It is essential that the location of every source is known at all times. One system ofachieving this is to have a log book in which an entry is made on each occasion that asource is moved, for example from the safe to theatre or from theatre to ward. Eachweek the contents of the safe should be checked against this record. This does notnecessarily involve the checking of every drawer or compartment within the safe. Areliable system of checking, which reduces the dose to staff to a minimum, makes use ofdrawers which are designed to operate an indicator (a ‘flag’) when opened. The weeklycheck is then a matter of inspecting which drawers or compartments have been openedand checking the contents of these against the records. Inspection of sources in the safeshould be achievable without the need to look directly at the sources. The use of a mirroror a lead glass screen makes this possible. A full audit of the sources against the inventoryshould be made annually.

9.6.1.1 Sources in transit

Transfer of sources should be made in lead trolleys or boxes which must remain firmlyclosed throughout the transfer. It is the responsibility of the person in charge of thetrolley to ensure, by observation and verbal warning, that other persons do not enter or


remain within the controlled area around the trolley. This same responsibility appliesalso to the person in charge of a patient containing radioactive sources.

9.6.1.2 On the ward

Sources on the ward are the responsibility of the person in charge who will have signeda receipt for the sources. It is essential that various forms of monitoring the presence ofthe sources takes place while they are on the ward. This can take the form of observation,if needles or applicators are used; it is more difficult with seeds. An associated part ofthe monitoring must always take place after the patient leaves the ward following theremoval of the sources. A radiation monitor must be used to check that no sources areleft in the ward or the patient’s bed linen. In the case of the removal of iridium wire bycutting catheters it is equally important to monitor the patient prior to discharge.

9.6.1.3 Discharge of the patient

In the case of permanent implants, it is necessary to check the patient, preferably bymeasurement, to determine that he/she can leave hospital by private car or ambulanceaccording to the regulations. Advice should be given, of course, about contact with otherpeople, particularly in the case of treatment with 125I because of its long half-life.

9.7 Other sources

9.7.1 Iodine-125 seeds

125I seeds are becoming more widely used in brachytherapy. 125I decays with a half-lifeof 60 days by electron capture with the emission of characteristic photons and electrons.The electrons are absorbed by the titanium wall of the seed, while the principal photonemissions are 27.4 and 31.4 keV X-rays and a 35.5 keV gamma ray.

These seeds may be used as a permanent implant in relatively inaccessible sites suchas the prostate, as a temporary implant where the seeds may be introduced throughcatheters, or for use in a surface applicator, such as for the treatment of conjunctivalneoplasms.

On receipt of 125I seeds, it is important to ensure that documentation is received statingthe number and activity of the seeds. This must be checked to agree with the order.Seeds are too small to allow coding to be inscribed on individual sources, thus, if seedsare of different activities, they must be delivered and stored in clearly-labelled, individualcontainers.

The seeds are always delivered in shrink-wrapped, screw-capped, glass vials inside asealed lead ‘securitainer’. This lead effectively shields more than 99.9 per cent of theemitted photons. If the seals are not intact, the seeds should be returned. 125I seeds havea very high structural integrity: they can withstand temperatures up to 138°C and pressuresof 35 psi (>2 atmospheres), but if they are subjected to rough handling, exposure tohigher temperatures or crushing, then a seed could be ruptured causing the release offree 125I.

On receipt of 125I seeds, the activity must be measured in a calibrator, and this activitychecked with that stated on the certificate. Since 125I emits low energy photons, it is


important that the calibrator has a jig designed to cause minimum absorption of theradiation, and which also allows accurate positioning of the seeds. Fine nylon wires maybe used within the jig which support and accurately position the seeds.

125I seeds are classified in the medium toxicity group B1 (BS 5288: 1976). Thosewhich are for clinical use are well below the maximum activity which requires separateevaluation of usage or design. The above publication recommends limits on sealed sourceperformance standards when tested with pressure, temperature, impact, vibration andpuncture, which impose limiting values in excess of those recommended by themanufacturers for 125I seeds; thus the manufacturers’ more stringent limits must beadhered to rather than BS 5288: 1976.

125I seeds are leak-tested prior to shipment, demonstrating a level of activity of lessthan 5 nCi of removable 125I. This must be confirmed by referring to the certificate. Nosource should be transferred or used clinically unless it has been certificated to havebeen leak-tested within a period of six months. Should any seeds be used clinically for aperiod greater than six months, then they must be leak-tested again before use andsubsequently at intervals not exceeding six months. Where seeds are used for temporaryimplants, leak-testing at more frequent intervals is recommended.

Wipe tests are impractical due to the size of the seeds; thus, either an immersion withboiling test or an immersion test (BS 5288: 1976) should be performed. The immersionwith boiling test requires boiling a liquid which will not attack the seed surface, such asdistilled water or a weak solution of detergent or chelating agent. The liquid containingthe seed should be boiled for ten minutes, the source removed and rinsed, then theprocedure repeated using fresh liquid for a total of three times. The final amount ofliquid is then measured for activity, and this must be less than 5 nCi for a leak-free seed.

Seeds used for temporary implants which may be loaded in plastic tubing must beremoved from this tubing very carefully and, if there is any possibility of a seed havingbeen scratched, it must be leak-tested prior to any further use.

Should a seed be found by a patient, it should be returned to the radiation physicistwho must check it for leakage as above since the seed’s history cannot be guaranteed.

9.7.2 Gold-198 grains (including gold grain (Marsden) gun)

Gold seeds are delivered in an cartridge containing 14 grains. Although there is provisionfor extracting the individual seeds and reloading them it is probably better to measurethe whole cartridge together on receipt of the grains. If cartridges are reloaded it may beappropriate to check each grain to ensure that it is from the same batch. The gun itselfhas several stilettes which push the grains out of the end of the needle. Before use theseshould be checked, using dummy gold grains kept for the purpose, to ensure that theindexer is correctly set. If the gun is in regular use this can be done once a month, but forirregular use the check should be carried out before each use.

9.7.3 Beta-ray sources: strontium-90 applicators

Beta-ray sources or plaques are used sporadically throughout the country. Beta-rayophthalmic applicators, both 90Sr and 106Ru, are used routinely in one or two centres.For 90Sr in particular there has been very little recent work in the UK on the dosimetry,and physicists have relied on early published work, manufacturer’s certification and


clinical experience. However, there have been several papers from the USA in recentyears showing discrepancies between methods of measuring output and the outputs statedon the manufacturer’s certificates. The methods of calibration used include TLD, film,extrapolation chambers, scintillators and radiochromic foil. Discrepancies in output ashigh as 50 per cent have been reported, probably mainly to do with the shape of theradiation distribution (peak dose rate versus average dose rate).

Care should be taken when interpreting results of independent measurements of thisnature. Treatment times that are well established and have only changed with the decayof strontium over a long period of time should not necessarily be changed arbitrarily if aphysicist measures an output which is different from the value stated on the certificate.

Alternative methods can be employed to check beta-ray plaques: autoradiographycan be used to establish the size and uniformity of the radiation field. If dose/distancecurves are to be quantified and plotted it is advisable to calibrate the film and thencorrect the density readings to absorbed doses from the measured density/dose curve.

References

AAPM (American Association of Physicists in Medicine) 1993 Report of AAPM TaskGroup 41 Remote Afterloading Technology ISBN 1-56396-240-3 (New York: AmericanInstitute of Physics)

Ali M M and Khan F M 1990 Determination of surface dose-rate from a Sr-90 ophthalmicapplicator Med. Phys. 17 416–421

BIR (British Institute of Radiology) 1993 Aird EGA, Jones CH, Joslin CAF, KlevenhagenSC, Rossiter MJ, Welsh AD, Wilkinson JM, Woods MJ and Wright SJ Recommendationsfor Brachytherapy Dosimetry. Report of a Joint BIR/IPSM Working Party (London: BritishInstitute of Radiology)

BS 5288: 1976 Specification of sealed radioactive sources (London: British StandardsInstitution)

Deshpande NA and Wilkinson JM 1994 Calibration of low activity caesium tubes andneedles traceable to the therapy level standard Br. J. Radiol. 67 194–199

Goetsch SJ, Attix FH, Pearson DW and Thomadsen BR 1991 Calibration of Ir-192 highdose-rate afterloading systems Med. Phys. 18 462–467

Goetsch S J, Wollin M and Olch A J 1993 Evaluation of three well-type ionisation chambersystems for calibration of iridium-192 HDR afterloaders Activity: NucletronBrachytherapy Journal 7(1) 3–7

ICRU (International Commission on Radiation Units and Measurements) 1985 Doseand Volume Specification for Reporting Intracavitary Therapy in Gynaecology ICRUReport 38 (Bethesda MD: ICRU)

IEC (International Electrotechnical Commission) 1989 Medical Electrical Equipment,Section 2.17 Specification for remote-controlled Automatically Driven Gamma-rayAfterloading Equipment (Geneva: IEC Publication 601-2-17)


IPSM (Institute of Physical Sciences in Medicine) 1992 Protocol for establishing andmaintaining the calibration of medical radionuclide calibrators and their quality control.Report 65 Quality Standards in Nuclear Medicine Chapter 5. Ed AH Smith and GC Hart.ISBN 0904-181642 (York: IPEM)

Jones C H 1988 Quality assurance in gynaecological brachytherapy Dosimetry inRadiotherapy Proc. IAEA Symposium (Vienna 1987) 1 275–290 (Vienna:IAEA)

Jones C H 1995 HDR microSelectron quality-assurance studies using a well-type ionchamber Phys. Med. Biol. 40 95–101

Meertens H 1984 A calibration method for Selectron sources Brachytherapy 1984. Proc.3rd Int. Selectron Users Meeting 1984 Ed RF Mould (Nucletron Trading BV) pp 59–67

NRPB (National Radiological Protection Board) 1988 Guidance Notes for the Protectionof Persons Against Ionising Radiations Arising from Medical and Dental Use (London:HMSO).

Pruitt J S 1987 Calibration of Beta-particle-Emitting Ophthalmic Applicators NBS SpecialPublication 250-9

Sayez JA and Gregory RC 1991 A new method for characterising beta-ray ophthalmicapplicator sources Med. Phys. 18 453–461

Sephton JP, Woods MJ, Rossiter MJ, Williams TT, Dean JC, Bass GA and Lucas SE1993 Calibration of the NPL secondary standard radionuclide calibrator for 192Irbrachytherapy sources Phys. Med. Biol. 38 1157–1164

CHAPTER 10

In vivo Dosimetry and Portal Verification

In the survey of current practice in the UK carried out in 1992 (see Appendix B), in vivodosimetry and portal verification showed the widest variability in practice. Since therewas no consensus of opinion, it has been necessary to base the recommendations containedin this chapter on the published papers and the views of the members of the workingparty. It was felt that practice in these areas was generally below the level that couldreasonably be expected in modern radiotherapy and that increased use should thereforebe recommended.

10.1 In vivo dosimetry

10.1.1 Introduction

In vivo dosimetry can form an important link in the quality assurance chain, but in orderto obtain accurate results considerable care is needed. Before conducting an in vivomeasurement consideration should be given to the purpose of the measurement. Thiswill dictate the accuracy required and whether build-up should be used. For example, ifthe measurement is to check the dose to sensitive normal tissues, larger uncertainties(e.g. 5 per cent) can be tolerated than when the measurement is intended to verify thedose given, when the aim should be 3 per cent accuracy. In the latter case more than onedosimeter may be needed to improve statistical accuracy. A measurement in an area ofhigh dose gradient will require careful positioning and this will be more difficult toachieve if more than one dosimeter is used, but the accuracy required will probably beless.

10.1.2 Indications for use

In vivo dosimetry is widely used throughout the UK to provide data on doses to criticalorgans close to field borders. Very little use has been made of the technique to verify thedosimetry in the centre of the radiation field, although a few UK hospitals are makingsome progress in this area (e.g. Adeyami and Lord 1997, Edwards et al 1997, Millwateret al 1997). There was, however, a clear recommendation that this should be done in theWHO publication on quality assurance in Radiotherapy (WHO 1988). In Europe a numberof reports have been published recently indicating the very considerable benefits to begained (Leunens et al 1990, Heukelom et al 1991). On the other hand, it has been arguedthat the uncertainties associated with in vivo dosimetry, together with the practicalimpossibility of making a direct measurement at the centre of the tumour, mean that a lotof time would be spent following up non-existent errors. Adeyami and Lord (1997) founda relatively higher proportion of ‘errors’ were due to the measurement system.

The European experience – particularly of the Leuven group (Leunens et al 1990) –


has been that a number of defects in practice have been identified as a result of in vivodosimetry. This makes it hard to avoid the conclusion that in vivo dosimetry provides theonly way to confirm that patients have really been given the intended dose. Errors detectedranged from errors in a treatment planning algorithm that could have been detected byphantom measurements to errors in patient measurements that could not. The followingrecommendations are made against this background:

1. Measurements in a phantom must always be made when:(a) a new planning technique is introduced;(b) a significant change is made to an existing technique;(c) a new planning system is introduced; or(d) a new treatment machine is commissioned.

2. In vivo measurements should be made following on from such phantom measurementson a representative sample of patients.

3. Entrance dose measurements should be made on a sample of patients on a routinebasis. These measurements should be restricted to one or two fields for any one patient.The sample should be shown to represent the full range of treatments carried out in adepartment, but emphasis should be placed on radical treatments. An annual programmeof such measurements is the minimum recommended, but more frequent measurementsare desirable.

4. In vivo dosimetry must be carried out as part of the quality assurance programme inany treatment programme that involves giving doses expected to reach the tolerancedose. Examples of this would include dose escalation studies. The frequency of suchmeasurements will depend on the estimated risk to the patient of any error. As aminimum, one measurement should be made during every individual course oftreatment. It would be good practice to measure the exit dose more frequently thanthis.

5. Exit dose measurements can be useful as a means of monitoring variability during acourse of treatment without significant perturbation to the radiation field. They providea useful confirmation of the stability of patient set-up. Combined with entrance dosemeasurements, they can also be used to estimate the dose in the centre of the patient.Such measurements are harder to interpret but allow the radiological thickness of thepatient and hence the dose to the target to be confirmed.

10.1.3 Methods

In vivo dosimetry can be carried out either with diodes or with TLD (see Chapter 8,Section 8.3). For measurements at the edge of the field TLD has the benefit of being lessenergy dependent and it is usually easier to place the dosimeter in the exact positionrequired. When measuring doses for verification of the target dose, however, diodeshave the advantage of providing an instant answer which allows a check of the set-up tobe made immediately in the event of a result which is outside the accepted limits. It isalso easier to measure the dose from an individual beam. For comparison with measureddoses it is necessary to calculate the expected entrance and exit doses for each beam.The entrance dose is simply the peak dose (i.e. at dmax) for the field size corrected forany accessories used. Exit dose calculations must account for the lack of backscatterbeyond the exit surface of the patient (see Section 10.1.3.2). This is not a correction tothe dose reading, but rather a correction to the planning computer calculation.

231In Vivo Dosimetry and Portal Verification

10.1.3.1 Entrance dose measurement

For measurement of the entrance dose to be useful it is necessary for full build-up to beprovided. If this is not done the dose measured will be very dependent on the field sizeand on any beam modifiers that affect the build-up curve. In addition, it is necessary forthe dose to be compared with a calculated value and it is only practical to calculate theexpected dose at the depth of the peak. Diodes are supplied with different thicknesses ofbuild-up depending on the energy at which it is intended to use them. In order to limitthe physical volume of the built-up detector, metal build-up can be used for energiesabove that of cobalt-60. Detectors should be calibrated by comparing the reading obtainedfrom the diode on the phantom surface to the dose at the peak. (It is common practice tocalibrate the diode so that it reads the dose at the peak directly, thus including an implicitcorrection for the difference in source-detector distance (SDD). For different SSDs asmall correction may be necessary to account for the ratio of the difference in SDDs, butfor a 200 mm change in SSD, this correction is only 1 per cent, even at 25 MV.) Whenmaking entrance dose measurements, it must be borne in mind that the measurementdoes perturb the beam – typically reducing the dose beneath the detector by 5 per cent.For this reason it is not recommended that detectors should be in position for more thana small fraction of the total irradiation time. If it is desired to measure the consistency ofdose delivery, dosimeters without build-up could be used or exit dose measurementscould be made. Entrance dose measurements can provide a check of:

• machine output;• wedge filters and other beam modifiers;• patient position in relation to the accelerator; and• monitor units set.

10.1.3.2 Exit doses

In using exit dose measurements, account must be taken of the reduction in dose at thesurface due to loss of backscatter. An appropriate methodology is described by Heukelomet al (1991) and Leunens et al (1990). Exit dose measurements will identify errors indensity corrections and in patient thickness measurements, but if the target volume is farremoved from the exit surface, these may provide a misleading estimate of errors indose to the target. The exit dose will also be affected by misalignment of the beam. Allthe factors listed under entrance dose measurements will change the exit dose, but errorsin the exit dose are harder to interpret.

10.1.3.3 Normal tissue doses

When measuring doses to normal tissues close to the beam edge, careful considerationneeds to be given to what is being measured. Electron contamination in the area justoutside the beam may lead to an overestimate of the dose at depth. It is appropriate touse some form of build-up material unless the tissue of interest is very superficial. Carefulpositioning of the dosimeter is essential if the results are to have any significance.Obtaining satisfactory measurements of eye doses is particularly difficult. Harnett et al(1987) have shown that external measurements of the lens dose may significantlyunderestimate the dose. They recommend placing the detector on the outer canthus ratherthan on the eyelid. (However, determination of the limiting dose may well have beenmade with surface measurements and these may also have underestimated the acceptabledose.)


10.1.3.4 Precautions to increase accuracy

Diodes are significantly affected by temperature and the temperature dependence increaseswith the dose delivered. In order to obtain a more stable calibration it is the practice ofsome suppliers to pre-irradiate them to a high dose. This has the effect of increasing thetemperature effect to about 3.5 per cent for the difference between room temperatureand the temperature of the patient’s skin. (If diodes are to be used exclusively for lowenergy photons, it may therefore be preferable to use unirradiated diodes. Withunirradiated diodes it is necessary to check the temperature dependence more frequently.)It has been shown (Andreo, private communication), using a thermistor mounted insidethe diode, that diodes reach an equilibrium temperature in about a minute and it is thereforenecessary to make a correction if they were calibrated at room temperature. Thetemperature correction for different diodes (even from the same batch) may well bedifferent and some diodes have been shown to have non-linear variations in sensitivitywith temperature. For measurements of temperature dependence the ideal is to have aspecially constructed water-filled phantom with a method of heating the water. A baby’sbottle warmer can also provide a simple way of heating the diodes since only relativemeasurements are required. TLD has the advantage of not being significantly temperaturedependent.

For both diodes and TLDs, small corrections are also necessary for field size and forbeam modifying devices in addition to the calculated changes in dose at the peak. This ispartly because the shape of the build-up curve varies with field size and with the use ofaccessories and the use of full build-up minimises these corrections.

TLDs must be packaged in a suitable container (e.g. a polythene sachet or tube) inorder to prevent contamination with grease, etc. It is also important to avoid scratchingthe surface of the TLDs when handling them. (Vacuum tweezers are the most satisfactoryway of picking them up.) Diodes can also be damaged by spirit based sterilising solutionsand in order to allow satisfactory hygiene it may be appropriate to package them also.

10.2 Portal verification

For radical radiotherapy, accurate geometric delivery of dose is at least as important asthe accuracy of the dose on the central axis. It is therefore essential that quality controlextends to determination of geometric accuracy. It has been shown theoretically (Webband Nahum 1993) that if even a small part of the tumour-bearing tissue is missed, theprobability of controlling the tumour becomes negligible. This is because if even a singleclonogenic cell is left undamaged, the tumour can regrow. Further work is needed toexamine the effect of the decreasing cell density progressing from the gross tumourvolume (GTV) to the clinical target volume (CTV) and to look at the effect of movementof the CTV within the planning target volume (PTV). Whatever the outcome of suchstudies, however, there can be no doubt that positioning accuracy is essential to tumourcure. In conformal radiotherapy the aim is to reduce the technical margin to the minimumwidth, since, for a typical 60 mm diameter target volume, an increase of 5 mm in themargin will increase the volume of tissue irradiated by 59 per cent. When tight marginsare being applied, the requirements for geometric accuracy are increased.


10.2.1 Random and systematic errors

A number of studies of field positioning errors have been carried out (see Chapter 1,Section 1.1.2). All have shown that, with the exception of stereotactically fixated brainfields, set-up accuracy varies significantly from one treatment to the next. This variationrepresents the random error.

In addition to these random errors, there may also be systematic errors. These can beof two kinds:

1. There may be a difference between the calibration of the simulator and the treatmentmachine which will lead to an error in the field centre for all patients.

2. For an individual patient there may be a difference in the way in which the patient ispositioned and this will lead to a systematic error for that patient alone.

If patient positions are to be altered as a result of verification films, it is important thatthe amount by which the field position is altered is compared to the expected randomerrors. A situation in which there is no systematic error can be converted into one inwhich there is, if a correction of the order of the random error is made on the basis ofonly one check film.

There may be some advantage, when using portal films, in carrying out the verificationon the second fraction as the set-up is likely to be faster and the patient may be morerelaxed.

10.2.2 Techniques to achieve good image quality

The position of the image detector relative to the patient can affect image quality.Traditional radiological practice is to place the film as close to the patient as possible toimprove the sharpness of the image. In making a megavoltage image with a portal imagingdevice, this is not usually the best solution. In diagnostic radiology scatter is reduced bymeans of a grid, but grids are not useful at megavoltage energies. Scatter may be reducedby increasing the distance between the patient and the image detector. A separation of0.4 m is sufficient. (A detailed study of the theory of megavoltage imaging is given bySwindell et al (1991).) For film imaging Bissonnette et al (1994) have shown that thereis an image-quality advantage in having the film as close to the patient as possible.However, it is helpful if the magnification of the portal film is the same as that of thesimulator film. A convenient standard source film distance is 1.4 m.

To reduce the dose needed to form an image, a cassette with lead screens can be used.The screen generates secondary electrons to produce build-up and high density materialsare used so that these secondary electrons are generated in close proximity to the film.Some improvement in the image is achieved with a copper screen on the patient side,and such a cassette is available commercially (Kodak L). When the film is placed veryclose to the patient the lead screen also has the advantage of stopping secondary electronsproduced in the patient. Roberts (1996) has conducted an evaluation of films availablefor portal imaging.

The detector must be held normal to the radiation beam to prevent image distortionand attachments to the accelerator head can be purchased to suspend the film perpendicularto the beam. To facilitate corrections to the patient set-up, it is necessary to be able toidentify the centre of the field. If the collimator edges will not all be visible on the film


lead markers can be attached to the X-ray head and removed once the image has beenmade. Markers are unnecessary if the detector is rigidly fixed in relation to the beam.

In order to make it easier to relate the portal film to the patient’s anatomy it is commonpractice to use a double exposure technique. An initial exposure is made with the treatmentfield size and any additional shielding. The field is then opened up to produce a 50 mmborder and a second exposure made with no additional shielding. Each exposure willrequire about 4 Monitor Units. In carrying out such procedures, it is important to avoidgiving excess dose to particularly sensitive structures, such as the eyes, which shouldremain shielded.

10.2.3 Methods of comparison with planning data

The major difficulty in verification is that megavoltage beam images have inherentlypoor contrast. A 10 mm thick bone that produces a contrast of 18 per cent at 50 kV willproduce a contrast of only about 2 per cent at 6 MV. Consequently it will not normallybe possible to identify the tumour on a megavoltage image (unless radioopaque markersare placed within the tumour). Decisions must therefore be based on bony landmarksand air cavities and it is necessary to establish the relationship of these to the targetvolume. This is usually done from simulator check films taken as the final verificationof the treatment plan. However, there are a number of pitfalls that must be borne in mindif these are to be used as a reliable basis for the analysis of set-up errors:

1. Many target volumes will have been defined on the basis of CT data. The sagittalposition of the centre of the target volume may be identified from a ‘scanogram’ (alsocalled ‘topogram’ or ‘scout view’ – the image generated by moving the patient throughthe stationary fan-beam). A scanogram differs from a simulator film in being formedby a beam that is divergent only in the transverse plane whereas the simulator beamdiverges in both directions. For this reason structural relationships will only be thesame at the beam centre.

2. Simulators are subject to inaccuracies in the same way that the treatment machinesare. This is a particular problem in the case of older simulators where the tolerancesspecified were often significantly less severe than those specified for treatmentmachines.

3. It is often difficult to establish the correctness of an oblique view taken on a simulatoron the basis of direct comparison with CT and it is usual to take these views on trust.

4. The assumption is made that the patient was in the same position on the simulator ason the CT scanner. As they become more familiar with the hospital environmentpatients often become more relaxed and this may affect the relationship of the tumourto the bony landmarks that are used to check the beam alignment on the simulator.

Although not all of these problems can be overcome, the fundamental rule should be totry to relate back to the criteria used by the radiotherapist when defining the target volume.Comparisons with CT based field definitions should ideally be made on the basis ofdigitally reconstructed radiographs (Sherouse et al 1990, Wong et al 1990) althoughthese, like the CT data themselves, suffer from inherently poor resolution in the sagittaldirection. The ultimate form of verification is to take a CT image in the treatment positionon the therapy machine, but this technology is still under development (Lewis et al 1992,


Nakagawa et al 1994) and the limitations of machine time will probably limit its use toresearch applications.

For digital images, which can include images digitised from radiographs, a number ofmethods of measuring the field placement error have been described in the literature(Shalev et al 1989, Meertens et al 1990, Bijhold et al 1991, Jones and Boyer 1991,Evans et al 1992, Gilhuijs and van Herk 1993):

1. Identification of point landmarks on the two images.2. Overlay of traced structures.3. Digital subtraction of images.4. Movie comparisons between images.5. Chamfer matching image registration.6. Correlation.

This is a rapidly expanding field and further development of automated techniques canbe expected. The choice of analysis method is usually tied to the software provided withthe portal imaging device, but Wong et al (1995) and Shalev et al (1996) have describeddevice independent systems.

10.2.4 Frequency of carrying out portal images

The minimum standard should be that some form of verification image (either from thesimulator or from the treatment machine) should exist for every new set-up with amegavoltage beam. For isocentric treatments this should consist of at least two films toestablish the position of the isocentre in three dimensions. For non-isocentric set-ups animage is needed for every beam. If the localisation of a superficial tumour has beenbased on surface landmarks a verification film may not be necessary. Images taken onthe treatment machine are more difficult to interpret, but most situations provide morecertain confirmation of treatment set-up than simulator films. For radical radiotherapywith tight margins a higher standard of localisation of the treatment beams is needed. Toachieve this a study must be carried out on a limited number of patients to establish themagnitude of random and systematic errors in the department. The view of EORTC isthat if conformal radiotherapy is being carried out verification should be carried out onan individual patient basis, with serial images. This is clearly an ideal to be aimed atunless it can be demonstrated that the use of some immobilisation device (e.g. astereotactic head frame) can reduce the random and systematic errors below an acceptablethreshold. It is recommended that a departmental standard be produced which describesthe accuracy which can be expected in that department for the different forms of fixationthat are used.

10.2.5 Transit dosimetry

A number of centres are investigating the use of portal imaging for dosimetric verification(Fiorino et al 1993, Evans et al 1995, Kirby and Williams 1995, Zhu et al 1995, Esserset al 1996, Hansen et al 1996, Boellaard et al 1997). When reliable results are availableby this method it can provide an appropriate replacement for the in vivo exit dosemeasurements already described. To achieve useful results it is necessary to know theposition of the source and the patient relative to the imaging device and to have an


accurately calibrated imager. In this application it is particularly important that theimaging device be placed sufficiently far from the patient to reduce scatter to a lowlevel. Calibration of the imager can be carried out by measuring the output with successivethicknesses of absorber placed on the treatment couch.

10.2.6 Commissioning and QC of imaging devices

Commissioning and quality control of imaging devices is relatively simple unless theimaging device is to be used for verification of dosimetry. Table 10.1 lists the tests thatshould be carried out.

Table 10.1. Recommended frequencies of QC tests for portal imaging devices.

Test and frequency Tolerance

WeeklyContrast resolution/sensitivitySpatial resolution

MonthlyMagnification 1%Position of the central axis of the machine 2 mmUniformity of the field

AnnuallyImage distortionReproducibility of positioning (for detachable devices) 2 mmContrast sensitivity

Portal imaging device quality control is still under development and it is thereforedifficult to say which is best. A portal imaging device should be able to see a 1 per centcontrast object of 5 mm diameter with a dose of 10 cGy at 6 MV.

Electronic portal imaging devices, like any other imaging system, require qualitycontrol to ensure their properties do not change significantly with time and that theirperformance remains within specification. Important properties are contrast resolution,spatial resolution, reproducibility and uniformity of response. It is also important toensure that the positioning of the detector relative to the treatment machine remainswithin tolerance and that the images produced are not distorted.

The visibility of an object depends on both its contrast and size. A variety of phantomshave been developed to measure contrast resolution of portal imaging devices. Some usea single object size and some use a series of object sizes. Designs include:

1. A set of holes of differing size and contrast (Wong et al 1993, Dong and Boyer 1994).2. A set of irregularly shaped objects of differing orientation and contrast (Lutz and

Bjarngard 1985).

Spatial resolution is generally measured using a high-contrast phantom with closelyseparated objects. Designs include:


1. Several sets of rectangular bars, where each set has a different spatial frequency(Rajapakshe et al 1996).

2. A ‘Crow’s foot’. This consists of a set of wedges which are arranged in a circle sothey meet at their thin end (Morton et al 1991).

The ‘crows foot’ phantom gives a simple, quick indication of spatial resolution, whilethe rectangular bar phantom can be analysed to get the form of the transfer function.

Contrast and spatial resolution should ideally be checked monthly. The results obtainedwill depend on both dose and magnification and thus a protocol for the exposure timeand source to detector distance is necessary.

Reproducibility is particularly important for the transit dosimetry applicationsdiscussed in Section 10.2.5. This is typically measured by integration of the intensityvalues within a region of interest in an open field image or an image of a uniform object.The variability of intensity should be ideally no more than 2–3 per cent (Kirby andWilliams 1993).

In measuring the uniformity of the field it is important to consider the non-uniformityof the primary radiation beam. For imaging purposes the aim will be to produce a uniformfield in spite of this, but for dosimetry the non-uniformity of the beam should be preserved.(Thus different calibration data and often different calibration techniques may be usedfor dosimetry and for imaging.)

For all applications of electronic portal imaging it is essential to ensure the positioningof the imager remains within tolerance. The positioning of the detector will vary withgantry angle, but as long as this fluctuation is reproducible, it may be corrected for(Kirby 1996). This has been measured using a set of LEDs placed inside the detector(Vos et al 1996). Phantoms have been developed to measure both detector positionvariation and X-ray/light field congruence (Kirby 1995, Luchka et al 1996).

Distortion may be measured by imaging straight edges placed at different parts of thefield. This should be done annually.

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Wong J, Yan D, Michalski J, Graham M, Halverson K, Harms W and Purdy J 1995 Thecumulative verification image analysis tool for offline evaluation of portal images Int. J.Radiat. Oncol. Biol. Phys. 33 1301–1310

Zhu Y, Jiang XQ and Van Dyk J 1995 Portal dosimetry using a liquid ion chamber matrix:dose response studies Med. Phys. 22 1101–1106

CHAPTER 11

Control of In-house Software

11.1 General background

Although the majority of radiotherapy centres use commercially available TPSs, manycentres use in-house developed software or spreadsheets for checking treatment plancalculations as part of their quality control system for Monitor Unit calculations or forparticular applications. This software should be subjected to the stringent controls thatwould apply to commercially available software. The software may be run on hardwareranging from mainframes to PCs and pocket calculators. The requirements for reliabilityof the hardware must also be considered as discussed in Chapter 4, Section 4.4.2.1.

It is generally acknowledged that an inordinate amount of time may be spent inmaintenance, enhancements and the addition of remedial features once a developmenthas been released. Careful planning and the use of techniques which are aimed at assistingin the future development and maintenance of the software will considerably reduce thetotal resources required over the software life cycle.

These principles are now widely taught, but they are not readily assimilated in thescientific community, where computer software is generally regarded as a tool to assistin a task. Recommendations are made through British Standard BS 7165 (BSI 1991)which contains useful checklists for all areas of software development.

The IPSM Computer Topic Group raised awareness amongst Medical Physicists in1989 by laying a set of ground rules (HPA 1989). Reassurance of the level of quality canbe gained by departmental managers by registering under the DTI TickIT scheme (DTI1992) which is an interpretation of the International Quality Management Standard BSEN ISO 9000 (1994). Useful guidance can be obtained from guides written by bodiessuch as the National Computing Centre (NCC) under programmes such as STARTS(STARTS Guide 1989, The National Computing Centre Ltd, Oxford Rd, Manchester M17ED).

Purchase of off-the-shelf software, such as a commercial Treatment Planning System,is not considered here, but similar principles should be applied when the product isspecified and when it is installed on site.

11.1.1 Aims for software quality

Fundamental requirements are accuracy, reliability, ease of use and ease of maintenance.These help make good use of resources and are established by:

1. A clear definition of what is to be achieved (specification).2. A description of methods which must be used for all work (code of practice).3. Monitoring of performance (quality control).4. Quality audit for non-trivial developments (quality assurance).

Software cannot be considered without taking into account the hardware systems on


which it runs. Essential elements in the total process are:

1. Adequate planning of all elements of the system before starting.2. Monitoring against defined standards by qualified personnel throughout the life cycle.3. Departmental policy to provide uniform methods and the assessment of quality.4. General controls, e.g. virus protection/user authorisation, etc.

11.1.2 Responsibility

The ultimate responsibility for any development must lie with the department managers,who must convince themselves that adequate controls are in place to ensure the best useof resources. The managers must take into account the whole life cycle of the software,together with the possibility that there may be a change of personnel in the support ofthe software when deciding on the level of control which must be used. Planning is alsoimportant as a task can only be controlled to the detail to which it has been planned.

Flexibility is often seen as one of the benefits of in-house software production, butthis should not be seen as a way of absolving responsibility for the long term requirementsof the department. Flexibility can be retained, but should be contained within a controlledenvironment.

Documentation is a key issue in providing continuity in a project. It provides apermanent record which is accessible to all and not subject to the distortions of memory.It is the responsibility of all involved in the project to show the origins, the developmenteffort and the fitness for purpose of each part. This provides anyone involved in futuremaintenance or enhancements with the opportunity of setting clear specifications andactions. Specifications need not be set in tablets of stone, but any change which is madeshould be controlled.

Adherence to standards can assist in assuring the managers and the end users that thework is progressing satisfactorily, but developers cannot be expected to undertake whatis easily seen as non-productive work without adequate training.

11.1.3 Limitations

Code is often inherited within a department in a poorly documented state, or written in alanguage or manner which is considered to be currently inappropriate. It may beimpractical to rewrite such code, due to the investment required and also the danger ofinserting problems into functioning code. In such cases serious consideration should begiven to using commercial packages to reverse engineer the code in order to verify thelogical pathways and the validity of the variables passed between modules. The value ofusing old code should be weighed against the liability incurred in the case of an error.

Every opportunity should be taken to seek practical ways to incorporate code into adepartmental quality system at an appropriate time such as at a major enhancement. Itmay be beneficial to incorporate current projects into the departmental system in anevolutionary way which does not prevent the products from being completed as originallyspecified.

Learning new languages for small projects would be inadvisable, but training shouldfeature strongly on the budget for any project, so that the best use can be made ofresources, both from within and outside the organisation.

243Control of In-house Software

11.2 Procedures for software control – overview

Small projects, especially tutorial exercises, often succeed with a flexible approach, buta lack of control will cause inefficiencies and failures in all but the most trivial ofdevelopments. The controls mentioned below may be tedious from an individual’sviewpoint, but are essential if one considers the long term benefits for the departmentwith changes of personnel and techniques.

It is recommended that each of the following topics is addressed for every exercise,whether it be a feasibility study, the production of new software, or an extension toexisting products:

1. Specification – a written understanding between the project developers and the peoplewho will use the product.

2. Project management – controlling the methods of development.3. Programming – consisting of :

(a) Analysis – what is to be achieved.(b) Design – how it will be achieved.(c) Coding – the task in hand.(d) Testing – confirming the predefined level of quality.

4. Documentation – should be developed throughout the project and not left until theend – documentation is the key to future evolution.

5. Project library – a regulated storage of project records and products.6. Implementation – must include a plan for training and user authorisation.7. Product control – reliability of operation, controlled modifications, security of code

and bug reporting.8. Back-ups – long term recovery plans.

None of these stages should be considered in isolation, and it should be emphasised thatcoding, the commonly conceived core of the work, takes up a relatively small part of theproject and only after careful planning.

11.3 Procedures for software control – one solution

Different approaches can be made to controlling development work, ranging from informalpeer review to formalised project management systems. The results of any method shouldbe recorded to ensure that there is a reference for future work.

It is important that no approach is enforced, as local situations and project size willrequire differing controls. However, in order to avoid leaving departments with only anebulous framework, a suggestion for a more formal approach to the eight points aboveis indicated here, leaving departments to extract useful concepts for their requirements.This formal approach has proven useful, even on small projects, in keeping the objectivesand the work patterns in focus.


11.3.1 Specification

1. The user’s (customer) specification must be clear and unambiguous.2. Any potential supplier (off-the-shelf software vendors, commercial software houses

and the in-house group) should respond point by point as to how the requirementswill be met. Clarification of the objectives should now be achieved.

3. Consideration will need to be given to the long term requirements (e.g. systemdevelopment, product research, maintenance capability, local expertise, etc.) indeciding the best route for supply of the product.

4. A specification may change, but any change should be formally controlled. Movinggoalposts is a common cause of failure.

5. The project documentation should be filed in a systematic fashion (Project library –see Section 11.3.5). It will include appropriate sections from the following:(a) a description of what the product should achieve;(b) identification of project manager and team;(c) the intended development tools;(d) the test methods to be used;(e) a hazard analysis;(f) software safety and data protection requirements;(g) (suitability of the hardware, including reliability in safety critical areas); and(h) a plan for the development and maintenance of the program in the long term.

6. If a firm specification cannot be achieved due to the work being innovative, a feasibilitystudy should be performed with the objective of defining the requirements. The studyshould be controlled using project management techniques such as those indicatedbelow and should have a definite goal. When this has been achieved the study shouldbe closed down, ensuring that the software does not drift into development and use.

11.3.2 Project management

The size of the project will dictate the size of team required to complete the task, buteven in the case of a one man operation the project should be carefully managed. Goodproject management will reduce the time to completion. Assistance with management ofsmall projects can be obtained from Park Place Training (SPOCE – Small Projects in aControlled Environment, Park Place Training, Poole, Dorset BH14 0HP).

There is concern that a minor project should not be overshadowed by unnecessarymanagement. Also, the resources available within a small department may demand theadaptation of techniques. If a program is purely for personal use and will not affectanyone else, then departmental controls could be eased. However, the decision as towhich development route to take must be a departmental one which bears in mind thatgood project control should improve the long term productivity.

1. Departmental codes of practiceCommon procedures which have been developed with peer group review encouragepeople to follow a departmental style and this increases understanding and reducesthe training requirements.(a) These procedures should contain:

(i) a policy which dictates approved and forbidden work practices for eachlanguage or system with explanations (e.g. create logical hierarchies of


directory structures which separate differing items such as sources,executables, data, etc.; avoid use of the GOTO statement in Fortran as syntaxwhich is less prone to creating errors usually exists);

(ii) agreed project standards (e.g. review frequency);(iii) a list of useful tools (e.g. Language Sensitive Editor) and techniques with

lists of useful references such as User Manuals;(iv) examples of suitable techniques for developments (e.g. templates available);(v) a list of responsible persons to whom the working team can turn to for

authority or advice on specified topics (e.g. the System Manager);(iv) a good system for dissemination of all information to all who need it; and(vii) any operational facilities (e.g. backups) provided by the System Manager.

(b) These codes of practice should be reviewed at least once a year and be incorporatedinto all current projects in a manner which reduces the risks to that project. Theresult of the review must be recorded.

(c) Conformance with these procedures should be monitored and recorded.(d) Anything which does not match up to the project standards should be recorded.

There should be a system for controlling deviations and the action taken followingtheir report.

2. Project construction(a) One person should be detailed to be responsible for the day-to-day running of

each project (Project Manager).(b) The project should be broken down into manageable stages, each one lasting for

no longer than 3 months.(c) The project should have a defined end point and be closed when this is achieved.

A temptation to extend the work should be resisted even if the development isplanned to continue. The creation of a new project will help to ensure that thework is adequately controlled.

3. Review proceduresThese should all be recorded (using a proforma will speed the process and encouragesa complete record).(a) End Stage Reviews – Reviews held at the end of stages with someone who is

independent of the day-to-day work. The project will dictate whether this reviewshould include anyone from outside the department, such as technical advisers orindependent quality assessors. The end user could provide valuable feedback andclarification at these points in time. Approval of one stage is required prior tocommencing the next. Records must be kept in the project library.

(b) Team Meetings – Regular, brief meetings with the working team detail the currenttasks and who will be performing them, and identify deviations at an early date.

(c) Manager Reports – Brief, written, intermediate reports collated from the notesfrom the team meetings, keep the department managers in touch with the projectprogress.

11.3.3 Programming

The object is to provide a comprehensible and structured product. Poor technique willhinder debugging, testing and development. The breakdown below develops a ‘top-down’


approach to the process which may not be suitable in techniques such as ‘object-oriented’programming, but an equally thorough system should replace it. Language specificreferences are numerous and likely to date quickly. Take advice locally for useful texts.

1. Analysis(a) Break down the broad objective into modular objectives. These modules may be

further subdivided as necessary.(b) A clear logical flow should be demonstrated between the modules.(c) The data flow must also be defined and clearly comprehensible, including the

input and output from each module.

2. Design – This phase may often be returned to during the remaining processes. Ensurethat records are kept of each version.(a) Do not write any code at this stage – use pseudo-code or English.(b) Define the names of modules and key variables and retain these throughout the

design.(c) Give variables meaningful names.(d) Start at the main module, which should describe the overall control and proceed

to each sub module.(e) Each module should perform one function, have a single entry and a single exit

point.(f) A walk through of the design by a reviewer and the user may highlight

discrepancies.

3. Coding(a) Use language standards where practicable to aid portability.(b) Use a language sensitive editor – it assists in creating a standard layout and speeds

development by providing the required language structures.(c) Supplement the code by comments, unless the language is self-commenting.(d) Complete a header with module name, called and calling modules, version number

and change log unless this information is provided by another mechanism.(e) Errors should be trapped, appropriately decoded and acted upon.(f) Check for potential maths errors (e.g. division by 0) and both data and array

bounds prior to use.(g) Where the program logic forks, code for default even if no action is to be taken.(h) If possible, pass data to and from a module via an argument and not by global

variables.(i) Declare data types explicitly.

4. Testing – A topic which is often poorly addressed (see Beizer (1984) for assistance).A thorough examination of the testing of a Treatment Planning System is provided byJacky and White (1990), where the physical validation of the system is separatedfrom the program verification.(a) Detail and approve tests before programming phase – consider automating the

tests.(b) Develop the tests before or during the coding and not afterwards.(c) Tests must include both data which is expected and out of bound data. Particular

attention should be paid to boundaries.(d) Test data should be specified and recorded.(e) Record all successes and failures.


(f) A code walk-through will assist in identifying weaknesses.(g) Test modules before insertion into final product.(h) After addressing deviations or failures, the program or module is retested and the

results recorded.(i) Site trials must pass through the formal controls of specification, testing and

reporting in conjunction with the user. A useful technique of designing softwarephantoms has been developed in Nuclear Medicine and may offer interestingpossibilities in Radiotherapy (Quality Assurance of Medical Software – COSTB2Project (Cosgriff PS et al., personal communication)).

(j) Systems tests should be designed and run by someone else, preferably the user.(k) Check that comments and documentation are accurate.

5. Tools(a) Source code control (code management) – assists the documentation by preserving

a historical record of the developments and contains inadvertent changes.(b) Navigation tools provide a powerful aid to assist documentation.(c) Module management (Make) facilities greatly speed compilation times.

6. Identification – Software and possibly data files should be identified by unique versionnumbers.(a) This indicates the historical progression of the software.(b) Can be used to identify the interdependence of the modules.(c) The program version number must be displayed on entry into the suite and also

on any hardcopy.

11.3.4 Documentation

Documentation is required on any project: use references where appropriate – do notrewrite existing work. Consider storing appropriate parts of the documentation in asubdirectory related to the sources, so that they migrate together.

Documentation need not be seen as needing volumes of paperwork. A few sentencesmay be all that are required, but they should be stored in a manner which makes themretrievable by all who may need them at any point in the future. Consider using a templateor proforma to reduce the work load, and once a system has been developed for oneproject, adapt it for future work.

The appropriate documents should be chosen from:

1. Design documentation including project results (see Section 11.3.5).2. Test documentation – specification and results.3. User documentation (Brockmann 1986).4. Maintenance documentation to assist future programmers.5. Physics manual (if the complexity of the process demands explanation).

11.3.5 Project library

All documents should be listed and any changes to the documentation should be recorded.The information should be readily available to all who require it.


1. The following documents should be stored on behalf of the Project Manager detailingthe historical evolution of software:(a) The documentation in Section 11.3.4.(b) Records of faults with action initiated.(c) Records of software modifications.(d) Records of document modifications.(e) Records of the controlled release of software.

2. Records stored by the user:(a) Clear records of the accuracy and reliability of the product during use.(b) Demonstration of the integrity of data.

3. Reconstruction of past versions:(a) Sources must be available to recreate previous implementations – use code

management or dated version numbers.(b) Library modules and command files which created working versions must be

stored and identified.(c) Details of the compilation process must be recorded.

11.3.6 Implementation of product

1. All software which relates to patients should be passed for use by a person or groupwhich is independent of the project. This person need not be thoroughly familiar withprogramming technique, but should be at a level of competence to provide anindependent view as to whether the quality defined by the department has beenachieved.

2. Sources for any released software must be stored securely, preferably on a differentdisk partition to any development software.

3. Users should be provided with all the necessary documentation and training to enablethem to use the system fully.

4. Consideration should be given to password control and licensing users.5. The location and version number of all current installations should be recorded.6. The user should thoroughly test the product, and all bugs which are not trivial should

be addressed and the software retested before final installation.7. Consideration should be given to preventing unauthorised alteration to the sources,

the executable images and the data.

11.3.7 Product control

1. A system should be established for users to report bugs or request changes.2. Minor changes should be collected together and only be addressed during a major

rewrite to reduce the effort required for testing the system.3. Changes must only be made in a controlled and documented manner.4. Store details of all changes and testing in the project library.5. Users must be advised of any changes and the effect on the product.


11.3.8 Backups

1. Make secure copies of all software and store off-site if necessary.2. Regular backups are essential – software investment often exceeds hardware costs

and is far less replaceable.3. Only make working copies from the master and record the event. This will help prevent

the unintentional initiation of duplicate development paths being created.

References

Beizer B 1984 Software system Testing and Quality Assurance (New York: van Nostrand)

Brockmann R J 1986 Writing Better Computer User Documentation (Wiley)

BS EN ISO 9000 1994 Quality systems (London: British Standards Institution)

BSI 1991 Recommendations for Achievement of Quality in Software BSI 7165 ISBN 0-580-18843-4 (London: BSI)

DTI 1992 Guide to Software Quality Management System Construction and Certificationusing EN 29001 ISBN 0-9519309-0-7 (HMSO: London)

HPA (Hospital Physicists Association) 1989 Guidelines for Software Safety HPA BulletinMarch 1989

Jacky J and White C 1990 Testing a 3-D Radiation Therapy Planning Program Int. J.Radiat. Oncol. Biol. Phys. 18 253–261

APPENDIX A

Procedures for the Definitive Calibration of RadiotherapyEquipment

Guidance on procedures for definitive calibrations in radiotherapy were contained in thefirst edition of Scope in March 1992, p. 33. However, since this publication may not beuniversally available, the guidance issued then by the Institute of Physical Sciences inMedicine is reproduced here. The opportunity has been taken to update the references,but the text is otherwise unaltered. (This guidance has also been included in the Healthand Safety Executive document PM77 (HSE 1998).)

The text is as follows:In order to establish good working practice for the definitive calibration of new

radiotherapy equipment or following a major modification of existing radiotherapyequipment, the IPSM issues this guidance. The guidance incorporates that issued by theIPSM in 1988, but extends the scope to include both external-beam radiotherapy treatmentmachines and radiation dose measuring equipment. It is assumed that current dosimetryprotocols will be followed (IPSM 1990a, IPEMB 1996a, 1996b) and that full qualityassurance programmes will be adhered to.

A definitive calibration of radiotherapy equipment is one which forms a baseline forsubsequent confirmatory measurements. Disagreement between subsequent measurementsand the definitive calibration may be an indication for a new definitive calibration(Recommendations 2.7 and 3.3).

Definitive calibrations of external-beam radiotherapy treatment machines or ofradiation dose measuring equipment must be carried out in the following circumstances:

External-beam radiotherapy treatment machinesTo determine the radiation output per monitor unit or per unit time:

(i) as part of the commissioning procedure of a new linear accelerator, cobalt machineor other external beam radiotherapy treatment machine;

(ii) following major repair or modification to external beam radiotherapy equipmentwhich might reasonably be expected to affect its calibration, for example, when anew dose monitor is installed in a treatment machine in which dose delivery iscontrolled by that monitor;

(iii) following the replacement of radioactive sources in cobalt or similar teletherapyequipment.

Radiation dose measuring equipmentTo derive a calibration factor:

(i) for new dose measuring equipment which is to be used in the definitive calibrationof radiotherapy treatment machines;

(ii) following major repair (e.g. thimble replacement) of dose measuring equipmentused in the definitive calibration of radiotherapy treatment machines.

251Procedures for the Definitive Calibration of Radiotherapy Equipment

Recommendations

1. General

1.1 The fundamental principle behind these recommendations is that any definitivemeasurement should be subjected to an independent check and that proceduresshould incorporate specific cross checks. Written procedures should be reviewedto ensure that this fundamental principle is adhered to.

1.2 Responsibility for definitive calibration must be vested in a physicist appropriatelyexperienced in radiotherapy physics to the Standard1 required by the Institute ofPhysical Sciences in Medicine.

1.3 Written procedures must be drawn up and followed. Measurements and observationsmust be fully documented.

1.4 Any parameters upon which the calibration depends, such as distance measurementand timer operation in cobalt treatment machines, must be checked according tothe procedures in the quality assurance programme.

1.5 All factors and quantities in any calculations must be written down even if they areunity.

2. External-beam radiotherapy treatment machines

2.1 The definitive calibration must be derived from two independent sets ofmeasurements made by two physicists experienced in radiotherapy physics2, usingdifferent dosemeters. All equipment must be removed and all the relevant treatmentmachine parameters (e.g. SSD and field size) must be changed and reset betweenthese measurements.

2.2 Each of the two dosemeters used in the definitive calibration must have beencalibrated according to Recommendations 3.1–3.6, although each dosemetercalibration may be referred to the same secondary standard dosemeter.

2.3 For teletherapy equipment employing a radioactive source, the definitive calibrationmeasurement must be compared with the supplier’s certificate of calibration. Dataderived from the certificate of calibration should not be regarded as a substitute forother recommended measurements, but any difference between these data and thedefinitive calibration must be reconciled.

2.4 Using the data to be supplied for clinical use, the response of a suitable dosemeterin a phantom must be calculated for a different treatment time or number of monitorunits from that used in the calibration. The definitive calibration is confirmed ifthe predicted reading is obtained within the limits of experimental uncertainty.

1 An appropriate standard for this responsibility is recently at least 6 years inradiotherapy physics and national assessment for competency by two members of theNational Panel of Assessors (IPSM 1990b).

2 An appropriate standard for this experience is currently at least 4 years in radiotherapyphysics and national assessment for competency by a member of the National Panelof Assesors (IPSM 1990b).


2.5 Subsequent confirmatory calibrations may use simplified measurement procedures.These procedures, and the ratios of measurements under the two sets of conditions,must be established at the time of the definitive calibration. Comparisons with anyprevious similar ratios must be made, and if the ratios differ by more than thelimits of experimental uncertainty, the difference must be reconciled.

2.6 Confirmatory measurements, according the Recommendations 2.1, 2.4 or 2.5 mustbe made at regular intervals not greater than one week for linear accelerators andone month for cobalt units (which accords with paragraph 7.20 and 7.22 of theGuidance Notes 1988), by staff under the supervision of a physicist experienced inradiotherapy physics.

2.7 When a confirmatory measurement of the output of a linear accelerator or other X-ray therapy unit is found to differ by 3 per cent or more from the expected value,the possibility of a contributory machine fault or of a measurement error must beconsidered. The appropriate action will depend upon circumstances in the individualdepartment, but may include carrying out a definitive calibration. The action takenmust in all cases be the responsibility of an experienced physicist as defined inRecommendation 1.2. Each department should have a written protocol definingthe procedure to be followed.

3. Radiation dose measuring equipment

3.1 A definitive calibration of radiation dose measuring equipment must be derived byan intercomparison between the field instrument and a reference dosemeter. Thereference dosemeter should normally be a secondary standard with a calibrationtraceable to the National Physical Laboratory. Exceptionally, a tertiary standardmay be used, in which case these guidelines must be rigorously followed at eachintercomparison.

3.2 In establishing the procedure for cross calibration of dosemeters, considerationmust be given to the principle described in Section 1.1. The following guidelinesshould be observed:(i) Two (or more) independent measurements must be carried out.(ii) Where the chamber is to be calibrated at more than one beam quality and the

relative calibration factors are known for chambers of that construction,requirement (i) can be met by demonstrating that the calibrations at thedifferent qualities are consistent within 1.5 per cent of the expected value.

(iii) If only one beam is available, a repeat calibration should be carried out inthat beam. However, constancy of the readings made with a strontium-90source at the previous definitive calibration may also be regarded as anindependent measurement in some circumstances.

(iv) Two physicists experienced in radiotherapy physics must be involved in themeasurements. Ideally, the calibrations should be carried out by differentphysicists, but it is sufficient that the second physicist should check the results.

(v) If a repair has been carried out, it is sufficient to show that no change in thecalibration factor has occurred at the extremes of beam quality to be used.

(vi) For an entirely new dosemeter, calibrations must be carried out for each beamquality. If the expected variation of calibration factors with beam quality is

253Procedures for the Definitive Calibration of Radiotherapy Equipment

not known, the calibrations must be repeated by an independent physicistexperienced in radiotherapy physics.

3.3. A check on the constancy of calibration of all instruments must be made using astrontium-90 check source before and after the definitive measurement. Anagreement within 1 per cent is necessary.

3.4 Where possible, the calibration factors obtained should be compared with thesupplier’s certificate of calibration. Data derived from the certificate of calibrationshould not be regarded as a substitute for other recommended measurements, butany difference between these data and the user’s calibration must be reconciled.

3.5 If practicable, the calibrated chamber should be used for the routine calibration ofa teleisotope source, and the consistency of the results with previous calibrationsconfirmed before it is used for other calibrations.

3.6 Paragraph 7.25 of the Guidance Notes (NRPB 1988) requires that a calibrationtraceable to the National Physical Laboratory should be carried out at least annually.To comply with that requirement, this annual calibration should cover arepresentative subset of the radiation qualities in use. If a change greater than 1 percent is observed, a new definitive calibration should be carried out.

References

HSE (Health and Safety Executive) 1998 Fitness of Equipment Used for Medical Exposureto Ionising Radiation Guidance note PM77 (second edition)

IPSM (Institute of Physical Sciences in Medicine) 1988 Procedures for the DefinitiveCalibration of External Beam Radiotherapy Equipment (York: IPEM)

IPSM (Institute of Physical Sciences in Medicine) 1990a Code of practice for high-energy photon therapy dosimetry based on the NPL absorbed dose calibration service.Phys. Med. Biol. 35 1355–1360

IPSM (Institute of Physical Sciences in Medicine) 1990b Guidance on assessment criteriafor medical physicists in different grades prepared by the Board of Assessors of theInstitute of Physical Sciences in Medicine HPA Bulletin December 1990 (York: IPEM)

IPEMB (Institution of Physics and Engineering in Medicine and Biology) 1996a TheIPEMB code of practice for electron dosimetry for radiotherapy beams of initial energyfrom 2 to 50 MeV based on an air kerma calibration Phys. Med. Biol. 41 2557–2604

IPEMB (Institution of Physics and Engineering in Medicine and Biology) 1996b TheIPEMB code of practice for the determination of absorbed dose for x-rays below 300 kVgenerating potential (0.035 mm A1 – 4 mm Cu HVL; 10–300 kV generating potential)Phys. Med. Biol. 41 2605–2626


APPENDIX B

Survey of Quality Control Practice in UK Hospitals CarriedOut by the Radiotherapy Physics Topic Group

Introduction

At the end of 1991, a survey was carried out by the Radiotherapy Physics Topic Groupwith the aim of reviewing current practice in UK hospitals. In total, 44 questionnaireswere returned. Taking into account responses which covered small groupings of centresthis corresponded to just over an 80 per cent return rate nationally. A very large majorityrequested that firm recommendations on check frequencies appear with IPSM sanction.This is not what is presented in this Appendix but is the intention of the main body of thereport. This appendix is a slightly abbreviated form of the report presented in Scope 149-61 (December 1992). Data on the equipment available and comment on theappropriateness of the test frequencies by the authors have been omitted. The report isreproduced here because it provides the basis on which some of the recommendationsgiven in previous chapters of this book are based. The editors of this book were responsiblefor the preparation of the original report.

The aim of the questionnaire was to elicit as broad an opinion as possible on therequired level of Quality Control checks on all physics related aspects of radiotherapy,and to relate this to what was actually being done.

There are many problems in designing a questionnaire. One of the most difficult isthat of interpretation, in that the frequency supplied for a given check means little withouta detailed description of the check itself. However, the preparation of a document givingsuch details and the expectation that respondents would wade through it, would havebeen a severe disincentive to achieving a useful conclusion from the study. The approachtherefore was to provide a detailed checklist which would act as a prompt for whateverinformation the user cared to supply. The request for information on the three levels ofchecking, namely the level aimed at, the minimum (safe) level, the desirable level in anideal situation and also the staff groups performing these checks, gave a good insightinto current practice in 1992. Views of individuals may have changed since then.

In response to general questions covering quality control on the full range of apparatusincluded in the survey, 39 per cent stated that they did not perform what they consideredto be the desirable level of checking. Furthermore, 33 of 37 centres said that theyoccasionally did not perform what they considered to be the minimum level and two ofthese admitted to not achieving this level frequently due to staffing difficulties. Aspectsof staffing, machine availability and the working of unsociable hours are vital factors inrelation to the standard of quality control which are not considered in the study, but forwhich the results of the study may have implications.

255Survey of Quality Control Practice in UK Hospitals

Megavoltage photons and electrons

Of the 44 returned questionnaires, 43 submitted responses to the megavoltage X-raysection and 31 of these also had an electron facility. Each centre supplied details of theirequipment. A total of 107 linear accelerators were covered, 48 of these with an electroncapability. The median accelerator age was 5 years with a mean of 6.8 years.

A great weight of opinion was in favour of a number of critical parameters beingchecked daily and there is undoubtedly an increasing need to demonstrate good practiceparticularly in the event of legal action. Physics departments could often not hope toachieve this level of checking and an encouraging use of other staff groups, particularlythe radiographers working on the machines, to do ‘quick checks’ was evident. Thephysicist historically performs a lower frequency of more detailed checks but increasinglyalso supervises written records from other staff groups and may in the near future berequired to have a more formalised response to problem reports from these sources.

Many of the checks listed in the study are interdependent and a different bias may beseen due to the practice in individual centres. For example, where daily calibrations areperformed using a Farmer ionisation chamber in a water equivalent phantom, anyrequirement for quick output checks may disappear. On the other hand the use of constancymeters may satisfy the pressure for increased monitoring while minimising the burdenon the Physics Department by permitting a relatively low frequency of calibration inaddition to the constancy meter checking. A similar situation may be seen when contrastingflatness or symmetry measurements, where the available equipment may dictate therespective frequencies. For example, if the department possesses a scanning or multi-detector array instrument which can be quickly set up, it may be routine to obtain apicture of the whole beam profile rather than do the ‘quick-check’ of measuring thesymmetry at a small number of pairs of points spaced equidistant from the central axis.The level of available equipment may increase the frequency of quality control checksor increase the level of information obtained for the same effort, and is a vital areawhere productivity gains might be achieved, providing the basic funding is forthcoming.

A possible criticism of results inferred from a questionnaire is the reliability of theinformation supplied. Unless accurate records are kept, the frequency of checks that areactually performed may differ markedly in some areas from that intended or imagined,due to breakdowns, holidays, unavailability of machine time or staff, or even the personalpreferences of the checker. Furthermore, checks may in some cases be implicit. Forexample since door interlocks and many other devices are used daily, it may be impliedthat these are checked. Obviously this is insufficient and only checks made explicitly aspart of a formal routine and preferably with a written record can be almost guaranteed tohave been performed properly. However, the request for the three levels of checks reducesthe need for complete accuracy since the disposition of the study was as much to currentopinion as to practice. The impression on statements of practice was one of candour,with many centres indicating that less critical areas of quality control (e.g. checks atdifferent gantry angles) are infrequently performed and minimum targets not achieved.


Detailed analysis of the questionnaire revealed a large spread in suggested checkfrequencies in many instances covering the full spectrum from ‘daily’ to ‘commissioningonly’. However, the modal frequency was usually immediately apparent so that it is fairto say that a consensus was achieved. The values quoted in Tables B.1–B.3 are the medianresults for what was considered to be the minimum level of checking. The medianfrequency is reported in preference to the modal, in order to give appropriate weight tothe views of centres who advocated frequencies towards the ends of the range.

Table B.1. Photon and mechanical checks at gantry 0° (median minimum frequencies).

Daily (quick check) Twice per week WeeklyOutput constancy Output constancy CalibrationLight field Crosswire Light fieldDistance meter Distance meterAxis lights Axis lightsCrosswire PointersDoor interlock Door interlockAudiovisual system Backup DosemeterMachine log

2 weeks Month–6 weeks 3 monthsEmergency-off Symmetry Flatness profileDeadman’s switch X-ray field Beam energyInterlock limits X-ray light coincidence Arcs (if used)Visual inspection AFC tuner Dosemeter reproducibility

Gantry isocentreDosimetry interlocksWedge factorWarning lightsMechanical fixturesAsymmetric collimators

Yearly Commissioning/repairPenumbra Dosemeter daily stabilityX-ray isocentre Leakage radiationDose linearity Field size outputCouch sag/vertical

Table B.2. Electron checks at gantry 0° (median minimum frequencies).

Daily (quick check) WeeklyOutput constancy Calibration

Month 6 weeks Yearly Commissioning/repairRadiation symmetry X-ray contamination Field sizeBeam energy Applicator output Applicator leakageDiaphragm positionsDisplay readoutsExcess dose rate trip


Table B.3. Photon and electron checks at gantry angles other than 0° (median minimum frequencies suggested byrespondents but not done).

Monthly Three-monthlyMechanical fixtures Photon symmetry and flatness

CrosswireLight field

Six-monthly Yearly Other annual checks mentionedPhoton output Photon energy Wedge factorsElectron output Electron energy Energy and % depth dose in waterElectron symmetry Electron flatness Assisted set-upPhoton opposing fields X-ray field size Head leakage

For photons at gantry angle 0°, the ‘minimum’ level was generally either the same orone time stop less frequent than that aimed for (see Table B.1). For electrons (see TableB.2) the ‘aim’ and ‘minimum’ level were closer but occasionally with the aim being lessthan the minimum. Checks at gantry angles other than zero degrees were generally veryinfrequently done for photons and virtually not at all for electrons (see Table B.3).However, there was a clear consensus that they were not only desirable but that there isa minimum requirement. The median ‘minimum’ frequencies given in Table B.3 thereforedo not actually reflect current practice for this group. The daily checks listed in TablesB.1 and B.2 are exceptional in that they are not minimum values as in the rest of thetables, but reflect what is either currently happening or a strong feeling that they aredesirable. These checks are then repeated under other headings where they are performedin greater detail or by a different staff group. It is worth singling out electron outputchecks from this list. Although daily checks are often performed, and are certainly thoughtto be desirable, the large number of available electron beams can present serious logisticalproblems.

Finally it was indicated that older machines or machines of different types (e.g. 90°or 270° bending, or scanned versus scattered electrons) may require different checkfrequencies in some tests. In any case the professional judgement of the local physicistwith a good knowledge of the units in his or her charge should not be compromised.

Cobalt-60

Of the 42 centres replying to the questionnaire, 31 supplied details of cobalt unit qualitycontrol procedures, the other 11 having no cobalt units. There was a wide range in thefrequency for a given check as reported by different centres. For example, the minimumfrequencies for radiation/light beam coincidence ranged from fortnightly to yearly, witha median value of monthly. However, the median minimum acceptable frequency andthe median ‘desirable’ frequency were either the same or the desirable was one ‘stop’higher: for example, the median minimum frequency for checking distance indicatorswas weekly, but the median desirable frequency was daily. The median frequencies aregiven in Table B.4.


Some tests, such as checks on arc therapy and back-pointer were, understandably,held by many to be inapplicable (presumably because they are not used). However, perhapsmore surprisingly, tests on items such as couch verticality, wipe tests and head leakagewere held to be inapplicable by some. Some centres felt that the output check was notapplicable, but those who did so performed regular output calibrations instead. Since theprocedures for checking and calibrating a cobalt unit are very similar, it is likely that thetwo terms are synonymous in many centres.

Superficial/Deep units

Replies to this section were received from 42 centres. The range of frequencies reportedfor a given check was again large: for instance, the responses to frequencies of checkingtimer linearity ranged from ‘daily’ to ‘only at commissioning’ (although that is an extremeexample). The median ‘desirable’ frequency was generally the same as the medianminimum frequency, or one ‘stop’ different (for example, the median minimum frequencyfor checking timer termination was weekly, but the median desirable frequency wasdaily). The median frequencies are presented in Table B.5.

A number of centres considered that certain functions require no checking, such asfield coverage, applicator factors, focal spot films, timer linearity, timer termination anddoor interlocking. However, presumably some of these parameters, such as applicatorfactors, were measured at least once, at commissioning. Only 12 centres check the outputat angles other than zero degrees.

Table B.4. Checks on Cobalt-60 units. (median minimum frequencies).

Weekly Monthly Three-monthlyLight field size Output check Arc therapyDistance indicators Output calibration Timer linearityAxis lasers Photon beam field size Couch verticalityCrosswire centring Radiation/light beam coincidenceTimer termination Isocentre (gantry)Emergency stop Isocentre (couch)Door interlock Mechanical fixturesBack pointer Limit switches

Deadman’s switchFront pointerMachine log

Yearly Source change/commissioningRadiation symmetry Depth dose/quality indexWipe test Head leakageManual source returnCouch sag

Other checks mentionedWedge interlocks Wedge factors Retention of time on power failureBackup timer Source transit time Output dependence on gantry angleCouch brakes Emergency stopwatch Warning signals (audible and visible)


Dosemeters

Replies to this part of the questionnaire were received from 42 centres. The results aregiven in Table B.6. There was strong agreement over the minimum frequency necessaryto calibrate field instruments (annually) and strontium-90 checks of secondary standardinstruments (immediately before and after use), but there was a wider variation on thefrequency of strontium-90 checks for field meters (ranging from two-weekly to yearly).

Table B.5. Checks on superficial/deep X-ray units (median minimum frequencies).

Daily Weekly MonthlyOutput check Timer termination Output calibrationDoor interlocks

Three-monthly Yearly CommissioningMechanical fixtures HVL Percentage depth dose

Field coverage Applicator factorsTimer linearity Applicator films

Focal spot films

Other checks mentionedTimer vs stopwatch Emergency off Retention of time on power failureFilter interlock Output dose rate monitor Room warning lightsEnd-error timer check

Table B.6. Checks on dosemeters (median frequencies).

Before and after use Each occasion used Three-monthlySr-90 check on Standard dosemeter Isodose plotter mechanical Sr-90 check on field dosemeter

Isodose plotter ratio check Phantoms

Yearly CommissioningField instrument calibration Thermometer

CT scanners and simulators

It was disappointing to note that the attitude of deliberately excluding physicists fromCT scanners, as was the case many years ago, still prevailed. Only about 25 per cent ofthose who used CT scanners did any extensive testing. Several did not know what tests,if any were done.

The tests most performed were image resolution, distortion and magnification andCT numbers. About 33 per cent of scanners had these checks carried out mainly monthlyand equally by radiographers or physicists. Tests at other gantry angles were usuallycarried out at the same time as gantry zero testing.

Most centres thought that their aim on Simulator QC frequency was desirable. There


was reasonable consensus on what this would be except when it came to fluoroscopicimage quality. Hardly any performed processor sensitometry. These latter two tests mayreflect people’s perception that diagnostic doses are negligible when therapy doses follow.Many simple daily tests were performed equally by technicians or radiographers, withthe more complex tests being carried out by technicians or physicists.

Tables B.7 and B.8 show the most favoured frequency with the choice of testerindicated in brackets.

Table B.7. Checks on CT scanners (most frequent responses).

Monthly Yearly CommissioningBeam light position (P) Focal spot (P) Filtration (P)kV/mA (P) Dose assessment (P)Collimator size (P)Couch movements (P)Gantry tilt (P)Image resolution/MTF (P)Image distortion (P)Image magnification (P)CT Nos Bone/water (P)Artefacts (T/R)Noise (P)CT No linearity (P)

Letters in brackets indicate who should carry out the checks (P=Physicist, T=Technician, R=Radiographer).

Table B.8. Checks on simulators (most frequent responses).

Daily Monthly Six-monthlyDistance indicators (T/R) X-ray field wire size (T/P) Fluoro image qualityCrosswire centring (T/R) Isocentre gantry (T/P) (rarely achieved)Axis lasers (T/R) Isocentre couch (T/R)Radiation/light field coincidence Isocentre X-ray (T/R) (T/P) Display readouts (T)Optical field wire size (T/R) Mechanical fixtures (T)

Checks at other gantry angles NeverCrosswire centring (T/P) Processor SensitometryRadiation/light field coincidence (T/P) Dose assessmentsDelineator sag/movement (T/R)

Letters in brackets indicate who should carry out the checks (P=Physicist, T=Technician, R=Radiographer).

Brachytherapy

Introduction

Forty-three centres returned questionnaires with details of their equipment relating tobrachytherapy, while 42 returned the brachytherapy section itself. Most centres used the


column provided to expand on their answers, while five enclosed their own protocols.It was intended that where centres had more than one protocol, e.g. for different

afterloading machines and manual afterloading, the questionnaire could be copied foreach set of replies. Only four centres actually did this, and therefore some of the replieswere a little ambiguous. It was necessary therefore to analyse separately those departmentswith and without afterloading machines, as those with machines tended to commentspecifically if their answers pertained to manual afterloading. Figures are also givenwhere check frequencies could be related to high or low dose-rate equipment.

Afterloading statistics

Ten of the 51 machines covered in the survey (20 per cent) were high dose-rate machines,29 (57 per cent) were low dose-rate and eight (16 per cent) were medium dose-rate. Fourmachines (7 per cent) were unspecified. [Here, low dose-rate is defined as <1 Gyh-1 atpoint A, medium dose-rate is >1 Gyh-1 at point A.]

Frequencies of individual tests

1. Source Strength: Centres with HDR afterloading equipment aimed to check the sourcestrength at intervals varying from ‘weekly’ through ‘monthly’ to ‘at commissioning’(i.e. approximately every four months), whereas what was thought to be the minimumand desirable frequencies varied between ‘monthly’ and ‘at commissioning’. At centreswith LDR/MDR afterloading equipment, replies for ‘aim’, ‘minimum’ and ‘desirable’frequencies ranged from ‘monthly’ to ‘at commissioning’, (i.e. approximately everyten years). These are summarised in Table B.9. For the other centres, caesium-137sources were measured at commissioning, either in air or in water, using film, TLD ora Farmer dosemeter. Iridium-192 coils were measured on delivery in a well chamber,with four centres measuring cut lengths.

Table B.9. Frequencies of checking of source strength.

Test frequency W M 2–3M 6M Y 2–3Y C NAim 1 4 1 – 8 – 8 –Minimum – – 2 – 5 3 9 1Desirable – 4 1 1 11 – 4 –

Numbers are the numbers of centres. (W=Weekly, M=Monthly, Y=Yearly, C=Commissioning, N=Never)

2. Inspection: Several centres were uncertain of the meaning of this question. Answersvaried from ‘before and after use’ by the technician to ‘occasionally’ by themanufacturer. In general, though, visual inspections of tubing integrity and equipment(for afterloading) and sealed sources were carried out before and after use.

3. Leak testing: There was a good consensus of opinion on this topic: the majority ofcentres with long half-life sources aimed to leak test annually and also view this asthe desirable frequency. The minimum was listed as every two years by three centres.Two centres also carried out immersion tests. Most centres with HDR sources aimedto leak test at three-monthly intervals and this is also the frequency they think desirable.


One centre carried out this test at monthly intervals, and one did not do any leak tests.Indirect methods are used for high dose-rate sources, and wipe tests are the method ofchoice.

4. Applicator inspection: In general, applicators were inspected before and after use forboth high and low dose-rate afterloading machines and manual afterloading equipment.There were several exceptions to this though, with the greatest interval being annually(four centres – two with afterloading machines and two without).

5. Mass: (Applicable only to Iridium-192 wire.) Six centres weighed iridium coils ondelivery.

6. X-ray of applicator: Most centres only radiographed the applicator as part of thetreatment verification and/or dose calculation. Three centres also radiographed theapplicator when damage is suspected. Several centres have transparent applicatorsand viewed this test as unnecessary. One centre verified the source position withinthe applicator at weekly intervals using autoradiograph and X-ray technique.

7. Autoradiograph of sources: The centres responding to this question entered the samefrequency interval in all three columns; ‘aim’, ‘minimum’ and ‘desirable’, but thefrequencies ranged from ‘daily’ (one centre) and ‘weekly’ (one centre), through‘monthly’ and ‘three-monthly’ (four centres) to ‘annually’ (four centres) and ‘atcommissioning’ (11 centres). Six centres autoradiographed cut lengths of iridium wirefor all treatments. Other comments were:

when damage is suspected;on construction of new source train;yearly or before use if use is infrequent.

8. Autoradiograph of the loaded applicator: Again, there was a wide variation in theresponse to this question: for centres with LDR and HDR machines, the frequencywhich was aimed for varied from ‘weekly’ to ‘three-monthly’, except for four centreswith LDR machines who aimed to check this annually. The desirable levels variedequally for both HDR and LDR equipment and one centre with manual afterloading,and this was between monthly and commissioning. In general, though, it was thoughtthat whatever the frequency of this check, the desired level was twice as often withthe mean being every three months.

9. Timers, door interlocks, warning lights and emergency-off: The frequency with whichthese checks are carried out was quite often dependent on the patient workload, aquestion which was omitted from the survey. The results are given in Table B.10 forthose centres with afterloading equipment. Checks carried out daily also include thosecarried out ‘before use’; likewise weekly checks also include twice-weekly and two-weekly checks.

10. Who is doing the quality control? The survey asked for information as to who carriesout all these checks: physicist, technician, radiographer or manufacturer’s agent. Insome instances, the same check was carried out at different intervals by differentpeople, and therefore Table B.11 shows the number of centres at which the differentprofessions were involved.


Additional checks

The check-list was not exhaustive and suggestions of other checks which were carriedout at some centres are detailed below:

rectal dosemeters;TLD checks;

Table B.10. Frequencies of checking of interlocks etc. for those centres with afterloading equipment.

Frequency of checks D W M 3M 6M/Y C

TimersAim 3 5 12 9 3 1Minimum 3 5 10 9 1 2Desirable 2 7 9 5 4 –

Door interlockAim 13 12 8 5 – –Minimum 12 9 8 4 – –Desirable

Warning lightsaim 12 12 6 3 – –minimum 7 9 8 2 – –desirable 9 13 4 2 – –

Emergency-offaim 8 7 7 3 – –minimum 6 6 8 3 – –desirable 6 7 6 2 – –

Numbers are the numbers of centres (D=Daily, W=Weekly, M=Monthly, Y=Yearly, C=Commissioning).

Table B.11. Personnel carrying out checks.

Check item Afterloading treatment machines Other afterloadingmethods/live loading

P T R M P T R M

Source strength 25 5 – – 3 3 – –Inspection 12 10 2 1 4 5 – –Leak testing 19 9 1 1 4 2 – –Applicator inspection 7 14 5 4 2 5 – –Mass – 2 – – 1 – – –X-ray of applicator 14 7 1 – – – 7 –Autoradiograph sources 14 8 1 – 1 – – 1Autoradiograph loaded applicator 17 6 1 – – – – –Timers 16 11 4 3 – – – –Door interlocks 10 10 9 2 – – – –Warning lights 10 11 8 1 1 – – –Emergency-off 10 9 6 3 – – – –

(P=Physicist, T=Technician, R=Radiographer, M=Manufacturer’s Agent).


source activity variance;source transit times;source position;manual source return;applicator couplings;applicator test for load/unload;source data (computer);independent radiation monitor/alarms;test run with dummy source;printer/paper;lamp test;audit - safe count,

ward safe – manual afterloading;iridium-192 cutter – contamination (before/after repair),

movements;CCTV;environmental monitoring;gold grain gun; andbeta source strength.

Summary

It was apparent from the survey that, for brachytherapy, there was a wide variation betweencentres in the number of checks performed and the frequency with which they werecarried out. This, in part, depended on the equipment available, whether it is HDR orLDR, or manual or not, and also on the patient workload.

Treatment planning systems

Introduction

Forty-two centres replied to this section of the survey representing 51 treatment planningsystems (TPS). Six centres were using in-house developed systems, and 11 centres hadmore than one system though in some cases a second system was being commissioned toreplace the first one. The survey requested the frequency with which checks should becarried out on:

1. The output data, i.e. the end result of the treatment planning process.2. The input/output devices used to get patient information in and out of the system.3. The input data that is stored in the treatment planning system.

The mean response rate to the questions was 38 per cent.


The output from the treatment planning system

The survey requested information on external beam calculations for photons (includingirregular fields) and electrons, combinations of the two modalities and brachytherapy.For photons and electrons, the frequency of checks was required for a single field isodose,multiple field isodose, obliquity and inhomogeneity corrections. Sixty-nine per cent (18/26) of centres replying do not use electron planning software, 76 per cent (19/25) do notuse combined electron and photon software, 50 per cent (11/24) do not use irregularfield software, and 11 per cent (3/27) do not use brachytherapy software. Figure B.1shows the histogram of check frequency against the number of centres, for a single photonbeam isodose; it is representative of all check frequencies in this section. In each casethe majority aim was to perform this check at commissioning of the system, and it wasalso the minimum frequency with which it should be checked. The desirable frequencyof checking the output data was less clear but was greater than the aim and minimumfrequency. Forty-two per cent of responders carried out checks at commissioning only,while 39 per cent aimed to carry out checks at intervals varying between checking eachtreatment plan and six monthly. The extent of the checking was not specified.

Figure B.1. Histogram of check frequency against the number of centres, for a single photon beam isodose. Thefrequencies were: D=daily, W=weekly, 1-6 M=one to six months, 1-5 Y=one to five years, C=at commissioning,and U/R=after software upgrades, repairs, or other (unspecified).

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Input and output devices

The next section dealt with input and output devices used to get patient information inand out of the system, namely the digitiser used for entering the patient outline and theplotter which produced the final isodose distribution used for the patient treatment. FigureB.2 shows a histogram of check frequency against the number of centres, for the plotter:the results were almost identical to those of the digitiser. The majority aim was to checkthese peripherals for each treatment plan, often by simply overlaying the input patientoutline with the treatment plan produced. The minimum frequency of checking thesedevices lay somewhere between weekly and quarterly and the desirable frequency betweendaily and fortnightly.

Beam input data

The third section, concerning input beam data, considered the frequency of checking thedata stored in the treatment planning system, and its applicability to the treatment machineconcerned. The survey requested the frequency of checks on the following items:

dataset – the stored data;depth dose data;check isodoses – testing the algorithm;tray and wedge attenuation factors – where used;TAR/TMR – when calculated from depth dose data;

Figure B.2. Histogram of check frequency against the number of centres, for the plotter. The frequencies were:P=every plan, D=daily, 1-2 W=one to two weeks, M=monthly, 3M=three monthly, Y=yearly, C=at commissioning,O=other unspecified).

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off axis ratios – where calculated;field size factors; andSSD factors.

For all questions the majority of replies aimed to check these at commissioning, and thiswas also the minimum frequency at which these data should be checked. The desirablefrequency was greater than this, between weekly and annually. Figure B.3 shows thehistogram for checks of the dataset; the results are representative of all items in thissection.

Quality control examiner

Of 33 replies, 75 per cent said that a physicist carried out the check, and a further 14 percent of checks were carried out by a physicist and a technician or radiographer. Usuallythe technician or radiographer performed the check more frequently than the physicist(Figure B.4). Radiographers were only involved with checking the output produced, andmanufacturers’ agents only checked the input/output devices.

Figure B.3. Histogram of check frequency against the number of centres, for the dataset. The frequencies were:P=every plan, D=daily, W=weekly, 1-6 M=one to six months, 1-Y=one to two years, C=at commissioning, U/R=after software upgrades and repairs.

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Discussion

Comparing the aim and minimum frequency for each question answered in all threesections demonstrated that in most cases the aim and minimum frequency were the same(Figure B.5). Comparing the aim and desirable frequency for each question answeredyielded a similar result, but a significant number of replies indicated that the desirablefrequency was greater than that which they aimed for – particularly for checks on datastored in the treatment planning system (Figure B.6).

Under ‘comments’, methods used for checking output data were:

1. Comparison with standard tables – especially brachytherapy plans, that were alsochecked for correct reconstruction of source positions against the X-ray film.

2. Checked against plan produced by independent TPS.3. Measurements on a standard phantom for a treatment planned on the TPS.4. Set of ‘standard’ outlines reentered and computed on a regular basis and checked

against a ‘gold’ standard (15 centres).5. Independent checks on the isocentre dose for each treatment plan either manually or

by independent computer system (18 centres).

Methods for checking input and output devices consisted of comparing the input withthe output in the following ways:

1. Overlaying the original patient outline on the computed treatment plan; this also checkson the operator entering the outline details.

Figure B.4. Pie chart to demonstrate the profession of the examiner. Phys – Physicist, Tech – Physics technician,Rad – Radiographer, Man.Ag – Manufacturers agent.

Physicist 70%

Phys/Tech 3%

Tech 6%

Rad 1%Tech/Rad 2%Man.Ag 1%

Phys/Rad 11%


Figure B.6. Histogram of aim versus desirable check frequency against the number of centres.

Figure B.5. Histogram of aim versus minimum check frequency against the number of centres.


Table B.12. Numbers of centres using in vivo dosimetry in 1992 and possible future users.

100% 50 – 100% 10-50% <10% 0% TotalsFuture 1992 Future 1992 Future 1992 Future 1992 Future 1992 Future 1992

At first treatment 4 3 4 0 12 9 13 27 1 2 34 41After geometry changes 2 2 5 0 10 5 14 27 2 2 33 36Weekly 0 0 0 0 3 0 12 7 11 21 26 28More frequently 0 0 0 0 1 1 11 7 13 20 25 28

2. Entering specified distances or a shape and comparing the hardcopy output with theinput.

3. Entering CT image of a standard phantom and checking values of CT numbers andfor any signs of image distortion.

Suggestions for checking the input data stored in the TPS were as follows:

1. Comparison with the reference dataset.2. A data sum check which could be incorporated as a feature in the software of

commercial systems.3. Remeasuring the beam data. (This should be done as part of the QC on the treatment

unit, but is not a method for checking the integrity of the stored data in the TPS.)

Conclusion

It appeared from the results of this section of the QC survey that there was a wide variationof views on the frequency with which checks on treatment planning systems should becarried out, unlike other sections of the survey on treatment machines where there arewell established guidelines or recommendations. (This was before the issue of IPSMReport 68, A guide to Commissioning and Quality Control of Treatment Planning Systemsin 1996.) However, despite the apparent lack of regular QC programmes, over 70 percent of centres replying performed individual checks on each treatment plan or had aprogram for recomputing standard plans on a regular basis.

Quality control of patient treatments

As can be seen from Tables B.12, B.14 and B.16, this is an area where practice differedwidely. It may be that this was because of different ways of achieving the same end.


Table B.13. Uses of in vivo dosimetry.

Eye doses (27) Electrons (4)TBI (18) Field matching (2)Irregular field dosimetry (8) Verification (2)Dose to testis (7) Routine verification (4)Breast (6) Medullablastoma (3)Dose to foetus in pregnancy (4) Critical organs (3)Oesophagus (1) Hip prosthesis (1)

Table B.14. Numbers of centres taking check films in 1992 and possible future use.

100% 50 – 100% 10-50% <10% 0% TotalsFuture 1992 Future 1992 Future 1992 Future 1992 Future 1992 Future 1992

At first treatment 3 1 5 5 12 13 6 16 0 4 26 39After geometry changes 5 1 4 3 9 11 7 16 0 5 25 36Weekly 2 0 1 0 5 0 7 12 6 18 21 30More frequently 0 0 0 0 0 0 8 3 12 25 20 28

Numbers represent the number of centres indicating that they use in vivo dosimetry for that purpose.

In Vivo Dosimetry

The questionnaire sought information on the current use of in vivo dose measurement asa quality control procedure. Table B.12 shows the number of responses for each category.It will be noted that the total number of responses to each part of the question was differentand it is important to bear in mind the totals column when interpreting the table.

Table B.13 shows the types of situations to which the technique was being applied.Responders were asked to indicate how in vivo dosimetry was used, so the numbersshould be regarded as a relative indication only. Only six centres said they were using invivo dosimetry for dose verification, while nine centres would consider it desirable (asmay be deduced from Table B.12). Only 3/9 considered that it should be repeated regularlythroughout the course of treatment. One centre commented that ‘routine use is desirablebut not technically or operationally feasible’. Several centres pointed out the desirabilityof making repeat measurements or making the measurement on the second fractionbecause the first fraction was likely to take longer to set up and be less typical of the restof the course of treatments.

Portal imaging

Table B.14 shows the responses to that part of the questionnaire dealing with the use ofimaging for verification and Table B.15 indicates the principle uses. Unfortunately thequestionnaire did not seek to consider the relationship between the simulator and thetreatment machine. Many would consider that adequate verification had been done onthe simulator prior to treatment so that port films on the treatment machine would only


Table B.15. Applications of portal imaging.

Complex/custom blocking (17)All blocked patients (2)Head and neck (13)Accuracy important e.g. pelvis (8)Tangential breasts (6)Any patients not simulated (4)

Numbers represent the number of centres indicating that they use in vivo dosimetry for that purpose

be necessary when simulator verification was not possible. The figures of those carryingout verification on the first treatment are similar to the figures for in vivo dosimetry.However, if the numbers who did not respond were added to those who never took filmsthere would be several more centres who did not regard this sort of verification asimportant – indeed, one response said it was of no concern to the physics department.The fact that fewer centres felt that there was a need for increased imaging may reflectthe view that an increase would only be practicable if an electronic device were available.However, one responder said that ‘all radically treated patients should be filmed at leastonce’.

Electronic portal imaging devices

The questionnaire did not spell out the details in this section and one of the respondentsindicated that many were not aware of the possibilities offered by these devices. Onlyten centres felt that these devices were desirable.

APPENDIX C

Bibliography

The following bibliography is not intended as a definitive guide, but rather as a summaryof particularly relevant documents published by official bodies. These are collected herefor quick reference. A more detailed bibliography is given at the end of each chapter.

British Government documents

Current legal framework

HSC (Health and Safety Commission) 1985 Statutory Instrument No. 1333 The IonisingRadiations Regulations 1985 (London: HMSO)

HSC (Health and Safety Commission) 1985 Approved Code of Practice. The Protectionof Persons Against Ionising Radiation Arising from Any Work Activity (London: HMSO)

NRPB (National Radiological Protection board) 1988 Guidance Notes for the Protectionof Persons Against Ionising Radiations Arising from Medical and Dental Use (London:HMSO)

HSC (Health and Safety Commission) 1988 Statutory Instrument No. 778 The IonisingRadiations (Protection of persons undergoing medical examination or treatment)Regulations (London: HMSO)

HSC (Health and Safety Commission) 1992 The Management of Health and Safety atWork Regulations 1992 (London: HMSO)

HSC (Health and Safety Commission) 1993 The Radioactive Substances Act 1993(London: HMSO)

HSC (Health and Safety Commission) 1993 The Ionising Radiations (Outside Workers)Regulations (London: HMSO)

HSE (Health and Safety Executive) 1998 Fitness of Equipment Used for Medical Exposureto Ionising Radiation Guidance Note PM77 (2nd edition) (London: HMSO)

HSE and HSC publications are available from HSE Books, PO box 1999, Sudbury, SuffolkCO10 6FS; Tel +44 1787 881165; Fax +44 1787 313995. the HSE web page can befound at: http://www.open.gov.uk/hse/hsehome.htm

Other publications

DHSS (Department of Health and social Security) 1978 Health Notice HN (Hazard)(78) 26 (London: Department of Health)


Bleehen 1991 Quality Assurance in Radiotherapy. Report of a Working Party of theStanding Subcommittee on Cancer of the Standing Medical Advisory Committee May1991 (London: Department of Health)

DTI 1992 Guide to Software Quality Management System Construction and Certificationusing EN 29001 ISBN 0-9519309-0-7 (HMSO: London)

NAMAS Directory of Accredited Laboratories NAMAS Executive, National PhysicalLaboratory, Teddington, Middlesex TW11 0LW

Documents in preparation

HSE (Health and Safety Commission) 1999 The Ionising Radiations Regulations 1999(London: HMSO) (IRRRev)

HSE (Health and Safety Commission) 1999 The Ionising Radiations Regulations 1999.Approved Code of Practice and Supporting Guidance (London: HMSO) (IRRRev ACOP)

HSE (Health and Safety Commission) 1999 The Radiation (Emergency Preparednessand Public Information Regulations (London: HMSO) (REPPIR)

European Community documents

EC 1989 Framework Directive 89/391/EEC Council Directive on the Introduction ofMeasures to Encourage Improvements in the Safety and Health of Workers

EC 1993 Council Directive 93/42/EEC 12 July 1993 Medical Devices Directive OfficialJournal of the European Communities OJ No L169/1

EC 1997 Council Directive 97/43/EURATOM 30 June 1997. Health Protection ofIndividuals Against the Dangers of Ionizing Radiation in Relation to Medical ExposureOfficial Journal of the European Communities OJ No L180/22

European Commission 1997 Radiation Protection 91. Criteria for Acceptability ofRadiological (Including Radiotherapy) and Nuclear Medicine Installations Chapter 7ISBN 92-828-1140-9 (Luxembourg: Office for Official Publications of the EuropeanCommunities)

Council Directives may be obtained from HMSO. Further information about the MedicalDevices Directive and its application in the UK is available from: Medical DevicesDirectorate, Department of Health, 14 Russell Square, London WC1B 5EP; Tel +44 171972 8253.

275Bibliography

International Standards

BSI (British Standards Institute) 1976 BS 5288 Specification: Sealed Radioactive Sources(London: BSI)

IEC (International Electrotechnical Commission) 1986 Medical Electrical EquipmentDosimeters with Ionization Chambers as used in Radiotherapy IEC Publication 731(Geneva: IEC)

IEC (International Electrotechnical Commission) 1986 Medical Electrical EquipmentDosimeters with Ionization Chambers as used in Radiotherapy IEC Publication 977(|Geneva: IEC)

IEC (International Electrotechnical Commission) 1987 Medical Electrical Equipment,Part 2: Particular Requirements for the Safety of Gamma Beam Therapy Equipment(Geneva: IEC Publication 601-2-11)

IEC (International Electrotechnical Commission) 1988 Medical Electrical Equipment,Part 2: Particular Requirements for the Safety of Gamma Beam Therapy EquipmentAmendment 1 (Geneva: IEC Publication 601-2-11 Amdt 1)

IEC (International Electrotechnical Commission) 1989 Medical Electrical Equipment –Medical Electron Accelerators. Functional Performance Characteristics IEC Publication976

IEC (International Electrotechnical Commission) 1989 Medical Electrical Equipment,Section 2.17 Specification for Remote-controlled Automatically Driven Gamma-rayAfterloading Equipment (Geneva: IEC Publication 601-2-17)

BSI (British Standards Institute) 1989 BS 5724: 2.11 Medical Electrical Equipment,Part 2: Particular Requirements for Safety, Section 2.11 Specification for Gamma BeamTherapy Equipment (London: BSI) [Identical to IEC 1987]

BSI 1991 Recommendations for Achievement of Quality of Software BSI 7165 ISBN 0-580-18843-4 (London: BSI)

BSI (British Standards Institute) 1991 BS 5724: 2.11 Medical Electrical Equipment,Part 2: Particular Requirements for Safety, Section 2.11 Specification for Gamma BeamTherapy Equipment. Supplement 1. Revised and Additional Text (London: BSI) [Identicalto IEC 1988]

BSI (British Standards Institute) 1993 BS 5724: 2.11 Medical Electrical Equipment,Part 2: Particular Requirements for Safety, Section 2.11 Specification for Gamma BeamTherapy Equipment. Supplement 2. Methods of Test for Radiation Safety (London: BSI)[Identical to IEC 1993]


ISO 8402 1994 BS 4778 Quality Vocabulary Part 1: 1987 International Terms (London:BSI)


ISO 9001 1994 BS EN ISO 9001: 1994 Quality Systems. Model for Quality Assurance inDesign, Development, Production, Installation and Servicing (London: British StandardsInstitution)

IEC (International Electrotechnical Commission) 1996 Radiotherapy Equipment –Coordinates, Movements and Scales. CEI/IEC 1217 First Edition 1996-08 Geneva: IEC(BS EN 61217)

IEC (International Electrotechnical Commission) 1998 (in press) Functional Safety ofElectrical/Electronic/Programmable Electronic Safety-related systems IEC Publication1508 (Geneva: IEC)

These standards are available from the British Standards Institute, Linford Wood, MiltonKeynes MK14 6LE. Lists of standards are available on the BSI web site: http://www.bsi.org.uk/bsis/index.htm

International dosimetry recommendations

ICRU (International Commission on Radiation Units and Measurements) 1969 Report14 Radiation Dosimetry: X Rays and Gamma Rays with Maximum Photon EnergiesBetween 0.6 and 50 MeV (Bethesda MD:ICRU)


ICRU (International Commission on Radiation Units and Measurements) 1985 Doseand Volume Specification for Reporting Intracavitary Therapy in Gynaecology (ICRUReport 38 (Bethesda MD: ICRU)

ICRU (International Commission on Radiological Units) 1987 Use of Computers inExternal Beam Radiotherapy Procedures with High-energy Photons and Electrons ICRUReport 42 (Bethesda MD: ICRU)

IAEA (International Atomic Energy Agency) 1987 Absorbed Dose Determination inPhoton and Electron Beams. An International Code of Practice (Technical Report Series277 (Vienna: IAEA)


ICRU (International commission on Radiological Units) 1993 Prescribing, Recordingand Reporting Photon Beam Therapy ICRU Report 50 T Landberg (Chairman), JChavaudra, HJ Dobbs, G Hanks, K-A Johansson, T Möller and J Purdy (Bethesda MD:ICRU)

IAEA (International Atomic Energy Agency) 1994 Calibration of Dosimeters Used inRadiotherapy: A Manual Technical Report Series 374 (Vienna: IAEA)

277Bibliography

IAEA (International Atomic Energy Agency) 1997 The Use of Plane-parallel IonisationChambers in High-energy Electron and Photon Beams: An International Code of PracticeIAEA Technical Report Series 381 (Vienna: IAEA)

ICRU publications are available from the International Commission on Radiation Unitsand Measurements, 7910 Woodmont Avenue, Bethesda, Maryland 20814, USA.

IAEA publications are available from the International Atomic Energy Agency,Wagramerstrasse 5, PO Box 100, A-1400, Vienna, Austria. Their web site is at: http://www.iaea.org/worldatom/inforesource

AAPM guidance

AAPM (American Association of Physicists in Medicine) 1983 A protocol for thedetermination of absorbed dose from high energy photon and electron beams Med. Phys.10 741

AAPM (American Association of Physicists in Medicine) 1984 Report 13 Physical Aspectsof Quality Assurance in Radiotherapy (New York: American Institute of Physics)

AAPM (American Association of Physicists in Medicine) 1990 Quality AssuranceMethods and Phantoms for Magnetic Resonance Imaging American Association ofPhysicists in Medicine Report No 28 (Maryland: AAPM)

AAPM (American Association of Physicists in Medicine) 1993 Purday JA, Biggs PJ,Bowers C Medical accelerator safety considerations: Report of AAPM Task Group 35Med. Phys. 20 1261–1275

AAPM (American Association of Physicists in Medicine) 1993 Report of AAPM TaskGroup 41 Remote Afterloading Technology ISBN 1-56396-240-3 (New York: AmericanInstitute of Physics)

AAPM (American Association of Physicists in Medicine) 1994 Almond PR, Attix FH,Humphries LJ, Kubo H, Nath R, Goetsch S and Rogers DWO The calibration and use ofplane-parallel ionisation chambers for dosimetry of electron beams: An extension of the1983 AAPM protocol report of AAPM Radiation Therapy Committee Task Group No.39 Med. Phys. 21 1251–1260


AAPM (American Association of Physicists in Medicine) 1994 Nath R, Biggs PJ, BovaFJ, Ling CC, Purdy JA, van de Geijn J and Weinhous MS AAPM code of practice forradiotherapy accelerators: Report of AAPM Radiation Therapy Committee Task Group45 Med. Phys. 21 1094–1121

AAPM (American Association of Physicists in Medicine) 1995 Radiation TreatmentPlanning Dosimetry Verification AAPM Report 55 (Task Group 23). American Instituteof Physics, Woodbury, NY.


AAPM publications are obtainable from AAPM, American Center for Physics, OnePhysics Ellipse, College Park, MD 20740-3843, USA. Their web site is at: http://aapm.org/pubs/index.html#pubs

BIR guidance

BIR (British Institute of Radiology) 1985 Criteria and Methods for Quality Assurance inMedical X-ray Diagnosis British Journal of Radiology Supplement 18 (London: BritishInstitute of Radiology)

BIR (British Institute of Radiology) 1989 Treatment Simulators British Journal ofRadiology Supplement 23 (London: British Institute of Radiology)

BIR (British Institute of Radiology) 1993 Aird EGA, Jones CH, Joslin CAF, KlevenhagenSC, Rossiter MJ, Welsh AD, Wilkinson JM, Woods MJ and Wright SJ Recommendationsfor Brachytherapy Dosimetry. Report of a Joint BIR/IPSM Working Party (London: BritishInstitute of Radiology)

BIR (British Institute of Radiology) 1996 Central axis depth dose data for use inradiotherapy. British Journal of Radiology Supplement 25

BIR publications are obtainable from the British Institute of Radiology, 36 PortlandPlace, London W1N 4AT; Tel +44 171 580 4085; Fax +44 171 255 3209. The BIR website can be found at: http://www.bir.org.uk

IPEM documents

HPA (Hospital Physicists’ Association) 1977 The Physics of Radiodiagnosis HPAScientific Report Series 6 (York: IPEM)

HPA (Hospital Physicists’ Association) 1980 Measurement of the PerformanceCharacteristics of Diagnostic X-ray Systems Used in Medicine: Part 1 X-ray Tubes andGenerators TGR-32 Part 1 (York: IPEM)

IPSM (Institute of Physical Science in Medicine) 1990 Supplement to Report 54.Commissioning and Quality Assurance of Linear Accelerators (York: IPEM)

IPSM (Institute of Physical Sciences in Medicine) 1990 Code of practice for high-energyphoton therapy dosimetry based on the NPL absorbed dose calibration service Phys.Med. Biol. 35 1355–1360

IPSM (Institute of Physical Sciences in Medicine) 1992 Protocol for establishing andmaintaining the calibration of medical radionuclide calibrators and their quality control,Report 65 Quality Standards in Nuclear Medicine Chapter 5, Ed AH Smith and GC Hart,ISBN 0904-181642 (York: IPEM)

279Bibliography

IPSM (Institute of Physical Sciences in Medicine) 1992 Survey of quality control practicein UK hospitals carried out by the radiotherapy physics topic group. Scope 1 49-61[Summary in Appendix B]

IPSM (Institute of Physical Sciences in Medicine) 1992 Procedures for the definitivecalibration of radiotherapy equipment Scope 1 No 1 [Reproduced in Appendix A]

IPEMB (Institution of Physics and Engineering in Medicine and Biology) 1996 Report68 A Guide to Commissioning and Quality Control of Treatment Planning Systems EdJE Shaw (York: IPEM)

IPEMB (Institution of Physics and Engineering in Medicine and Biology) 1996 TheIPEMB code of practice for electron dosimetry for radiotherapy beams of initial energyfrom 2 to 50 MeV based on an air kerma calibration Phys. Med. Biol. 41 2557–2603

IPEMB (Institution of Physics and Engineering in Medicine and Biology) 1996 TheIPEMB code of practice for the determination of absorbed dose for x-rays below 300 kVgenerating potential (0.035 mm Al–4 mm Cu HVL; 10–300 kV generating potential)Phys. Med. Biol. 41 2605–2626

IPEMB (Institution of Physics and Engineering in Medicine and Biology) 1996 Report74 Application of the Medical Devices directive. Guidance Notes (York: IPEM)

IPEM publications are obtainable from The Institute of Physics and Engineering inMedicine, PO Box 303, York YO1 2WR. Tel +44 1904 610821 Fax +44 1904 612279

Miscellaneous recommendations

NACP 1980 Procedures in external radiation therapy dosimetry with electron and photonbeams with maximum energies between 1 and 50 MeV Acta. Radiol. Oncol. 19 Fasc 1.

NCRP (National Council on Radiation Protection and Measurements) 1981 (Thirdreprinting 1993) Report 69 Dosimetry of X-ray and Gamma-ray Beams for RadiationTherapy in the Energy Range 10 keV to 50 MeV (Bethesda MD: NCRP)

NCRP (National Council on Radiation Protection and Measurements) 1989 Report 102Medical X-Ray, Electron Beam and Gamma Ray Protection for Energies up to 50 MeV(Bethesda MD: NCRP)

NPL (National Physical Laboratory) 1998 Guide to the Measurement of Pressure andVacuum ISBN 0 904457 29 X (London: Institute of Measurement and Control) Obtainablefrom the Institute of Measurement and Control, 87 Gower Street, London WC1E 6AA)

RCR (Royal College of Radiologists) 1998 Report of the generic radiotherapy workinggroup. Royal College of Radiologists Clinical Oncology Information Network (COIN).(London: RCR) In draft July 1998.

World Health Organisation 1988 Quality Assurance in Radiotherapy (Geneva: WHO)


INDEX

AAPM see Task GroupAcceptance testing 11,34,46,72Accuracy

Achievable 12-14,214and Precision 7-8Dose distribution 9,11-12,62,72,88,191,214Dose 4-5,12,72,88,150,176Geometric 3-4,9,12,48,51,62,87,

89,146,232-235Position see Accuracy, GeometricRequirements for 4-10,159

Action level 11,97-98,144,159Activity see Air kerma rateAFC 125,127Afterloading see Remote (or Manual) AfterloadingAir kerma rate 69,77,91,152,202,

207-208,211-212,214,216Algorithm testing 61,69,246Algorithms, Dose calculation (types) 63Alignment errors, Causes of 110Angular dependence 178Applicator 120,121,124,165,213Arc therapy 66,68,74-75,116,144,157Archive 83Asymmetric fields 66,82,127-129Attenuation 69,73,91Au-198 226Audible alarm 99,155Audit 13,27-32,79Audit, Clinical trials 29Audit, National Audit Centre 30Audit, Objectives 30Audit of sources 224Automatic Frequency Control 125,127Autoradiography 204,212,215-216,227,262

Back pointer 105Backscatter factor 163,165Backup timer 100,167Backup 79,82,249BANG gel 91Barometer 119,136,194-195Barriers 99,202Beam

asymmetry 111-113bending 126-127data 60-68flatness 96,100,111-112,120,122-124,126,255generation models 63geometry see BEVintensity modulation see IMRTmodels 63quality 29,116,154,168-169spread 68

symmetry 96,100,111-112,120,124,126,255Beam’s Eye View see BEVBending system 126-127BEV (beam geometry) 66,75,76,89Bibliography 273-279BIM see IMRTBJR Supplement 23 34,41Bleehen Report 20-27Block Tray 39,63,90,100,107Blocks 64,66,74,81,87-88,90Bolus 73,81Brachytherapy

calculation data 69,220equipment test frequency 223,261-262plan checking 91-92,222planning test frequency 222planning tests 76,219-223protection 202-204,213,219,223radiographs 207,214sources 209source calibration 207-212sources test frequency 223TPS, Commissioning 69,76-77,219-223treatment planning 69,76,91-92,219-223

BS5750 see ISO9000Build-up cap 114,210Build-up data 64Build-up 61,64,209

Caesium-137 202-203,212,216-217Calculation algorithms 63Calibration,

Brachytherapy source 207-212Cobalt unit 153-154Definitive 113,153,250-253Diodes 187-188,231-232Linear accelerator 113,120Output 26-27,113,120,153,164-166Radiation monitors 223TLD 184-188,231-232X-ray 163-166

Catheter 213CFRT see Conformal radiotherapyCharged particle equilibrium 209CHART 29Clinical checks 86Clinical Target Volume see Target volumeCobalt emergency procedure 155-156,159Cobalt source construction 150Cobalt source position 152Cobalt unit, Calibration 153-155Cobalt unit, Frequency 159-160,257-258Cobalt units 150-160Cobalt-60 brachytherapy 209,211

281Index

Code of practice see Dose, ProtocolsCollection efficiency 113,130-131,137,177,191Collimator differences 62,129Collimator rotation 104Collimator settings for electrons 125,142Colpostat 213Commissioning 10-11,70-78,203-212

Brachytherapy calculations 77,219Electron calculations 75Photon calculations 73-74Brachytherapy sources 215

Compensator 64-65,73,81,87-88Computer control 132-133Computer hardware 57,79Computerised Tomography see CTConformal radiotherapy 89-91,143Consistency, dosemeter 115Constancy check 48,113,121,153,

163,165,166,193,255Contamination, Radioactive 151,158,215Contouring, CT 46Contrast see Image contrastControlled area 212,223Correction factors, Planning 64,87-88Correction of field positioning errors 233Couch isocentre 38-39,103Couch position readouts 39,45,52,107Couch under load 39,45,101Couch vertical movement 39,107Cross calibration 179Crosswire tests 36,104Crow’s foot 237CT,

Axis lights 44Couch deflection 45Couch position 45,85Couch top 44Frequency 49Helical scanning 49High resolution 43Number see Hounsfield unitsPixel size 43,48Planning computer interface 46,48,60,66,80,85Quality control 43-49Scan diameter 43,48Simulator see Simulator CTSpiral scanning 49Tolerances 49

CTV see Target volume

Dark current 184,188Deadman’s switch 100,155Decay correction 214Definitive calibration 113,153,163,

210-211,250-253Depleted Uranium 151Depth dose 65,68,71,73,75,78,115,154,197

Detector size 61-62,180Detectors 61-62Detex paper 205Diaphragm see CollimatorDigitally reconstructed radiograph 90,234Digitiser 70,79,92,139Diode 114,122,165,181-182,184,187-189,230-232

Dark current 184,188Ageing see Diode, pre-irradiationCalibration 187-188,230-232Energy dependence 61,187Frequency of checks 189Pre-irradiation 184Temperature effect 184,232

Distortion, Display monitor 71,80Distortion, MR 50-52,85Distortion, Simulator CT 57Documentation 76,99,118,119,126,

215,214,242,247-248Dose see also Calibration

Accuracy see Accuracy, DoseConsistency 115Distribution 88Effects 4-6,8Fractionation 6In vivo measurement 3,12–14,79,91,120,

179,182,229-232,270-271Intercomparisons 12,14,182Linearity 115,168,178Normal tissue 231Prescription 6,86Protocols 10-11,113,120,164-166,250Rate 113Reference point 6,13Reporting 6Stability see Dose, Consistency

Dose per pulse 113,116,130-132Dose Volume Histogram 7,90-91Dosimetry intercomparisons seeDose,

IntercomparisonsDosimetry protocols see Dose, ProtocolsDosimetry interlocks 100,154DRR see Digitally Reconstructed RadiographDual dosimetry 100,111,136-137,154DVH see Dose Volume HistogramDynamic MLC 143Dynamic wedge 133-135

Ears see HornsEffective point of measurement 62,120,180Electrical safety 101,190Electrometers 175Electron

algorithms 67-68beam data 67-68,75-76density 43,47-48,57,65,66,73,81energy 67,121-122


gun 125,127Electronic Portal Imaging Device 113,141,236Emergency-off 99Energy see Photon or Electron energyEntrance doses 230-231EPID see Electronic Portal Imaging DeviceEquipment see Test equipmentErrors, Combination of 7,9-12Errors, Systematic and random 7-8,11,233European Commission 11,97European Network for Quality Assurance 28Exit doses 65,230-231Field junction 68,128Field of view 43Field positioning error, Correction of 233Field defining wires 37-38Film dosimetry 196-197Filters 100-101,166-167Finger doses 219Flatness 96,100,111-112,120,122-124,126,255Flattened area 112,123Flattening filter 112,116Focal spot 41,170-171Focus coils 126-127Fortin barometer 194Frequency,

Basis of recommendations 118,120,121,150,254-272

Brachytherapy equipment 223,261-262Brachytherapy planning 80,222Brachytherapy sources 223Cobalt unit 159-160,257-258CT scanner 49Diode 189Dynamic wedge 134-135Ionisation Chambers 176Linear accelerator electron 120,122,257Linear accelerator X-ray 117-118,134-135,256MLC 143MR 54Phantoms 196Plotting tanks 192,259Portal imaging 236Simulator 42Stereotactic tests 145TLD 188TPS 80,222,265-267X-ray unit 173,258-259

Front pointer 105GafChromic film 205Gamma alarm 155Gantry angle 39,105Gantry rotation, Effect on dose 114,121,150Gaps in treatment 96Gel dosimetry 91Geometric accuracy 3-4,9,12,48,51,62,

87,89,146,232-235

Glancing fields 65Gold-198 226Gross Tumour Volume (GTV) see Target volumeGuidance Notes 119,151,152,158,252Gun, Electron 125,127Half value layer 168-169,195Half-life 64,69,202Handling of TLD 184,232Hardware see Computer hardwareHDR 201-202Helical scanner see Spiral scannerHigh dose rate interlock see Interlocks,

DosimetryHigh Dose Rate see HDRHorns 116,124Hounsfield units 43,47-48,57,65-66,73,81Humidity 195HURED see Hounsfield unitsHVL 168-169,195Hygrometer 195IAEA 29ICRU 38 69ICRU 48 44ICRU 50 1-2,6-7,86IEC recommendations 10-11,31,34,41,64,97,101,

112,119,122,151,156,158,176Image

accuracy 47-48,57,80-81comparison 235contrast, CT 43,47contrast, MR 54intensifier 39-40,56quality, MR 50quality, Radiographic 39-41transfer 48,60,80-81

IMPACT 43Implicit checks 99,255IMRT 91,143In vivo dosimetry 3,12-14,79,91,120,179,

182,229-232,270-271Indicator lights 101,203Inhomogeneity correction 66,68,73,75,

81,87-88,90Intensity Modulated Radiotherapy see IMRTIntercomparison, Dosemeter 179Intercomparison, Dosimetry 13,30Interlocks 42,99-101,124,142,155,

166-167,171,202,212Dosimetry 100,154Filter 100,166-167Overriding 99

Iodine-125 225-226Ionisation chambers 61,120,175-181

angular dependence 178linearity 178Quality Control 179stability check 178

283Index

tests required 176Ionisation chamber, Monitor 111,113-114,

135-137,168Ionising Radiation Regulations 158,223IPSM phantom 31IPSM Report 54 96,118,120,122,159Iridium loader 218-219Iridium wire 215-216,218-219Iridium-192 201-225Irregular field see Blocks and Multileaf CollimatorISO9000 20-27,241Isocentre, Cobalt unit 152-153Isocentre, Linear accelerator 101-105,108-111Isocentre, Radiation 108-111,152-153Isocentre, Simulator 36,38

Jaw see CollimatorJunction, Field 68,128

Klystron 125,127KV constancy 159

Lasers 37,44-45,102LDR 201-202Lead tray 39,63,90,100,107Leak test 151,158,215-216,226Leakage between leaves 138,142Leakage current 177,191Leakage from cobalt head 152Left/Right registration 47-48,81Legislation 120-121,158,223,273-274Level of checking 98Light field 105-107,110-111,128,142,170Light source 105,107,110-111Limit switches 100Linear accelerator electron,

Frequency 120,122,257Tolerance 122

Linear accelerator photon,Frequency 99,117-118,135,143,256Tolerance 117-118

Linear accelerator,Calibration 113,120Constancy check 113,121,255Couch 107Crosswires 104Quality Control 98-146

Linearity 115,168,178Live loading 223-224Low Dose Rate 201-202Low energy X-ray see X-rayLung correction 65,73,75

Magnetic Resonance Imaging see MRMagnetron 125,127Magnification, MR and CT 43,48,51Magnification, Simulator 39,84,86

Maintenance 47Manchester system 213-214Manual afterloading 214-219Margins 5-6,90Marker sources 207Matching fields 68,127Matrix, Calculation 220MDR 201-202Measuring range, Electrometer 178Mechanical inspection 156,171,190Medium Dose Rate see MDRMedium energy X-ray see X-rayMLC see Multi-leaf collimatorMonitor ionisation chamber 111,113-114,

135-137,168Monitor Unit calculation 3,64,67,74-75,82,87-88,241Monte Carlo 67,68MR,

Distortion 50-52,85Frequency of checks 55Gel dosimetry 91Quality Control 50-55,85

MU see Monitor UnitMulti-leaf collimator 66,73,82,90,133,137-143

Non-coplanar treatment 66Normal Tissue Complication Probability 8,232Normal tissue dose measurement 231NTCP see Normal Tissue Complication ProbabilityNuclides for brachytherapy 202-203

Oblique incidence 65,73,81Off-axis calculation 81,88,128Off-axis data 61,65,68,73,115,182Optical distance indicator 37,105,156Optical field trimmer 106,110Optimisation 88Organ movement 9,85Output factor, Electron 67Output factor, Photon 64,115,154Overriding interlocks 99Ovoids 213

Paris system 213-214Patient support unit see CouchPDR 201-202Pencil beam 67Penumbra 61-68,75,90,108,110,129,191Percentage depth dose 65,68,71,73,75,78,115,154,197Phantoms,

Audit 29,31,67Beam data 62CT 44-49Frequency of checks 196Materials 44,50,62,120,122,196MR 50

Photon beam data 62-67


Photon energy 116Pinhole 170Pixel size 43,48,51Plan checking 84,87-89Plan normalisation 6,86Planning Target Volume see Target VolumePlotter 70,79Plotting tank 62,189-193Plotting tanks, Frequency 192,259Polarising voltage 177,191Polarity correction 61,177Pointer see Front pointer, Back pointerPortal imager test frequency 236Portal imaging 9,12-13,141,233-237,271-272Positioning accuracy 3-4,9,12,48,51,62,

87,89,146,232-235Practical range 67Precision, and Accuracy 7-8Pressure 119,136,194-195PRF see Pulse Repetition FrequencyPrinter 70Processor tests 79Process,

Radiotherapy 1-3Treatment Planning 83

Profile data 61,65,68,73,115,182Programming 245-247Project library 247-248Project management 244-245PTV see Target volumePulse Repetition Frequency 116Pulsed Dose Rate 201-202QART 20-27Quality Index 29,116,154Quality Manual 23,25Quick check, Definition 98,113Quick check, Flatness 112Quick check, Mechanical isocentre 103-105Quick check, Optical field indication 105Quick check, Radiation field alignment 108Radiation field alignment 37-38,56,107-111,

129,141-142,170,197Radiation isocentre 108-111,145,153Radiation monitors, Calibration 223Radiation protection for brachytherapy 203-203,

213,219Radiation safety see Radiation protectionRadiation source 110,152,203Radioactive contamination 151,158,215Radiographic tests 41Radionuclide calibrator 208,210-211,215,217,226Radionuclide stability check 178Radionuclides for brachytherapy 202-203Radiosurgery see Stereotactic radiotherapyRadiotherapy process 1-3RAKR see Reference Air Kerma RateRandom and systematic uncertainties 7-8,233

Range finder 37,105,156-157Range, Electron 67Range, Instrument 178Recombination correction 113,130-131,

137,177,191Reentrant contours 66,74,82Reentrant ionisation chamber 208,210-211,

215,217,226Reference Air Kerma Rate 69,77,91,152,202,207,

208,211-212,214,216Reference data 65-66,68Reference plan 82Reference point 6,13,86Registration, CT and MR 85Relocatable frame 144Remote afterloading commissioning 203-212Remote afterloading machine data 203Remote afterloading 201-214Repairs, Checks following major 96-97,250Report 54, IPSM 96,118,120,122,159Report 68 (IPEMB) 79Responsibilities 86,83,96,99,118,219,

242,251,255,262,267-268RF source 125-127Right/Left registration 47,48,81Rotation therapy 66,68,74-75,116,144,157Rulers 195Ruthenium-106 226-227Safe, Isotope storage 224Safety devices 42,99-101,145-146,155Safety, Electrical 101,190Scale conventions 64,71Scan plane 44,52-53Scanogram 85,234Scatter correction 64,66,69,74,91Scatter integration 63-64,74Scattering foil 101,120Scout view see ScanogramSecond check 89,179,251Secondary standard instruments 179-180,

210,216,259Security see SafetySegmentation 46Semiconductor detector see DiodeService mode 132Setup instructions 88Shadow tray 39,63,90,100,107Signal-to-noise ratio, MR 53Simulator CT,

Distortion 57Frequency of checks 57Quality control 55-57

Simulator film 84Simulator,

Acceptance testing 34Couch 38-39Crosswires 36

285Index

Daily checks 35,38,42Field wires 37Focal spot 41Frequency of checks 42Gantry angle 39Image intensifier 39-40Lasers 37Optical distance indicator 37Quality control 34-43,55-57Radiographic tests 41Shadow tray 39Tolerances 34,42

Slice position, CT 45Slice position, MR 52Slice warp, MR 53Software control 243-249Software quality 241-242Software updates 43,60,77,80,190,193Software 57,81-82,135,193Software, In-house 241-253Solid water 62,122,164,166,196Source

audit 224change, Brachytherapy 207change, Cobalt 154,159,250-251diameter 64filtration 69inventory 224transfer 224radiation 110,152,203

Spatial resolution 237Spiral scanner 49SSD correction 66,73,81Staging 1,5Standard deviation, Effective 7Standards 10-12,31Star film 109Statutory requirements 120-121,158-159,223,273-274Steering 126-127Stem effect 177,191Stereotactic radiotherapy 110,116,144-146Stereotactic test frequencies 145Stopwatch 119,157,167Stored beam systems 63Strontium check 178Strontium-90 226-227Supralinearity 187Swept beam electrons 129-132Symmetry 96,100,111-112,120,123-124,126-127,255Systematic and random uncertainties 7-8

TAR 71,73Target volume 1,5,84,86,232,234Task Group 23 60Task Group 39 10Task Group 40 10,79,118,120,159

TBI 182TCP see Tumour Control ProbabilityTemperature 119,136,176,184,194,232Test equipment,

Beam data acquisition 61-62,189-193Cobalt unit 157CT 44-49Linear accelerator 37,103,104,105,

108,112-113,116,119MR 50Simulator alignment 35-36,42Simulator radiography 41X-ray 172

Test Frequency see FrequencyThermal equilibrium 176Thermoluminescent Dosimetry see TLDThermometer 119,136,194Three-dimensional planning 63,66,74-75,

87,89-91TickIT 241Timer, Cobalt unit 154-155Timer, X-ray unit 167-168Tissue Air Ratio 71,73Tissue Maximum Ratio 71,74-75TLD 28-29,181-189,230-232

Calibration 184-188,231-232Frequency of checks 188

TMR 71,75Tolerance level 11,34,42,49,97-98,117-

118,121,122,144,159-160Tolerances, CT 49Tolerances, Simulator 34,42Topogram see ScanogramTotal Body Irradiation 182TPR 20/10 29,116,154TPS see Treatment Planning SystemTransfer, Image 48,60,80-81Transfer, Setup data 71,89,90,138Transit dosimetry 235-236Transit time 154,203,204Treatment plan, Quality control 84-89Treatment Planning System

Acceptance testing 72Beam data 60-78Commissioning test summary 73-74Commissioning 70-77,219-223Quality control 77-83Test frequency 80,222,265-267

Treatment verification 91,229-237Tumour Control Probability 8-9,232

Uncertainties 12-13Uniformity, Brachytherapy source 204-205Uniformity, MR image 54Uniformity, Photon and electron 96,100,111-112,

120,122-124,126,255Uniformity, X-ray beam 169-170


Unsealed ion chamber 136Uranium 151

VDU 71,80Verification film 233Verification 3,13-14,84Verification, Treatment 91,229-237Virtual source 67Viruses 82Volumes 1-3,5-6,9,232Volume growing 89

Wall thickness 202Water tank 62,189-193Wedge factors 64-65,73,81,87,114,134,156Wedge interlocks 100-101,156Well ionisation chamber 208,210-211,

215,217,226WHO recommendations 10-11,97,118,120,

159,163,169,229Wipe test 151,158,215-216,226Work instructions 27

X-raycalibration 163-166constancy check 163-166focal spot 170-171test frequencies 173,258-259uniformity 169-170