Applied

Physic

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Rad

iation

On

co

log

y rev

ised ed

ition

Stanton & StinsonMedical Physics Publishing

Robert Stanton

Donna Stinson

AppliedPhysicsfor

RadiationOncologyr e v i s e d e d i t i o n

Table of Contents

Preface xiiiAcknowledgments xv

1. Matter and Energy 1

Matter 1Force 3Energy 3

2. Radiation and Its Properties 7

The Structure of an Atom 8Subatomic Particles 10Ionizing Radiation 11Linear Energy Transfer 12Electromagnetic Radiation 14Wave-Particle Duality 15Wavelength and Frequency 15

3. The Production of X-Rays 19

The Discovery of X-Rays 19Collision Interactions 20Characteristic Radiation 22Radiative Interactions (Bremsstrahlung) 24Filters Used in Conventional X-Ray Therapy 26

vii

Beam Direction as a Function of Incoming Electron Energy 28Beam Direction Dependency on X-Ray Target Design 29

4. Radiation Quality 33

X-Ray Intensity 33Beam Divergence 34Beam Attenuation 34Attenuation Coefficients 41

5. X-Ray and -Ray Interactions with Matter 45Attenuation Coefficients 45Coherent Scatter 47Photoelectric Effect 48Compton Effect 50Pair Production 53Pair Annihilation 55Photonuclear Interaction 55Energy Absorption 57

6. Principles of Radiation Detectors 59

Measurement of Radiation 59Gas Ionization Detectors 60How to Use a Survey Meter 65Scintillation Detectors 68Neutron Dosimeters 69Thermoluminescent Dosimeters 69Diode Detectors 73MOSFET Detectors 74

7. Determining Radiation Intensity 7777

The Importance of Standardized Radiation Measurement 78The Roentgen as a Unit of Exposure 78Kerma 79Conventional X-Ray Machine Calibrations 80Radiation Absorbed Dose 83The fmedium Factor 84Cavity Theory 86Dose Equivalent 88

viii Table of Contents

8. Why Use Higher Energy Beams? 93

Disadvantages of Low-Energy Machines 94Penumbra Size 95Inability to Use Isocentric Techniques 97Advantages of Megavoltage over Orthovoltage Beams 98Skin Sparing 100Electron Equilibrium 100Disadvantages of Megavoltage 101

9. Linear Accelerators 103

Accelerator Guides 104Waveguides 105Power Sources 109Bending Magnets 111The Raw Electron Beam 115X-Ray Beam Production 115X-Ray Beam Flattening Filters 115Photon Beam Collimation 118Electron Beam Production 118Electron Scattering Foils 119Electron Beam Collimation 120Monitor Chambers for Photon and Electron Beams 122Helical Technology 123 The Linear Accelerator Console: The Operator Interface 124Quality Assurance 124

10. Other High-Energy Machines 127

Cobalt-60 Machines (Radionuclide Teletherapy) 128Timer Error 130Penumbra 132Quality Assurance of Cobalt-60 Machines 134Cyclotrons 136Heavy Particle Therapy 137

11. The Geometry of Photon Beams 141

Similar Triangles 142Magnification 144Abutting Fields 147

Table of Contents ix

Non-Midplane Structures 149Perpendiculars 150Planes 150Simple Beam Arrangements 155Isocentricity 157Conventional Beam Blocking 165Multileaf Collimation 170IMRT 171

12. Photon Beam Dosimetry 175

Dose and Distance Terms 176Dose Fractionation 177Quantities Used in Treatment Calculations 178Backscatter Factor 178Output Factor 180Equivalent Square Fields 182Equipment Attenuation Factors 183Patient Attenuation Factors 184Depth Dose 185Tissue-Air Ratio (TAR) 188Tissue-Maximum Ratio (TMR) 194Isodose Curves 196Dose Profiles 197Moving Field Calculations 199Computers 201

13. Electron Beam Dosimetry 203

Electron Beam Interactions 203Electron Beam Characteristics 205Electron Beam Profiles 208Gaps and Abutting Fields 208Electron Dose Measurements 209Treatment Calculations 209Irregularly Shaped Fields 211Tissue Inhomogeneities 213Inverse Square Law 214

x Table of Contents

14. Treatment Planning 217

Tumor Targeting Vocabulary 218Aims of Treatment Planning 218What Treatment Planning Includes 220Patient Alignment Devices 221Patient Positioning Aids 222Body Contours 222Isodose Distributions 226Oblique Incidence Corrections to Isodose Distributions 227Isodose Summations 229Treatment Techniques 230Stationary or Fixed Beam Treatment 230Moving Fields Treatment 233Tissue Inhomogeneities 235Tissue Compensation 238Wedge Filters 240Standard Treatment Calculation 244Beam On Time Calculations (Timer Settings) 244Monitor Unit Calculations 247

15. Clinical Applications in Treatment Planning 251

More Field Nomenclature 252Mixed Beams 253Tangents 256Field Weighting 260Normalization 260Non-Coplanar Beams 265Three-Dimensional Treatment Planning 267Conformal Methods 268IMRT 270 IGRT 272 Gated Radiation Therapy 272

16. Brachytherapy 277

Introduction 278Radium 278Radium Substitutes 278Radioactive Sources 279

Table of Contents xi

Applicators 281Afterloading 283Single Plane, Double Plane, and Volume Implants 284Permanent Implants 284Implant Dosimetry 286Remote High-Intensity Afterloading 288Specific Implant Techniques 289Radiation Safety with Implants 289

17. Radiation Safety 293

Recommendations and Regulations 294Measurement of Occupational Radiation Dose 295Radiation Risk 296Maximum Permissible Dose Equivalents 302Personnel Monitoring 304Time, Distance, and Shielding 306Radioactive Materials 310Radiation-Producing Machines 316Signs 321

Appendix 1: Signs and Symbols 325

Appendix 2: Constants and Units 327

Appendix 3: Glossary 331

Appendix 4: Answers to Problems 343

Appendix 5: Dosimetry Tables 351

Appendix 6: Radioactive Nuclides 359

Appendix 7: The Elements 367

Periodic Table 372

Index 375

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Preface

In this edition of Applied Physics for Radiation Oncology, the authors have tried tomaintain the basic character of our earlier text as an introduction for radiation ther-apists. Most of the basic physics chapters remain unchanged, as the basic principleshave not changed, and because of our belief that therapists should be taught physicsprinciples to help them understand the technologies they apply to patients.

We have both expanded several chapters to add new techniques and removedsome sections of only historic interest in others.

The collaborative team that produced this book includes those who helpedproduce previous versions including Rodger Holst, Joyce Keil, and MichaelNunno and a new contributor, Daniel Januseski, who updated the radiationsafety chapter. We, the authors, guided the entire project and hope that our yearsof experience helped produce a textbook that is easy to read and helps studentsto learn.

Robert StantonDonna Stinson September 2009

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14Treatment Planning

Tumor Targeting Vocabulary Aims of Treatment Planning What Treatment Planning Includes Patient Alignment Devices Patient Positioning Aids Body Contours Isodose Distributions Oblique Incidence Corrections to

Isodose Distributions Isodose Summations

Treatment Techniques Stationary or Fixed Beam

Treatment Moving Fields Treatment Tissue Inhomogeneities Tissue Compensation Wedge Filters Standard Treatment Calculation Beam On Time Calculations

(Timer Settings) Monitor Unit Calculations

Student Objectives

1. List five types of target volumes and explain the importance of each.

2. Identify three aims of treatment planning.

3. Describe the process of: visualization, localization, and field selectionand placement.

4. Describe field verification and documentation.

5. State the equations for timer and monitor unit calculations, andidentify the type of equipment used for each.

217

Treatment planning is the process of determining the best method of treating atumor with radiation. The major objective of treatment planning is to ensurethat the tumor receives a uniform radiation dose while healthy tissue and criti-cal structures are protected. Other important objectives of treatment planningare to develop reproducible setups and maintain patient comfort.

Treatment planning includes determining the volume to be treated and thendesigning appropriate radiation fields to treat that volume. It begins before thefirst radiation treatment and continues throughout a course of therapy to ensurethat the intended plan is being implemented. A treatment plan may sometimesbe changed during a course of therapy to compensate for changes in a patientscondition.

Professionals from several areas work together in developing the best treat-ment plan for each patient. These professionals include radiation oncologists,physicists, treatment planners (dosimetrists), and radiation therapists.

Tumor Targeting VocabularyAll of the individuals involved need to use the same vocabulary when discussingand planning patient treatment. Tumor dose (TD, or prescribed dose, DRx)is the dose to be delivered to the disease being treated. The patient work-up,defines the gross tumor volume (GTV), the palpable or visible extent of tis-sue that makes up the tumor. The clinical target volume (CTV) is the grosstumor plus a margin to include any areas of direct but subclinical disease spread.During the process of treatment planning, the planning target volume (PTV)is determined. It includes the clinical target volume plus the needed margin toensure delivery of dose to the target when motion, setup variations, etc. are takeninto account. Determining the tumor and target volumes are two of the key clin-ical decisions of the radiation oncologist. Once the treatment plan is developed,two other volumes can be determined. Treated volume (TV) is the volume oftissue that actually receives the tumor dose; it may be larger but should not besmaller than the clinical target volume. The irradiated volume is the entire vol-ume of tissue hit by any portion of the rad