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Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Attention
The aim of the Dose Datamed II workshop in Athens was to present preliminary data, collect feedback from the audience and to work towards final results
and conclusions.
Therefore, all the data and statements made in this presentation are preliminary and might change in the future.
Please use the data with care and indicate its preliminary character!
We would appreciate any feedback made based on the data presented with an email at:
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
How to estimate typical effective doses for X-ray procedures?
Paul Shrimpton
Health Protection Agency
Chilton, UK
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Sum of effective doses from all x-ray procedures/ examinations in a year
Population dose from X-ray procedures
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Summation over each type of examination
Exam 1: N1, E1
i
n
1i iE ENS ×=∑ =
Exam 2: N2, E2
Exam 3: N3, E3 Exam 4: N4, E4
Population dose from X-ray procedures
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Effective dose, E
Need representative (typical) values of effective dose for each examination category
But what is effective dose?
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
• Developed by the International Commission on Radiological Protection (ICRP) as part of its system for controlling sources of exposure (ie, compliance with dose limits or constraints for workers and public) (ICRP 103, 2007)
• …. But OK for assessment of population doses from diagnostic medical exposures and their inter-comparison
• Transforms particular exposure into an equivalent uniform whole body exposure that allows comparison and summation of doses, whether whole or partial body, or from external and internal sources
• E defined for a population of all ages and both sexes, on the basis of mean doses to a reference man and a reference woman (ICRP 110, 2009)
Purpose of effective dose, E
• It should not be calculated for individuals, or patients ….
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
ICRP Effective Dose (E) and Tissue Weighting Factors (2007)
thyroid - 0.04
liver - 0.04colon - 0.12
lungs - 0.12
gonads - 0.08
stomach - 0.12
oesophagus - 0.04
skin - 0.01
E = Σ wT DT
T
where DT is mean dose to tissue T and the product wT DT is summed over all tissues
breast - 0.12
Tissue weighting factors (wT) derived for whole population
wT
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Organ ICRP 1977 ICRP 1990 ICRP 2007
Gonads 0.25 0.20 0.08
Bone marrow (red) 0.12 0.12 0.12
Lung 0.12 0.12 0.12
Breast 0.15 0.05 0.12
Thyroid 0.03 0.05 0.04
Bone surfaces 0.03 0.01 0.01
Remainder 0.30 0.05 0.12
Colon - 0.12 0.12
Stomach - 0.12 0.12
Bladder - 0.05 0.04
Liver - 0.05 0.04
Oesophagus - 0.05 0.04
Skin - 0.01 0.01
Salivary glands - - 0.01
Brain - - 0.01
ICRP 1977 (Report 26)ICRP 1990 (Report 60)ICRP 2007 (Report 103)
Some influence on values of effective dose for medical exposures:
Ratio E2007/ E1990 varies by a few 10’s % for common examinations (0.6 - 1.4)
Evolution of ICRP tissue weighting factors (WT)
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
X-ray examination (on adults)
HeadChestAbdomenPelvisLumbar spine
CT headCT chestCT abdomenCT abdomen & pelvis
Ratio E2007 / E1990
1.361.000.910.620.91
0.841.141.090.98
Changes by a few 10’s of % for particular examinations
Influence of changes in tissue weighting factors
But ( … by chance?), only small effect (+2%) on estimated population dose from x-rays in UK for 2008 (Hart et al, 2010; HPA-CRCE-012)
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
• Can’t measure effective dose (E) directly!
• Need range of organ doses (also difficult to measure directly)
• Can only estimate for reference patients (as part of a dose model)
• Need sophisticated dosimetry
• In practice, derive E from simpler monitoring quantities
How can we estimate effective dose?
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
• ESD for a single radiograph (mGy)• DAP for a radiograph or complete examination (Gy cm2)
Dose-Area Product (DAP)
Entrance Surface Dose (with backscatter) (ESD)Patient
Practical quantities for monitoring conventional X-ray examinations
By calculation or measurement
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
For each scan sequence and complete examination: Dose-length product DLP (mGy cm)
It is IMPORTANT to know the underlying reference standard CT dosimetry phantom (16 cm or 32 cm diameter) for dose values displayed on the CT console since this affects the numerical value by a factor of ~2
Practical quantities for monitoring CT X-ray examinations
IAEA©
Practical measurement using 100 mm pencil chamber (as modified for broad beams (NT>40 mm, IEC 2011) in air and standard CT dosimetry phantoms:
For each scan sequence: Volume weighted CT dose index CTDIVol (mGy)
+50 mm
-50 mm
CTDI = D ( z ) d z NT1 ∫
CTDIFREE AIR, NT
x
CTDIFREE AIR, NTref ( where NTref ≤ 20 mm )
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Normalised organ and effective doses
Need to employ dose models to relate typical organ and effective doses for particular types of X-ray exposure to practical monitoring quantities:
• Air kerma - Mammography• ESD (with backscatter) - Radiography• DAP - Radiography/ Fluoroscopy• CTDIAir - CT• CTDIVol - CT• DLP - CT
Require examination- and technique-specific organ (and effective) doses normalised to these monitoring quantities
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Measure
Eg: TLDs/ MOSFETS in physical anthropomorphic phantom
Methods for deriving normalised organ doses from X-ray examinations
Calculate – Monte Carlo simulation
MIRD family
Voxel
Eg: PCXMC (STUK), HPA (NRPB), Helmholtz Centrum Munich (GSF))
ConventionalCT
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
ICRP 103 (2007) Use Voxel Adult Male (AM) and Adult Female (AF) (ICRP Report 110) for average organ doses
AF AM
Deviations in E for use of:
Adult MIRD (mathematical phantom) eg HPA18+
HPA18+
Estimates for AM & AF for common CT examinations differ by a few 10’s of % relative to MIRD values (0.7 – 1.4)
Influence on E of differing anthropomorphic dose models
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Relative normalised effective doses (E103) for standard CT examinations (mean data over range of scanners (120-130 kV))
CT examination Relative effective dose, E103
Anthropomorphic phantomHPA18+ AM AF AM+AF
Brain 1.0 0.9 1.2 1.1Chest 1.0 1.2 1.4 1.3
Abdomen 1.0 1.4 1.3 1.4Pelvis 1.0 0.7 1.2 1.0
Whole body 1.0 1.0 1.1 1.1
Changes by few 10’s of %
Variation in E calculated for different adult phantoms
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Influence on E of differing anthropomorphic dose models
Paediatric reference patients (Eg University of Florida voxel phantoms
¾y M 4y F 8y F 11y M 14y M
ICRP 103 (2007) Use Voxel Adult Male (AM) and Adult Female (AF) (ICRP Report 110) for average organ doses
AF AM
Deviations in E for use of:
Adult MIRD (mathematical phantom)
HPA18+
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Normalised effective doses for Siemens Sensation 16 (whole body CT exposure)
University of Florida voxel phantoms
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Summary – Influences on estimates of E
Estimates of effective dose depend on:
• Exposure conditions (kV, filtration, FSD, field size & position, mAs)
• Definition of E (tissue weighting factors)
• Dose model assumed for reference patient (eg anthropomorphic phantom, bone)
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Estimating representative values of E for national practice
Need typical values of E for each X-ray examination category • Consider heterogeneity in dose for any sub-categories• Focus on adult patients (relatively small numbers of paediatric exposures)
Sources of data• National patent dose survey
Sufficiently large sample of hospitals/ clinics and x-ray rooms to reflect national variations in practice (equipment, technique, etc)10-20 patients per room/ facility of average size (eg 60-80 kg) undergoing standard examination (mean doses for indication of typical practice) Mean of X-ray room mean doses (ESD, DAP, DLP) for each exam for representative sample of X-ray rooms
• Local/ regional survey in country (less reliable)
• Published literature (less reliable) eg, EC RP 154 (2008), UNSCEAR 2008 (2010), HPA-CRCE-012 (2010)
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Estimating representative values of E for national practice
Converting mean practical dose quantities into effective doses Use appropriate coefficient for typical exposures conditions (and sum contributions from individual components according to typical technique)
• E/ ESD Radiography• E/ DAP Radiography/ Fluoroscopy• E (mean glandular dose)/ Air Kerma Mammography• E/ DLP CT
Sources of data – Eg:PCXMC (STUK, Finland)EC RP154 (2008)ICRU Report 74 (2005) Patient dosimetry for x-rays used in medical imagingIAEA TRS 457 (2007) Dosimetry in diagnostic radiology: international code of practiceUNSCEAR 2008 (2010)ImPACT & CT-Expo (CT)HPA (Wall et al, 2011; HPA-CRCE-028)Helmholtz Centrum Munich, FDA ….. etc
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Normalised effective doses in relation to UK practice *
ExamChest PACheat LATCervical spine APCervical spine LATThoracic spine APThoracic spine LATLumbar spine APLumbar spine LATAbdomen APPelvis APBarium swallowBarium enemaBarium follow-throughIVUCardiac angiography
E/ ESD (mSv/ mGy)0.130.0900.0350.0230.0940.0310.120.0270.130.10 - - - - -
E/ DAP (mSv/ mGy cm2)0.160.130.190.120.240.0930.220.0920.180.140.230.120.130.180.16
*Radiation risks from medical x-ray examinations as a function of the age and sex of the patientBF Wall, R Haylock, JTM Jansen, MC Hillier, D Hart and PC ShrimptonReport HPA-CRCE-028 (2011); www.hpa.org.uk
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Normalised effective dose from CT along phantom axis for various phantoms
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Normalised effective dose EDLP (mSv mGy-1 cm-1)
E103 = EDLP DLP ( mSv )
Adult
0.0018
0.016
0.016
0.015
Head
Chest
Abdomen
Abdo & Pelvis
10 y
0.0027
0.015
0.016
0.015
5 y
0.0034
0.021
0.022
0.020
1 y
0.0056
0.030
0.033
0.029
0 y
0.0092
0.044
0.053
0.048
Shrimpton 2004, NRPB-PE/1/2004 (updated data)(See also Deak et al 2010, Radiology 257(1): 158-166 …. etc …. )
Estimation of effective dose from CT
Exam region
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Uncertainties• Basic dose measurements: 10-20% (95% confidence level)• Variations between hospitals and limited sample size:
Random (Systematic - selection bias)
• Conversion coefficients (CC): match to exposure conditions (radiation quality, anatomy exposed, exam technique)
Illustrative overall uncertainties EC RP 154 2008 (Hart & Wall, 2002; NRPB-W4)Sample size (95% CL)
>100 rooms ± 10%20-100 rooms ± 25%5-19 rooms ± 50%
Estimating representative values of E for national practice
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
Uncertainties• Basic dose measurements: 10-20% (95% confidence level)• Variations between hospitals and limited sample size:
Random (Systematic - selection bias)
• Conversion coefficients (CC): match to exposure conditions (radiation quality, anatomy exposed, exam technique)
Illustrative overall uncertainties EC RP 154 2008 (Hart & Wall, 2002; NRPB-W4)Sample size CC Overall (95% CL)
>100 rooms; good CC match ± 10% ± 10% ± 14%20-100 rooms; good CC match ± 25% ± 10% ± 27%5-19 rooms; good CC match ± 50% ± 10% ± 51%>100 rooms; poor CC match ± 10% ± 25% ± 27%20-100 rooms; poor CC match ± 25% ± 25% ± 35%5-19 rooms; poor CC match ± 50% ± 25% ± 56%Foreign data only +100% / - 50%
Estimating representative values of E for national practice
Workshop on European Population Doses from Medical Exposure24-26 April 2012, Athens, Greece
How to estimate typical effective doses for X-ray procedures?
• Effective dose (E) can’t be measured
• Derive from practical dose measurements using dose models
• Need to be aware of underlying dose models and assumptions
• Determine typical values of ESD, DAP, DLP for examinations from wide scale surveys
• Estimate representative values of E for each type of examination using standard conversion coefficients appropriate for typical exposure conditions and technique
• Have awareness of likely uncertainties in dose estimates!
Summary