How to estimate typical effective doses for X-ray …ddmed.eu/_media/workshop:o5.pdf · How to estimate typical effective doses for X-ray procedures? ... skin - 0.01 E = ΣwT DT T

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:

[email protected]

mailto:[email protected]


How to estimate typical effective doses for X-ray procedures?

Paul Shrimpton

Health Protection Agency

Chilton, UK


Sum of effective doses from all x-ray procedures/ examinations in a year

Population dose from X-ray procedures


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


Effective dose, E

Need representative (typical) values of effective dose for each examination category

But what is effective dose?


• 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 ….


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


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)


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)


• 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?


• 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


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 )


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


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


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


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


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+


Normalised effective doses for Siemens Sensation 16 (whole body CT exposure)

University of Florida voxel phantoms


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)


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)



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


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


Normalised effective dose from CT along phantom axis for various phantoms


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


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%



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%




• 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



Paul Shrimpton

Health Protection Agency

Chilton, UK

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How to estimate typical effective doses for X-ray …ddmed.eu/_media/workshop:o5.pdf · How to estimate typical effective doses for X-ray procedures? ... skin - 0.01 E = ΣwT DT T