1. No category

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]
Workshop on European Population Doses from Medical Exposure
24-26 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 Exposure
24-26 April 2012, Athens, Greece
Population dose from X-ray procedures
Sum of effective doses from all x-ray procedures/ examinations in a year
Workshop on European Population Doses from Medical Exposure
24-26 April 2012, Athens, Greece
Population dose from X-ray procedures
Summation over each type of examination
SE = ∑i =1 N i × E i
n
Exam 1: N1, E1
Exam 2: N2, E2
Exam 3: N3, E3
Exam 4: N4, E4
Workshop on European Population Doses from Medical Exposure
24-26 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 Exposure
24-26 April 2012, Athens, Greece
Purpose of effective dose, E
•
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)
•
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)
•
It should not be calculated for individuals, or patients ….
•
…. But OK for assessment of population doses from diagnostic medical
exposures and their inter-comparison
Workshop on European Population Doses from Medical Exposure
24-26 April 2012, Athens, Greece
ICRP Effective Dose (E) and Tissue Weighting Factors (2007)
wT
E=
Σ wT DT
T
where DT is mean dose to
tissue T and the product
wT DT is summed over all
tissues
oesophagus - 0.04
thyroid - 0.04
lungs - 0.12
skin - 0.01
breast - 0.12
stomach - 0.12
liver - 0.04
colon - 0.12
gonads - 0.08
Tissue weighting factors (wT) derived for whole population
Workshop on European Population Doses from Medical Exposure
24-26 April 2012, Athens, Greece
Evolution of ICRP tissue weighting factors (W T)
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)
Workshop on European Population Doses from Medical Exposure
24-26 April 2012, Athens, Greece
Influence of changes in tissue weighting factors
X-ray examination (on adults)
Ratio E2007 / E1990
Head
Chest
Abdomen
Pelvis
Lumbar spine
1.36
1.00
0.91
0.62
0.91
CT head
CT chest
CT abdomen
CT abdomen & pelvis
0.84
1.14
1.09
0.98
Changes by a few 10’s of % for particular examinations
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 Exposure
24-26 April 2012, Athens, Greece
How can we estimate effective dose?
• 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
Workshop on European Population Doses from Medical Exposure
24-26 April 2012, Athens, Greece
Practical quantities for monitoring conventional X-ray examinations
By calculation or measurement
Dose-Area Product
(DAP)
Patient
Entrance Surface Dose
(with backscatter)
(ESD)
• ESD for a single radiograph (mGy)
• DAP for a radiograph or complete examination (Gy cm2)
Workshop on European Population Doses from Medical Exposure
24-26 April 2012, Athens, Greece
Practical quantities for monitoring CT X-ray examinations
Practical measurement using 100 mm pencil
chamber (as modified for broad beams
(NT>40 mm, IEC 2011) in air and standard CT
dosimetry phantoms:
+50 mm
CTDIFREE AIR, NT
1
CTDI = NT D ( z ) d z x
CTDIFREE AIR, NTref
-50 mm
∫
For each scan sequence:
Volume weighted CT dose index
IAEA©
( where NTref ≤ 20 mm )
CTDIVol
For each scan sequence and complete examination:
Dose-length product
DLP
(mGy)
(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
Workshop on European Population Doses from Medical Exposure
24-26 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 Exposure
24-26 April 2012, Athens, Greece
Methods for deriving normalised organ doses from X-ray examinations
Measure
Eg: TLDs/ MOSFETS in
physical anthropomorphic
phantom
Calculate – Monte Carlo simulation
Eg: PCXMC (STUK),
HPA (NRPB), Helmholtz Centrum Munich (GSF))
MIRD family
CT
Conventional
Voxel
Workshop on European Population Doses from Medical Exposure
24-26 April 2012, Athens, Greece
Influence on E of differing anthropomorphic dose models
ICRP 103 (2007)
Use Voxel Adult Male (AM) and Adult
Female (AF) (ICRP Report 110) for
average organ doses
Deviations in E for use of:
Adult MIRD (mathematical phantom)
eg HPA18+
AF
AM
HPA18+
Estimates for AM & AF for common CT examinations differ by a few 10’s of %
relative to MIRD values (0.7 – 1.4)
Workshop on European Population Doses from Medical Exposure
24-26 April 2012, Athens, Greece
Variation in E calculated for different adult phantoms
Relative normalised effective doses (E103) for standard CT examinations
(mean data over range of scanners (120-130 kV))
CT examination
Brain
Chest
Abdomen
Pelvis
Whole body
Relative effective dose, E103
Anthropomorphic phantom
HPA18+
AM
AF
1.0
0.9
1.2
1.0
1.2
1.4
1.0
1.4
1.3
1.0
0.7
1.2
1.0
1.0
1.1
AM+AF
1.1
1.3
1.4
1.0
1.1
Changes by few 10’s of %
Workshop on European Population Doses from Medical Exposure
24-26 April 2012, Athens, Greece
Influence on E of differing anthropomorphic dose models
ICRP 103 (2007)
Use Voxel Adult Male (AM) and Adult
Female (AF) (ICRP Report 110) for
average organ doses
Deviations in E for use of:
Adult MIRD (mathematical
phantom)
AF
¾y M
4y F
AM
8y F
HPA18+
11y M
14y M
Paediatric reference patients
(Eg University of
Florida voxel phantoms
Workshop on European Population Doses from Medical Exposure
24-26 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 Exposure
24-26 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 Exposure
24-26 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 Exposure
24-26 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
E/ DAP
E (mean glandular dose)/ Air Kerma
E/ DLP
Radiography
Radiography/ Fluoroscopy
Mammography
CT
Sources of data – Eg:
PCXMC (STUK, Finland)
EC RP154 (2008)
ICRU Report 74 (2005) Patient dosimetry for x-rays used in medical imaging
IAEA TRS 457 (2007) Dosimetry in diagnostic radiology: international code of practice
UNSCEAR 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 Exposure
24-26 April 2012, Athens, Greece
Normalised effective doses in relation to UK practice
Exam
Chest PA
Cheat LAT
Cervical spine AP
Cervical spine LAT
Thoracic spine AP
Thoracic spine LAT
Lumbar spine AP
Lumbar spine LAT
Abdomen AP
Pelvis AP
Barium swallow
Barium enema
Barium follow-through
IVU
Cardiac angiography
E/ ESD (mSv/ mGy)
0.13
0.090
0.035
0.023
0.094
0.031
0.12
0.027
0.13
0.10
-
*
E/ DAP (mSv/ mGy cm2)
0.16
0.13
0.19
0.12
0.24
0.093
0.22
0.092
0.18
0.14
0.23
0.12
0.13
0.18
0.16
*Radiation risks from medical x-ray examinations as a function of the age and sex of the patient
BF Wall, R Haylock, JTM Jansen, MC Hillier, D Hart and PC Shrimpton
Report HPA-CRCE-028 (2011); www.hpa.org.uk
Workshop on European Population Doses from Medical Exposure
24-26 April 2012, Athens, Greece
Normalised effective dose from CT along phantom axis for various phantoms
Workshop on European Population Doses from Medical Exposure
24-26 April 2012, Athens, Greece
Estimation of effective dose from CT
E103 = EDLP DLP
Exam region
Head
Chest
Abdomen
Abdo & Pelvis
( mSv )
Normalised effective dose EDLP
Adult
10 y
5y
0.0018
0.0027
0.0034
0.016
0.015
0.021
0.016
0.016
0.022
0.015
0.015
0.020
(mSv mGy-1 cm-1)
1y
0y
0.0056
0.0092
0.030
0.044
0.033
0.053
0.029
0.048
Shrimpton 2004, NRPB-PE/1/2004 (updated data)
(See also Deak et al 2010, Radiology 257(1): 158-166 …. etc …. )
Workshop on European Population Doses from Medical Exposure
24-26 April 2012, Athens, Greece
Estimating representative values of E for national practice
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%
Workshop on European Population Doses from Medical Exposure
24-26 April 2012, Athens, Greece
Estimating representative values of E for national practice
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%
Workshop on European Population Doses from Medical Exposure
24-26 April 2012, Athens, Greece
How to estimate typical effective doses for X-ray procedures?
Summary
• 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!
Workshop on European Population Doses from Medical Exposure
24-26 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 Exposure
24-26 April 2012, Athens, Greece