Dosimetry in digital mammography - Linköping University · 2014-04-07 · Dosimetry in digital...
Transcript of Dosimetry in digital mammography - Linköping University · 2014-04-07 · Dosimetry in digital...
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Dosimetry in digital
mammography
Professor David Dance
NCCPM, Royal Surrey County Hospital, Guildford, United kingdom
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Outline• Why do dosimetry?
• History
• Essentials of European breast
dosimetry protocol
• Practical breast dosimetry
• Uncertainties and limitations
• Key points
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Why do dosimetry?
• To control the dose
– no higher than necessary
– BUT high enough to ensure that adequate image quality is obtained
– essential part of quality control
• optimise equipment selection, design and use
• To estimate risk
– comparison of risk and benefit
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History
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UK DOSE SURVEY 1981
• 5 cm phantom
• Entrance surface dose – TLD
• Dose range 0.9 to 45 mGy (!)
• kV range 24- 49 kV
• HVL range 0.2 mm Al to 1.7 mm Al!
• Entrance dose is a poor measure
of risk!
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Transmission through 5 cm breast
0.001
0.01
0.1
1
15 20 25 30 35
Adipose
Gland
Energy keV
Tra
nsmission
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Alternative dose measures
• Mid-breast dose
• Mean breast dose
• Average glandular dose(Mean glandular dose)
– Karlsson in 1976
– Recommended by ICRP in 1987
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Essentials of European
breast dosimetry protocol
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Original dosimetry
requirements in UK
• AGD: risk related dose measure
• Easy to implement
• Applicable for patient and phantom dosimetry
• Standard phantom for QC
– Equivalent to a typical compressed breast
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Average glandular dose• Cannot measure AGD on patients
• Incident air kerma K (without backscatter) can be easily measured or estimated
• Need conversion coefficients which relate K to AGD
• Used in: IPSM Report 59 1989
European Protocol 1996
AGD = K g
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Focal spot
Compression plateSimple breast phantomBreast support etc
Calculation of g
using Monte Carlo model (Dance 1990)
Calculate energy deposited in breast tissues
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Simple breast modelHammerstein 1979
• 5 mm adipose shield region
• Central region with mixture of glandular and adipose tissues
• Fraction by weight of glandular tissue in central region is known as the glandularity
Hammerstein et al suggested the use of 50% glandularity for dosimetry
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g-factors• g-factor is for 50% glandularity
• In principle g-factors depend upon
target/filtration and kV
Simplification• Tables of g-factor just based on HVL & breast
thickness
• For spectra in use at that time worked within ±
5%
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g-factors
0
0.2
0.4
0.6
0.8
2 4 6 8
HVL 0.3 mm
HVL 0.6 mm
Breast thickness cm
g-fa
ctor
mGy/m
Gy
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Improved formulation in 2000
Dance et al 2000, 2009, 2010, 2014
• Real breasts don’t all have 50%
glandularity – factors calculated for
0.1-100%
• Wider range of spectra used
AGD = Kgcs
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• c corrects for glandularity
=1 for 50% glandularity
• s corrects for X-ray spectrum
=1 for Mo/Mo
• Used in:
– UK (IPEM report 89 2005)
– European Guidelines (latest
supplement is 2013)
– IAEA Code of Practice (TRS457)
AGD = Kgcs
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c-factorsTabulated against HVL, breast thickness
and glandularity
0.30 0.35 0.40 0.45 0.50 0.55 0.60
HVL (mm Al)
0.6
0.8
1.0
1.2
1.4
c-fa
ctor
0.1%
25%
50%
75%
100%
5 cm breast
But what glandularity should be used?
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Breast glandularity (UK)
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s-factors
S Max ErrorMo/Mo 1.000 3.1%
Mo/Rh 1.017 2.2%
Rh/Rh 1.061 3.6%
Rh/Al 1.044 2.4%
W/Rh 1.042 2.1%
W/Ag 1.042 4.6%
W/Al s-table needed
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“Standard breasts”
• For quality control and inter-system comparison need a simple phantom or phantoms
• Cheap and easily reproducible (PMMA)
• Should be approximately equivalent to typical compressed breast(s)
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Breast/PMMA equivalence
• Same incident air kerma at PMMA
surface and breast surface
– Obtained by Monte Carlo simulation
– Validated by measurements with
breast tissue equivalent phantoms
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PMMA for standard breast
dosimetry
PMMA mm 20 30 40 45 50 60 70
Breast mm 21 32 45 53 60 75 90
Gland’y % 97 67 41 29 20 9 4
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PMMA equivalence age 50-64Dance et al, 2000
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Practical breast dosimetry
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European Guidelines
Fourth Edition
Supplements 2013
Method:
Section 2b2.3
g,c and s factors:
Appendix 5
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Practical dosimetry using
blocks of PMMA (6 monthly)
Objective is to simulate exposure of ‘standard breasts’
• Stage 1: determine exposure settings using blocks of PMMA
• Stage 2: determine incident air kerma & HVL
• Stage 3: calculate AGD
• Stage 4: review results
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Stage 1 - Exposure parameters
for blocks of PMMA
• Use usual clinical
exposure settings
• Record mAs, kV
and target/filter
• Repeat for 20, 30,
40, 45, 50, 60, 70
mm blocks
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Exposure parameters for blocks
of PMMA – Notes I
• PMMA blocks
– Thickness correct within ± 0.5 mm
• PMMA and breast equiv thicknesses are
different
– If AEC works on thickness, add expanded
polystyrene spacer to adjust total thickness (do not
cover chamber with spacer)
– If AEC works on attenuation, spacer should not be
necessary
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Exposure parameters for
blocks of PMMA – Notes II
• Apply standard compression
• AEC on mid-line and closest to chest wall
• If digital AEC looks at densest region of breast,
exposures may not be a good match to those
obtained clinically
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Stage 2 - Determination of
incident air kerma and HVL
g, c and s depend upon the HVL
K and HVL are determined with the
compression paddle in place
AGD = Kgcs
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Determination of incident
air kerma - I
• Incident air kerma at upper surface of PMMA– Measurement of tube output at a known
distance from tube focus
– Apply inverse square law to calculate air kerma at top surface of PMMA
• Measurement is made with paddle in contact with the dosimeter
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Determination of incident
air kerma - II
• Use a calibrated dosimeter suitable for
mammography
• Position dosimeter on line joining focal spot
to a point on the mid-line of the breast
support 6 cm from chest wall edge.
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Determination of incident
air kerma - II
• For dosimeters with backscatter rejection
position dosimeter on breast support
• For dosimeters without backscatter
rejection
position the dosimeter higher up to avoid or
reduce the effect of backscatter
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Tables A5.1 & A5.2
give g and c vs
PMMA cm & HVL
Table A5.4
gives s
AGD = KgcsStage 3
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Stage 4: review results
0
1
2
3
4
5
6
7
2 3 4 4.5 5 6 7
acceptable
achievable
PMMA thickness cm
AGD m
Gy
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0
5
10
15
20
25
30
35
0.5 1 1.5 2 2.5 3
MGD (mGy)
Nu
mb
er
of
syste
ms
98% < 2.5 mGy
AGD: 53 mm breast (45 mm PMMA)
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Patient dosimetry
• Measure tube output and HVL with paddle in place
• Record exposure parameters for patient series (e.g.50 or more)
– kV & target/filter
– mAs
– compressed breast thickness
• Calculate AGD
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Patient dosimetry – Note
• Accuracy of displayed thickness
should be checked by applying a
typical force (e.g. 100 N) to rigid
material of known thickness
• Accuracy of ±2 mm or better
required
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Tables A5.5 –A5.7
give g and c vs
breast cm & HVL
c for age 50-64 or 40-49
Table A5.4
gives s
AGD = Kgcs
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Average patient AGD for different
digital systems: 2010 UK data –Oduko et al IWDM 2010
DR CR FS0.0
0.5
1.0
1.5
2.0
2.5
Image receptor type
AG
D (
mG
y)
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Uncertainties & Limitations
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Uncertainties in AGD using
PMMA (typical)
• Dosimeter calibration 5%
• AEC and dosimeter
reproducibility 2%
• Uncertainty in g due to
3% uncertainty in HVL 2%
• Chamber positioning (2mm) 1%
• Phantom thickness (0.5mm) 2%
• s-factor approximation 4%
• Overall 7%
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Errors in using tabulated
equivalent PMMA thickness
• Exact PMMA thickness required for
equivalence varies with beam quality.
• Error is largest for highest kVs and
thickest breasts.
– 0-60 mm breast: up to 1.0 mm PMMA
– 80 mm breast: up to 1.7 mm PMMA
– 100 mm breast: up to 2.4 mm PMMA
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Errors in AGD associated with non-
equivalence of tabulated PMMA
thickness (W/Al spectrum)
20 30 40 50 60 70 80 90 100
0
5
10
1530 kV 35 kV 40 kV
Breast thickness (mm)
Err
or
%
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Air kerma measurement
• Measurements should be made with the dosimeter in contact with paddle (to mirror MC model)
• Scatter contributes e.g. ~7% to air kerma for 2.4 mm polycarbonate paddle
• Some semi-conductor dosimeters are collimated and will not record scatter
Correction is then needed (Hemdal, Rad Prot Dosim. 2011)
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Uncertainties in patient AGD typical
and assuming breast model is correct
• Dosimeter calibration 5%
• Dosimeter etc. reproducibility 2%
• Uncertainty in g due to
3% uncertainty in HVL 2%
• Patient thickness (2 mm) –
impact on g and ISL 3%
• s-factor approximation 4%
• Overall, for one exposure 7%
PLUS Standard error on mean from sample of 50 patients about 7%
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Some limitations of the model
• Heel effect ignored
• Simple breast geometry
• Breast composition data
(Hammerstein) based on
– 4 samples glandular tissue
– 8 samples adipose tissue
– 6 samples skin tissue
– results show considerable variability
in composition
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Effect of adipose shield
thickness on g
2 4 6 80.95
1.00
1.05
1.10
1.15
1.20
2.5 mm adipose shield
Thickness (cm)
g /
g(s
tan
dard
)
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Effect of glandular tissue
composition on g
2 4 6 80.85
0.90
0.95
1.00
1.05
1.10High O
Low O
Thickness (cm)
g /
g(s
tan
dard
)
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“Real” distributions of
glandular tissue
• The distribution of glandular tissue in
patients will not be uniform and large
differences in conversion factors can be
expected.
• Modelled using breast phantoms from
Predrag Bakic (Dance et al, RPD 2005)
• Differences in AGD of up to ~40% found
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European vs USA breast
model (Wu)
• Adipose shield replaced by 4mm skin– Different initial attenuation
– Different volume containing glandular tissue to share energy absorbed
• Unclear if dose from photons scattered in paddle included
• Different cross section compilation
• Differences of up to 16% in conversion coefficients
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Key points
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Key points
• The conversion factors are influenced by the model parameters.
• Whatever breast model is chosen for QC dosimetry, it need only be an approximate representation of the true situation. The present European and American models give standard procedures and are fine for QC.
• Large errors can occur if either model is used to estimate AGD for individual patients. But the models are fine for QC of patient doses.
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Acknowledgements
• Recent work has been funded under the
EU 6th Framework Programme – the
HighRex project and the UK funded
OPTIMAM and OPTIMAM2 projects
• I am grateful to my colleagues at
NCCPM for provision of data, and for
discussions