patient dosimetry in mammography and...
TRANSCRIPT
John M. Boone, Ph.D., FAAPM, FSBI, FACR
Professor and Vice Chair (Research) of Radiology Professor of Biomedical Engineering
University of California Davis Medical Center Sacramento, California
Patient Dosimetry in Mammography
and Tomosynthesis: What to measure, why and how
2013 ICTP/IAEA Training Course on Radiation Protection of Patients Trieste
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Mammography and Tomosynthesis Dosimetry
Mammography
Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues
Tomosynthesis
Differences between tomo and mammo
Summary
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U.S. Breast Cancer Statistics (2006)
212,290 new cases 40,970 deaths 1 / 8 women will get breast cancer (12.5%)
Breast Cancer Lung Cancer
Incidence*
31% 12%
Mortality*
15% 26%
* of all cancers Ravdin, et al., NEJM 2004 3
Jemal A, et al., Cancer Statistics 2006
dea
th r
ate
per
1
00
,00
0
1930 2002
U.S. Cancer Mortality (1930-2002)
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CC
CC MLO
MLO
Mammography: Standard of Care
Annual screening dose accumulation
average size breast
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Mammography and Tomosynthesis Dosimetry
Mammography
Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues
Tomosynthesis
Differences between tomo and mammo
Summary
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Digital mammography unit at UC Davis
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Breast Dose is highly dependent upon the x-ray spectrum used. Different Anode/Filter combinations are used.
ANODE Mo Mo Rh W W W
FILTER Mo Rh Rh Rh Ag Al
S/F
Tomo
Digital
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In addition to the Anode / Filter combo, the kV and HVL need to be well characterized
compression paddle
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X-ray Tube Voltage Evaluation
non-invasive kV meter
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In addition to the Anode / Filter combo, the kV and HVL need to be well characterized
0.1 mm thick Al filters (99.999%)
ion chamber calibrated to mammo energies
air gap
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HVL as a function of tube voltage
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ion chamber
Establishing x-ray tube output
mGy air kerma per 100 mAs at 50 cm
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X-ray output (@50 cm) versus tube voltage
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kV
HVL’s
glandular fraction
breast thickness
anode / filter combo
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Validation of Breast Thickness Accuracy
breast phantoms (slabs)
4.1 cm
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From Bloomquist, et.al. Med Phys 33: 719-736, 2006
Estimating Entrance Air Kerma
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Estimating Entrance Air Kerma
Recorded in patient’s record (digital)
anode filter kV mA exposure time compressed breast thickness
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Estimating Entrance Air Kerma
air kerma
dgN values normalized glandular dose coefficients
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Estimating Entrance Air Kerma
@ Anode/Filter kV:
air kerma per 100 mAs @ 50 cm = k
ESK = k × (mAs/100) × ISL
50 cm 2
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SID T
ISL
breast thickness usually ~65 cm
SID - T
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Mammography and Tomosynthesis Dosimetry
Mammography
Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues
Tomosynthesis
Differences between tomo and mammo
Summary
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Mean Glandular Dose
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Mean Glandular Dose
glandular tissue
skin
adipose tissue
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Mean Glandular Dose (Monte Carlo Calculations)
glandular tissue?
skin
adipose tissue?
glandular fraction
homogeneous mixture
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Mammography and Tomosynthesis Dosimetry
Mammography
Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues
Tomosynthesis
Differences between tomo and mammo
Summary
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DgN values are based on MC Studies
assumed 4 mm skin thickness
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DgN values are based on MC Studies
assumed 4 mm skin thickness
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Monoenergetic MC eval of E deposition
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MGD =
men
r
men
r glandular
breast comp
Energy Imparted
Mass
Converting Energy imparted to dose….
f-factor (SI)
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Mean Glandular Dose (Monte Carlo Calculations)
ESK
MGD
DgN = ESK
MGD
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Dose Calc with Normalization by ESE
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To convert the monoenergetic DgN values to realistic polyenergetic values, spectral models are used to weight the
monoenergetic values
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DgN Tables (poly)
The units used in DgN Tables have varied over the years and by the country of origin. These units are mRad / R, but mGy/mGy
is in more common usage today 34
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2 cm 4 cm
6 cm 8 cm
DgN versus monoenergetic x-ray energy
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Mammography and Tomosynthesis Dosimetry
Mammography
Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues
Tomosynthesis
Differences between tomo and mammo
Summary
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t = 4 mm?
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t
Developed algorithms to segment skin from breast CT images and measure thickness
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Skin Thickness measurements were relatively precise for each women (18%)
Measurements from the same women
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Measurements from 51 different women
Skin Thickness measurements ranged from 0.9 mm to 2.3 mm (m = 1.45 mm)
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Left / Right Comparison provided a consistency check, with good results
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DgN values versus skin thickness
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Change (in %) in DgN values versus skin thickness, Relative to the assumption of t = 4 mm
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Mammography and Tomosynthesis Dosimetry
Mammography
Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues
Tomosynthesis
Differences between tomo and mammo
Summary
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Mean Glandular Fraction
100% 0% 50%
high low medium
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risk assessment & dosimetry
validation of 2D approaches (M. Yaffe)
%
Breast Density Analysis
segmentation
glandular
adipose
including skin
excluding skin
UC Davis breast CT N = 191
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Data from UC Davis Breast CT
Den
sity
an
alys
is d
on
e o
n m
amm
ogr
ams
at
Un
iver
sity
of
Toro
nto
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UCD & UT data combined (N = 2831)
Median ≈ 16 %
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UCD & UT data combined (N = 2831) Cumulative Distribution
50%
75%
90%
95%
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Breast density decreases about 5 % per decade
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Areal Fraction versus Volume Fraction
areal fraction (%)
volu
me
frac
tio
n (
%)
0%
100%
0% 100%
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DgN for 0% Glandular Breast
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DgN for 100% Glandular Breast
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2 cm 4 cm 6 cm 8 cm
5 cm
(265-237)/237 = 12%
50%
16%
increase in “average” DgN coefficient of:
12%
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Mammography and Tomosynthesis Dosimetry
Mammography
Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues
Tomosynthesis
Differences between tomo and mammo
Summary
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Tomosynthesis (limited angle tomography)
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Conventional versus Tomosynthesis
Conventional Tomosynthesis Conventional Tomosynthesis
Similar display characteristics
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Tomosynthesis (limited angle tomography)
±7.5°
Combo Tomo / Mammo mode output
mammography
tomosynthesis
mR/mAs or mGy per 100 mAs
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Physical measurements of dose in Tomo/Mammo
Polyethylene is an excellent surrogate for adipose
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Output of real time exposure meter
filter changes & grid is inserted
mammo exposure 15 tomo exposures
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Point dose measures: Tissue Air Ratios
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Mammography and Tomosynthesis Dosimetry
Mammography
Why measure breast dose? Basic Concepts of Breast Dosimetry (how) Mean Glandular Dose (MGD) (what) DgN coefficients Skin Thickness Issues Breast Density Issues
Tomosynthesis
Differences between tomo and mammo
Summary
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Mammography and Tomosynthesis Dosimetry Summary
Complete characterization of the x-ray system is necessary (kV accuracy, HVL, and air kerma / 100 mAs @ anode/filter) Ionization chambers must be mammo beam compatible thin windowed and calibrated Practical Assessment requires table of DgN values specific to the conditions of the actual exam Current DgN tables may slightly under-estimate dose due to % glandular fraction and skin thickness issues Tomosynthesis is similar enough to the geometry of mammo anode/filter combo may differ; different DgN tables needed 70
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