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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
New ICRU* Report on CT Dosimetry
2013 ICTP/IAEA Training Course on Radiation Protection of Patients Trieste
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*International Commission on Radiation Units (and Measurement)
ICRU Report 87: Radiation Dose and Image Quality Assessment in CT
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Disclosures Paid Consultant to:
Varian Imaging Systems
Alston and Bird LLC
CardioInsight
DXray
Royalty Income from:
Lippincott Williams and Wilkins
Samsung Corporation
Research Funding from:
Stanford Research Institute
University of Pittsburgh
Siemens Medical Systems
Hologic Corporation
National Institutes of Health (NIBIB)
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Introduction & Historical CT Dose Metrics
Dose dependency on patient size
Dose and CT scan length
Phantoms and radiation meters
ICRU extension to AAPM Report 111
Summary
ICRU Report on CT Dosimetry
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position
do
se
Z=0 -Z +Z
50
100
50
1( )
mm
mm
CTDI D z dznT
100 mm
CTDI - based Dose Metrics
CTDI100 (center & peripheral)
The Tools
CTDIw
CTDIvol
DLP
The Metrics
The Methods
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CTDI is a good measure of CT dose to a large plastic
phantom, but is not a stand-alone metric for patient dose
etcetera….
Med Phys 2003
Introduction & Historical CT Dose Metrics
Dose dependency on patient size
Dose and CT scan length
Phantoms and radiation meters
ICRU extension to AAPM Report 111
Summary
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ICRU Report on CT Dosimetry
32 cm
r = 1.19
47”
120 cm waistline
28 cm ≈ 35”
88 cm waistline
9 UC Davis Data
Dose Dependency on patient size
32 cm PMMA phantom 1138 cm2
average patient 615 cm2
85% bigger by
mass
r = 1.19 r = 1.0
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practical methods to correct dosimetry estimates for patient size
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Studied Size-dependent CT Dose
TG-204 Approach
Four Independent Research Groups
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Family of physical phantoms Cynthia McCollough, Mayo Clinic
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standard phantoms Tom Toth & Keith Strauss
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Anthropomorphic Monte Carlo phantoms Mike McNitt-Gray, UCLA
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Monte Carlo phantoms (1 – 50 cm) John M. Boone, UC Davis
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Figure 2
AP
lateral effective diameter
circle of equal area
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patient size
do
se
CTDIvol
32 cm PMMA
normalization point
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corr
ecti
on
fac
tor
CTDIvol
32 cm
after normalization
1.0
patient size
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Co
nve
rsio
n F
acto
r
water-equivalent diameter (cm)
32 cm 120 kV
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0 1 5 10 15
age in years
Co
nve
rsio
n F
acto
r 16 cm 120 kV
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CTDIvol is indicated on most scanners…..
CTDIvol mGy 13.2
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determine patient size
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determine patient size
Dw
CTDIvol (mGy) × = SSDE (mGy) f
air kerma absorbed dose
SSDE conversion factor
Size-Specific Dose Estimate (SSDE)
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SSDE conversion factor
CT Radiograph
Example of SSDE calculation from localizer view
5.40 mGy = CTDIvol
SSDE = 5.4 mGy × 2.5
SSDE = 13.5 mGy
Example of SSDE calculation from localizer view
Introduction & Historical CT Dose Metrics
Dose dependency on patient size
Dose and CT scan length
Phantoms and radiation meters
ICRU extension to AAPM Report 111
Summary
32
ICRU Report on CT Dosimetry
Problems with CTDIvol
32 cm diameter PMMA phantom
10 cm scan length 33
15 cm phantom
L
10 cm
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JM Boone, Dose spread functions in computed tomography: A Monte Carlo study”, Med Phys 36, 4547-4554 (2009)
How long are the scatter tails in CT?
62%
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JM Boone, Dose spread functions in computed tomography: A Monte Carlo study”, Med Phys 36, 4547-4554 (2009)
How long are the scatter tails in CT?
10% 10%
80% ~130 mm
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JM Boone, Dose spread functions in computed tomography: A Monte Carlo study”, Med Phys 36, 4547-4554 (2009)
How long are the scatter tails in CT?
1% 1%
98% 400 mm
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Dose profiles as a function of Scan Length
scan length
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Equilibrium Dose as a function of Scan Length
Deq
Do(L
)
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Integrating thimble chamber 41
TG-111 Method
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TG-111 Method
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TG-111 Method
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TG-111 Method
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h = scatter / primary
P P S
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Introduction & Historical CT Dose Metrics
Dose dependency on patient size
Dose and CT scan length
Phantoms and radiation meters
ICRU extension to AAPM Report 111
Summary
47
ICRU Report on CT Dosimetry
ICRU / AAPM (TG-200) Dosimetry Phantom
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phantoms
section A section B section C
Phantom is polyethylene 60 cm long by 30 cm in diameter
Each section is 20 cm long and weighs 13.7 kg (30 lbs)
compared to 32 cm diameter PMMA: 14.4 kg (5% lighter)
Total phantom 41.1 kg (90 lbs)
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50
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Integrating thimble chamber
real time probes
signal
time
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18 mm*
The thimble chamber with
real time (>1000 Hz) readout rates
*active length
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Introduction & Historical CT Dose Metrics
Dose dependency on patient size
Dose and CT scan length
Phantoms and radiation meters
ICRU extension to AAPM Report 111
Summary
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ICRU Report on CT Dosimetry
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real time thimble chamber 57
ICRU Method
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beam profile 59
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ICRU method
TG-111 method
Introduction & Historical CT Dose Metrics
Dose dependency on patient size
Dose and CT scan length
Phantoms and radiation meters
ICRU extension to AAPM Report 111
Summary
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ICRU Report on CT Dosimetry
CTDI-based methods need to be updated
SSDE is a method for adjusting for patient size
Scan length dose dependencies h(L)
Longer phantoms and faster radiation meters
Methods for automatic size detection
ICRU CT Report Available Q3 2013
TG-111/ICRU
TG-220
TG-204/ICRU
TG-200/ICRU
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ICRU Report on CT Dosimetry
Figures from CT Chapter
2012 edition
Introduction & Historical CT Dose Metrics
Dose dependency on patient size
Dose and CT scan length
Phantoms and radiation meters
ICRU extension to AAPM Report 111
Summary
65
ICRU Report on CT Dosimetry
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Cobra: Boone
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Computer simulation
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Computer simulation
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Physical Measurement
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