Patient / Organ Dose in CT
Patient specific and organ dose estimation
H.D. Nagel
Dr. HD Nagel, Science & Technology for RadiologyBuchholz / Germanywww.sascrad.com
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Topics
• CTDI & patient dose
• SSDE
• Organ dose estimation
• Dosimetry software
• Limitations of organ dose assessment
• What is organ dose really needed for ?• How patient-specific must it be ?
• Summary & conclusions.
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Standard CT Dosimetry Phantoms
Body phantom32 cm PMMACT# = 126 HU
WED = 36.0 cm
Head phantom16 cm PMMA
CT# = 126 HUWED = 18.0 cm
WED = water equivalent diameter
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32cm CTDIphantom
= 36 cm WED
ADAM40x20=28.3cm (eff.)
CTDI vs. Organ Dose
➡ Differences (ca. 30%) mainly due to male phantom diameter < 36 cm
* water equivalent of 32 cm PMMA ** effective diameter of ADAM phantom
36*/28.3**= 1.27
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deff = dlat ⋅ d front
SSDE
from: AAPM report 204 table 10
16cm PMMA
32cm PMMA
ADAMADC reference diameter
corr. factor = 36cm / deff
SSDE = CTDIvol ⋅ corr. factor (deff )
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Reference Diameters in CT ADC
Reference WED (body range) for average adults in automatic dose control:• CareDose 4D (Siemens): 33 cm on average• DoseRight (Philips): 33 cm
➡ SSDE correction factor = 36/33 = 1.1➡ CTDIvol under-estimates SSDE for average adults by 10% only.
from: Somatom Definition user manual
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Effective Diameters for Children
from: AAPM report 204 fig. 10
CF=1.5
CF=1.9
CF=2.3
Pädiatric range: CF = 1.9 ± 20% => SSDE = CTDIvol16 ± 20%
CTDIvol16 ± 20%16cm PMMA
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Limitations of SSDE
• Patient‘s size varies along the scan range
‣Which diameter is representative? Average, peak or central?
• Lateral diameter often not accessible
‣ Reconstructed FOV generally smaller than patient‘s cross section
‣ Calculation based on a.p. diameter alone presumes constant aspect ratio
• SSDE based on geometric, not on water equivalent diameter
‣Average CT# of e.g. chest from -100 to -300 HU
‣WED significantly smaller than deff by 10 to 30%
➡ Not necessary for average and obese adults (SSDE ≈ CTDIvol)➡ For pediatrics CTDIvol16 ≈ 2 · CTDIvol32 would do as well➡ Meaningful for slim adults only (or SSDE ≈ 1.5 · CTDIvol).
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Organ Dose Estimation
• Much more complicated than effective dose assessment
‣Huge number of conversion factors required
• Without appropriate software very time-consuming
‣Which one to use?
‣How user-friendly?
‣How accurate?
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0
0.5
1.0
1.5
2.0
2.5
Organ / Range
CTD
I100
vs.
Org
an D
ose
CTDI100 vs. organ dose
CTDI vs. Organ Dose
0
0.5
1.0
1.5
2.0
2.5
Organ / Range
CTD
I100
vs.
Org
an D
ose
0
0.5
1.0
1.5
2.0
2.5
Liver(Liver)
Organ / Range
CTD
I100
vs.
Org
an D
ose
CT-Expo
0
0.5
1.0
1.5
2.0
2.5
Liver(Liver)
Thyroid(Neck)
Organ / Range
CTD
I100
vs.
Org
an D
ose
CT-Expo
0
0.5
1.0
1.5
2.0
2.5
Liver(Liver)
Thyroid(Neck)
Eye Lenses(Sinuses)
Organ / Range
CTD
I100
vs.
Org
an D
ose
CT-Expo
0
0.5
1.0
1.5
2.0
2.5
Liver(Liver)
Thyroid(Neck)
Eye Lenses(Sinuses)
Brain(Brain)
Organ / Range
CTD
I100
vs.
Org
an D
ose
CT-Expo
0
0.5
1.0
1.5
2.0
2.5
Liver(Liver)
Thyroid(Neck)
Eye Lenses(Sinuses)
Brain(Brain)
Liver(Entire Trunk)
Organ / Range
CTD
I100
vs.
Org
an D
ose
CT-Expo
0
0.5
1.0
1.5
2.0
2.5
Liver(Liver)
Thyroid(Neck)
Eye Lenses(Sinuses)
Brain(Brain)
Liver(Entire Trunk)
Organ / Range
CTD
I100
vs.
Org
an D
ose
CT-ExpoImPACT-PDC
0
0.5
1.0
1.5
2.0
2.5
CTD
I300
vs.
Org
an D
ose
CT-ExpoImPACT-PDC
Liver(Liver)
Thyroid(Neck)
Eye Lenses(Sinuses)
Brain(Brain)
Liver(Entire Trunk)
Organ / Range
CTDI300 vs. organ dose
Scanners:- Elscint CT Twin - Picker PQ2000- GE LightSpeed 16 - Siemens Sensation 16- Philips Brilliance 16 - Toshiba Aquilion 16
Conditions:- 120 kV- 10 mm beam width
➡ ADAM/EVA are slim -> differences for average patients even smaller➡ With CTDI300 significant over-estimation of organ doses.
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CTDI vs. Organ Dose (Head) Scanners:
- Elscint CT Twin - Picker PQ2000- GE LightSpeed 16 - Siemens Sensation 16- Philips Brilliance 16 - Toshiba Aquilion 16
Conditions:- 120 kV- 10 mm beam width
➡ CTDIvol16 ≈ organ dose for head scans independent of age group ➡ Overestimation of brain doses due to partial shielding by skull.
CTDI100 vs. organ dose
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CT-Expo - Strengths and Weaknesses
• Strengths
‣ For almost all scanners, regularly updated
‣ Excellent assessment of CTDIvol and DLP with corrections for voltage, colli-
mation, effective mAs, overbeaming, overranging, z-axis modulation (generic)
‣ 3 age groups (adult, child (7 y), baby (8 w)), both genders
‣All relevant dose descriptors
‣Quick assessment of organ and effective doses from CTDIvol and DLP
• Weaknesses
‣ 1 adult size only, 2 pediatric sizes only
‣Mathematical phantom, not voxel-based
‣Gantry tilt, angular and sectorial dose modulation not accounted for.
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CT Dosimetry Software (Mobile)
Organ and effective dose40 types of examination
Input: CTDIvol and DLP only
Organ/effective dose + SSDE13 types of examination
Input: CTDIvol, DLP, circumference
... plus many other apps for iOS and Android, often not recommendable.18
www.caldose.org
• Web-based MC calculation• Fast (< 60 s)• 9 different patient sizes• 2 genders, voxel-based
• First tests: much left to do‣ few fixed scan ranges only‣ limited number of scanners‣ larger discrepancies in CTDIvol
CT Dosimetry Software (Web-based)
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Limitations of Organ Dose Assessment
• Most software tools based on mathematical phantoms‣ Voxel phantom-based solutions often regarded as more realistic
• Most software solutions based on one patient size only‣ However, differences in organ doses for adults are within ±15% only
• Individual habitus not taken into account‣ Composition, organ size and location can vary substantially‣ Large uncertainties for organs at or close to the border of scan range
±15%
Heart
±15%
Lungs
±15%
Breasts
Organ dose assessment performed with CALDose_XCT for thorax CT examination
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What is Organ Dose Needed for?
• Comprehensive assessment of organ doses rarely required
‣Only in research studies for organ-based risk estimates
‣Often with poor knowledge of underlying scanners and protocol settings
• In daily practice for a few purposes only
‣Uterus dose of pregnant women for decision making (abortion of pregnancy)
‣Selected organs for decision making of usefulness of protective devices.
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How much Patient-Specificity ?
• Individual habitus can vary substantially‣ Composition, organ size and location
male/178cm50kg/ BMI 16
male/178cm75kg/BMI 24
male/178cm79kg/BMI 25
male/178cm105kg/BMI 33
• Possible solution‣MC calculation based on individual voxel patient‣ Requires whole body scan with individual segmentation of organs
• Enormous effort plus additional radiation exposure.22
How much Patient-Specificity ?
• Uterus dose‣Never an issue (i.e. < 20 mSv) if outside scan range‣Otherwise never above 100 mSv
• Usefulness of protective devices‣Organ doses for eye lenses, thyroid, breasts, ovaries and testes‣ Coarse estimates sufficient for decision making
• CT doses limited to stochastic risk in > 99.9%‣ Completely different from therapy planning‣ Risk estimates with uncertainty of factor 1.5 to 2 more than sufficient.
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Summary & Conclusions
• CTDIvol is a reasonable (±20%) estimate of organ dose‣ In case of average and obese adults and‣ For organs fully inside the scan range
• SSDE in full detail no big help‣ Pediatrics: CTDIvol16 ≈ 2 · CTDIvol32 does the same within ±20%‣Slim adults: 1.5 · CTDIvol32 does the same within ±20%
• Software tools useful for organs not fully inside the scan range‣ But subject to larger uncertainties due to patient-specific factors‣Not much improved when based on multiple voxel phantoms‣ Resulting doses should be regarded as informative only
• Individual dose assessment‣Not really needed for assessment of stochastic risks‣ Estimates with correction for 3 sizes (as above) by far sufficient.
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