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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Radiology 2011; 259:311–316

CTDI & Patient Dose

?

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CT-Expo v2.4

CTDI vs. Organ DoseExample: Liver dose in upper abdomen scan

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GSF Phantoms ADAM and EVA

ADAM40 cm x 20 cmdeff = 28.3 cm

EVA37.6 cm x 18.8 cmdeff = 26.6 cm

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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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SSDE

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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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CT Dosimetry Software

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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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Radiology 2011; 259:311–316

Objection, Your Honor !

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