Patient / Organ Dose in CT Patient specific and organ dose estimation H.D. Nagel Dr. HD Nagel, Science & Technology for Radiology Buchholz / Germany www.sascrad.com 1
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. 2
CTDI & Patient Dose? Radiology 2011; 259:311 316 3
CTDI vs. Organ Dose Example: Liver dose in upper abdomen scan CT-Expo v2.4 4
GSF Phantoms ADAM and EVA ADAM 40 cm x 20 cm d eff = 28.3 cm EVA 37.6 cm x 18.8 cm d eff = 26.6 cm 5
Standard CT Dosimetry Phantoms Body phantom 32 cm PMMA CT# = 126 HU WED = 36.0 cm Head phantom 16 cm PMMA CT# = 126 HU WED = 18.0 cm WED = water equivalent diameter 6
CTDI vs. Organ Dose 32cm CTDI 36*/28.3** phantom ADAM 40x20=28.3cm = 1.27 = (eff.) 36 cm WED * water equivalent of 32 cm PMMA ** effective diameter of ADAM phantom Differences (ca. 30%) mainly due to male phantom diameter < 36 cm 7
SSDE 8
SSDE SSDE = CTDI vol corr. factor (d eff ) d eff = d lat d front 16cm PMMA corr. factor = 36cm / deff ADAM ADC reference diameter 32cm PMMA from: AAPM report 204 table 10 9
Reference Diameters in CT ADC from: Somatom Definition user manual 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 CTDI vol under-estimates SSDE for average adults by 10% only. 10
Effective Diameters for Children CF=1.5 16cm PMMA CF=1.9 CTDIvol16 ± 20% CF=2.3 from: AAPM report 204 fig. 10 Pädiatric range: CF = 1.9 ± 20% => SSDE = CTDI vol16 ± 20% 11
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 d eff by 10 to 30% Not necessary for average and obese adults (SSDE CTDI vol ) For pediatrics CTDI vol16 2 CTDI vol32 would do as well Meaningful for slim adults only (or SSDE 1.5 CTDI vol ). 12
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? 13
CT Dosimetry Software 14
CTDI 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 2.5 2.5 CTDI100 vs. Organ Dose 2.0 1.5 1.0 0.5 CT-Expo ImPACT-PDC CTDI300 vs. Organ Dose 2.0 1.5 1.0 0.5 CT-Expo ImPACT-PDC 0 Liver (Liver) Thyroid Eye Lenses Brain (Neck) (Sinuses) (Brain) Organ / Range Liver (Entire Trunk) 0 Liver (Liver) Thyroid (Neck) Eye Lenses (Sinuses) Organ / Range Brain (Brain) Liver (Entire Trunk) CTDI 100 vs. organ dose CTDI 300 vs. organ dose ADAM/EVA are slim -> differences for average patients even smaller With CTDI 300 significant over-estimation of organ doses. 15
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 CTDI 100 vs. organ dose CTDIvol16 organ dose for head scans independent of age group Overestimation of brain doses due to partial shielding by skull. 16
CT-Expo - Strengths and Weaknesses Strengths For almost all scanners, regularly updated Excellent assessment of CTDI vol and DLP with corrections for voltage, collimation, 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 CTDI vol 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. 17
CT Dosimetry Software (Mobile) Organ/effective dose + SSDE 13 types of examination Input: CTDI vol, DLP, circumference Organ and effective dose 40 types of examination Input: CTDI vol and DLP only... plus many other apps for ios and Android, often not recommendable. 18
CT Dosimetry Software (Web-based) Web-based MC calculation Fast (< 60 s) 9 different patient sizes 2 genders, voxel-based www.caldose.org First tests: much left to do few fixed scan ranges only limited number of scanners larger discrepancies in CTDI vol 19
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 Organ dose assessment performed with CALDose_XCT for thorax CT examination ±15% ±15% ±15% Heart Lungs Breasts 20
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. 21
How much Patient-Specificity? Individual habitus can vary substantially Composition, organ size and location male/178cm 50kg/ BMI 16 male/178cm 75kg/BMI 24 male/178cm 79kg/BMI 25 male/178cm 105kg/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. 23
Summary & Conclusions CTDI vol 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: CTDI vol16 2 CTDI vol32 does the same within ±20% Slim adults: 1.5 CTDI vol32 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. 24
Objection, Your Honor! Radiology 2011; 259:311 316 25