SU-D-103-03: Converting Size Specific Dose Estimate (SSDE) to Patient Specific Organ Dosimetry: Application to a Pediatric Population

Abstract

Purpose: Develop a methodology to convert size specific dose estimate (SSDE) to pediatric patient specific organ dosimetry. Methods: Each thoracic and abdominal-pelvic organ within a family of four anthropomorphic phantoms (ATOM, CIRS) developed to emulate a wide range of pediatric patient body habitus was measured for absolute dose using MOSFET dosimeters; e.g., a series of 884 measurements were made in 21 different organs spanning the thorax of an anthropomorphic phantom developed to emulate a 10 yr patient. A Scout scan was performed for each phantom wherein the phantoms thoracic and abdominal-pelvic effective diameters were calculated pursuant to the AAPM Report 204 methodology. The phantoms effective diameter was matched with a pediatric patient population average effective diameter, and imaged according to that patient populations weight based CT scan parameters; thus, normalizing CTDIvol between phantom and representative patient populations. A correction factor was developed for each phantoms organ dose as it related to the phantoms calculated SSDE (both thoracic and abdominal-pelvic). The SSDE-to-Absolute dose correction factors were then applied to an IRB approved patient cohort of 492 patients (7.6 ± 5.4 yrs; range 2 mo to 28 yrs and mean weight was 35 ± 33 kg; range 4–147 kg) to convert patient specific SSDE to organ dose. Results: Organ dose linearly correlated with patient effective diameter with an average R2 of 0.96. In general, chest organ doses were greater than the chest SSDE by 10–40%, the abdomen organ doses were greater than the abdomen SSDE by 10–20%, but organ doses in the pelvis were lower by 0–20%. Conclusion: Correction factors were established to convert SSDE to absolute organ dose. Organ dose values linearly correlated with patient size. SSDE, when calculated based on Report 204, generally underestimates actual patient chest and abdomen organ dosimetry from 10–40%, but overestimates pelvic organ dose by 0–20%.