Investigation of American Association of Physicists in Medicine Report 204 size-specific dose estimates for pediatric CT implementation

Abstract

Purpose: To compare five methodologies the American Association of Physicists in Medicine Report 204 used to calculate size-specific dose estimates (SSDEs) for pediatric computed tomography (CT).

Materials and methods: The institutional review board waived consent for this HIPAA-compliant retrospective study. The five SSDE methodologies were investigated for calculation variation based on volumetric CT dose index (CTDI), or CTDI(vol), of chest, abdominal, and pelvic CT. SSDE calculations were derived from a predominantly pediatric population of 186 patients retrospectively and consecutively analyzed from June through November 2011. Eighty (43%) of the 186 patients were female, and 106 (57%) were male. Mean patient age was 8.6 years ± 6.3 (standard deviation), the age range was 1 month to 28 years, and mean weight was 37.7 kg ± 33.1, with a range of 3.4-146.6 kg. SSDE conversion factors were derived from anteroposterior (AP) and lateral dimensions measured on the patient's CT radiograph. The measurements were either used independently, or as a summation, or to calculate the patient's effective diameter; additionally, SSDE was derived on the basis of the patient's age (International Commission on Radiation Units Report 74 data). SSDE conversion factors were applied to CTDI(vol) data that corrected for both 16- and 32-cm-diameter CTDI phantom measurements. SSDE data were summarized by using the patient's originally prescribed weight-based CT scanning protocols. Data were summarized by using descriptive statistics.

Results: SSDEs derived from individual measurements varied 2%-12%. The combination of measurements (sum or effective diameter) varied 0.9%-2%. The age approach varied by an average of 2% (in the younger population [0-13 years]), but up to 44%, with an average of 18% (in the older population [14-18 years]). No SSDE correction was required for patients of varying size who weighed 36 kg or less when CTDI(vol) was measured by using a 16-cm CTDI phantom or for patients weighing 100-140 kg when CTDI(vol) was measured by using a 32-cm phantom. CTDI(vol) measured by using a 32-cm phantom in patients weighing between 36 and 100 kg and patients weighing more than 140 kg differed from SSDE by an average of 35%. An average difference of 1% was found between male and female SSDE-corrected values when the two sexes were compared within the same CT weight scanning categories.

Conclusion: The combination of AP and lateral measurements should be used to determine SSDE correction factors when possible. For pediatric patients, CTDI(vol) calculated with a 32-cm phantom requires SSDE conversion to more accurately estimate patient dose; CTDI(vol) calculated with a 16-cm phantom for pediatric patients weighing 36 kg or less does not require SSDE conversion.