Room: AAPM ePoster Library
Assessment of size-specific dose estimates (SSDE) for patients undergoing CT scans requires determining water equivalent diameter (Dw) of the region of interest (ROI). The Dw is determined as an average for Dw values assessed for each slice inside the ROI, which means calculating Dw for a large number of slices reaching to hundreds. This process, however, is time-consuming and requires advance software to accomplish the calculation. The aim of this study was to investigate the suitability of using a single slice at the center of ROI to assess Dw average.
A MATLAB code was developed to calculate Dw of each slice for trunks of 351 pediatric and adult phantoms created based on CT images of patients. The trunk was divided into six scan regions: chest, abdomen, pelvis, chest-abdomen, abdomen-pelvis, and the whole trunk. Dw of the central slice and Dw average were assessed for each scan region by the code, which allowed assessment of the differences.
The percentage differences between Dw average and the central slice were found to be comparable for pediatric and adult phantoms, where the differences were in the ranges of (-10.1 to 12.7)% and (-11.3 to 12.8)%, respectively, with the average being ±5.4% and ±6.0%. Lower differences were in the chest region, whereas the pelvic region produced the largest differences. Beside the scan region, the differences were affected by gender and patient size. Generally, the differences were larger for males and increased with patient size.
Although the use of Dw of the central slice gave a reasonable estimation for Dw average for some regions, it should not be applied without consideration for the significant differences found for other regions that reached to ±13%. Correction factors or using correlation between Dw at the center and the average may be considered as practical approaches.