The accuracy of estimated organ doses from Monte Carlo CT simulations using cylindrical regions of interest within organs

The purpose of this study was to investigate the accuracy of Monte Carlo simulated organ doses using cylindrical ROIs within the organs of patient models as an alternative method to full organ segmentations. Full segmentation and placement of circular ROIs at the approximate volumetric centroid of liver, kidneys and spleen were performed for 20 patient models. For liver and spleen, ROIs with 2cm diameter were placed on 5 consecutive slices; for the kidneys 1cm ROIs were used. Voxelized models were generated and both fixed and modulated tube current simulations were performed and organ doses for each method (full segmentation and ROIs) were recorded. For the fixed tube current simulations, doses simulated using circular ROIs differed from those simulated using full segmentations: for liver, these differences ranged from -5.6% to 10.8% with a Root Mean Square (RMS) difference of 5.9%. For spleen these differences ranged from -9.5% to 5.7% with an RMS of 5.17%; and for kidney the differences ranged from -12.9% to 14.4% for left kidney with an RMS of 6.8%, and from -12.3% to 12.8% for right kidney with an RMS of 6.6%. Full body segmentations need expertise and are time consuming. Instead using circular ROIs to approximate the full segmentation would simplify this task and make dose calculations for a larger set of models feasible. It was shown that dose calculations using ROIs are comparable to those using full segmentations. For the fixed current simulations the maximum RMS value was 6.8% and for the TCM it was 6.9%.