Room: Room 205
Purpose: Whole-body effective doses are calculated using average organ doses with tissue-specific weighting factors. This methodology may underestimate risk to partially irradiated organs that show considerable dose inhomogeneity. Previous work on organ dose heterogeneity has explored limited protocols and phantoms. Here, we investigate dose heterogeneity for chest and abdominopelvic protocols across 58 adult and 56 pediatric XCAT phantoms through a virtual clinical trial platform.
Methods: A set of 58 adult and 56 pediatric whole-body XCAT phantoms were used to simulate clinical scan protocols (GE LightSpeed VCT multislice CT scanner) using a GPU-accelerated Monte Carlo simulation toolkit (MCGPU). Each scan was executed using 107 photons per projection and 36 projections per rotation. Dose-volume histograms were generated for each organ and scan to calculate the heterogeneity ratio â€“ i.e., the ratio of the 95th percentile dose level to the mean dose. The effective diameter was also recorded for each phantom to evaluate trends in heterogeneity with patient size.
Results: Organs fully within the CT scan range exhibited a heterogeneity ratio of 2.19 or lower. Organs partially in the beam or outside the beam showed greater heterogeneity between 1.02 and 11.92. A direct correlation was observed between effective body diameter and dose heterogeneity for the following organs and protocols: Ribs and thoracic vertebrae, chest; liver and lumbar vertebrae, abdominopelvic; pelvic and sacral bones, pediatric abdominopelvic.
Conclusion: The 95th percentile dose can be over 11 times the mean organ dose for chest and abdominopelvic protocols in pediatric and adult phantoms. The work demonstrates that details may be hidden by average dose estimates and suggests the need to consider intra-organ dose heterogeneity in CT dose calculations, particularly in the case of sensitive tissue (e.g., bone marrow) and populations (e.g., pediatric).