Monte Carlo Simulation of Effective Dose in Fluoroscopy and Computed Tomography Procedures

The overarching goal of this project was to investigate organ dose assessment and variability using Monte Carlo methods to study two areas of medical imaging – fluoroscopy and computed tomography. Namely, these studies were intended to (1) provide estimates of the dose incurred by fluoroscopy-guided spinal injection procedures, and (2) investigate dose heterogeneity in chest and abdominopelvic computed tomography (CT) scans for a range of patient sizes. Fluoroscopy dose estimates were calculated using GEANT4, by recreating the patient procedures of six lumbar-sacral epidural injections. Computed tomography dose was estimated with a GPU-accelerated Monte Carlo package, MCGPU. Both simulations used a library of digital human (XCAT) phantoms, which were previously derived from real-patient CT scans. The fluoroscopy simulations suggest that smaller patients have a higher effective dose per dose area product, and the overall results agreed with previous experimental measurements. Variation of absorbed dose within a given organ was calculated for chest and abdominopelvic CT protocols. It was found that the 95th percentile dose can be over 11 times the mean organ dose in pediatric and adult phantoms. Furthermore, if the organ dose is calculated using only voxels within the beam or all the voxels within an organ, the result can change the result by a factor of 8. The change in dose was found to be higher for organs that have smaller fractions within the beam. Several models of tissue-weighted dose were also investigated, following similar methods to those used for effective dose. It was found that these tissue-weighted dose calculations can vary by up to 13% depending on whether the out of field dose is included. We also found that the results were not significantly affected by the pitch or projections per rotation. The results have shown that dose-volume details may be hidden by average dose estimates and suggested the need to consider intra-organ dose heterogeneity in CT dose calculations, particularly in the case of sensitive tissues (e.g., bone marrow) and populations (e.g., pediatric).