| The initial step in this work was to accurately build the Mevion system in its entirety in the simulation space. Each of the 24 options, including 14 range modulator wheels, 3 second scattereres, 8 first scattering foils, 24 range absorbers and 24 post absorbers, were modeled according to manufacturer provided blueprints. To ensure that the Mevion simulation could accurately characterize the radiation field exiting the beam nozzle, a benchmark study was conducted comparing simulated data to real beam data taken during commissioning. Pristine Bragg peaks, spread out Bragg peaks and lateral profiles were simulated for the beam options and compared to the analogous commissioning data. There were excellent agreements between the two data sets, where the distal depth of 90% all matched within 1 mm, the distal 80%-20% matched within 2 mm for each beam option, spread out Bragg Peak widths all matched within 3 mm, and the flatness and symmetry of the lateral profiles agreed within 1% of the beam commissioning data.;Using the benchmarked simulation model of the Mevion system, an in-room neutron study was conducted. The presence of the cyclotron in the treatment vault creates a potential for elevated neutron contamination dose to the patient during treatment. This potential was investigated by irradiating a water phantom in simulation using the deepest, largest and smallest beam option, and scoring neutron energy fluences produced at distances of 20, 40, 60, 80, 100 and 150 cm from isocenter, and at angles of 0, 45, 90 and 135 degrees relative to the beam path around the water phantom. The influence of field size on neutron production was also studied, where at the same locations neutron dose equivalents were scored for field sizes ranging from 0 x 0 to 26.5 x 26.5 cm2. The acquired neutron energy fluences were combined with ICRP report number 74 fluence to ambient dose equivalent conversion factors to calculate the final neutron dose equivalent per Gy of proton delivered to isocenter. The calculated neutron dose equivalents were also compared to measured neutron dose equivalents using the SWENDI-II neutron detector.;Calculated neutron dose equivalents ranged from 8 mSv/Gy to less than 1 mSv/Gy. Measured neutron doses were lower, in some cases by an order of magnitude. The data shows the neutron dose equivalent generally decreases as the distance from isocenter increases. The neutron dose equivalent additionally increases along with an increased angle around the water phantom. The field size also influences neutron production, as smaller field sizes tend to increase the amount of contamination neutron dose.;One of the more prominent roles Monte Carlo simulations play in radiation therapy is in patient specific dosimetry. In this study, a clear workflow was developed to simulate a patient treatment and compare the 3D dose distributions to that calculated in the treatment planning system. Beginning from loading the specific patient beam parameters and patient CT data set into the simulation space, to a final visualization and comparison of the 2D doses calculated from simulation and the treatment planning system, this work successfully demonstrates the ability to conduct patient specific applications.;Using this workflow, a study of the effects of patient anatomy and range compensator on the dose per monitor unit calibration was conducted. The patient scatter factor (PSF), defined as the ratio of dose in patient to that in a homogeneous phantom, and compensator scatter factor (CSF), defined as the ratio of dose in phantom both with and without the compensator in place, are two terms largely ignored in monitor unit calibrations due to the difficulty of physical measurement. PSFs and CSFs were calculated for 8 brain, 8 lung, 11 pancreas and 11 prostate fields using the pencil beam algorithm treatment planning system. PSFs and CSFs from a sample of 3 fields from each site were also simulated using the developed workflow and compared to their treatment planning system counterpart. For the prostate and lung cases, the tissue type in the treatment field was quantitatively analyzed based on their HU values within the 50% isodose contour for each field and then plotted against the corresponding PSF value. The interquartile range (IQR) of compensator thickness values within a small region of interest along the central beam axis was calculated to evaluate the impact of compensator shape on the CSF. (Abstract shortened by ProQuest.). |
