SU‐E‐T‐60: Development and Validation of a TOPAS Model of a Spot Scanning Proton Therapy Nozzle

Purpose: To create a Monte Carlo model of a commercial spot scanning proton therapy nozzle and validate pencil beam dose distributions simulatedusing the model. Methods: We used TOPAS (Tool for Particle Simulation), a Monte Carlo simulation tool based on the Geant4 Toolkit, in our simulations. In TOPAS, we implemented the geometry and material composition of the nozzle, based on information found in the literature (Gillin et al. 2010, Med. Phys. 37:154; Sawakuchi et al. 2010, Med. Phys. 37:4960). The following components were modeled: vacuum window, profile monitor (multi‐wire ionization chamber), helium chamber, main and sub dose monitors (parallel plate ionization chambers), spot position monitor (multi‐wire ionization chamber) and snout. We scored dose to medium using the standard scorers in TOPAS. No variance reduction techniques were used. Simulated dose distributions for several energies were compared to ionization chamber measurements. Source parameters, including size, angular spread and energy spread, were taken to be free parameters due tothe large uncertainties in the literature‐provided data. Results: For energies ranging from 72.5 to 206.3 MeV, simulated pencil beam ranges agreed within 2.2 mm with experimental data. Simulated full width at half‐maximum values for in‐air lateral profiles agreed within the experimental uncertainties (∼0.5 mm) for three validated energies (72.5, 173.7 and 221.8 MeV). Conclusions: A spot scanning proton therapy nozzle has been modeled using TOPAS. Further work is underway to improve the agreement between simulated and experimental data of ranges and lateral profiles through adjustments of the source parameters and physics list. Because TOPAS handles CT datasets, once validated, the model developed in this work may be used to perform retrospective studies of the impact of dose and linear energy transfer distributions in patient outcome. This model may also be used to investigate the response of radiation detectors exposed to scanning proton beams. Natural Sciences and Engineering Research Council of Canada and Ontario Graduate Scholarship Program, Ontario Ministry of Training, Colleges and Universities.