As proton therapy becomes increasingly popular, so does the need for Monte Carlo simulation studies involving accurate beam line modeling of proton treatment units. In this study, the 24 beam configurations of the Mevion S250 proton therapy system installed recently at our institution were modeled using the TOolkit for PArticle Simulation (TOPAS) code. Pristine Bragg peak, spread out Bragg peak (SOBP), and lateral beam profile dose distributions were simulated and matched to the measurements taken during commissioning of the unit. Differences in the range for all Percent Depth Dose (PDD) curves between measured and simulated data agreed to within 0.1 cm. For SOBP scans, the SOBP widths all agreed to within 0.3 cm. With regards to lateral beam profile comparisons between the measured and simulated data, the penumbras differed by less than 1 mm and the flatness differed by less than 1% in nearly all cases. This study shows that Monte Carlo simulation studies involving the Mevion S250 proton therapy unit can be a viable tool in commissioning and verification of the proton treatment planning system.

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For passive scattering proton therapy systems, neutron contamination is the main concern both from an occupational and patient safety perspective. The Mevion S250 compact proton therapy system is the first of its kind, offering an in‐room cyclotron design which prompts more concern for shielding assessment. The purpose of this study was to accomplish an in‐depth evaluation of both the shielding design and in‐room neutron production at our facility using both Monte Carlo simulation and measurement. We found that the shielding in place at our facility is adequate, with simulated annual neutron ambient dose equivalents at 30 cm outside wall/door perimeter ranging from background to 0.07 mSv and measured dose equivalents ranging from background to 0.06 mSv. The in‐room measurements reveal that the H*/D decreases when the distance from isocenter and field size increases. Furthermore, the H*/D generally increases when the angle around isocenter increases. Our results from in‐room measurements show consistent trends with our Monte Carlo model of the Mevion system.

Purpose: To calculate in-and out-of-field neutron spectra and dose equivalent, using Monte Carlo (MC) simulation, for a Mevion gantry-mounted passively scattered proton system in craniospinal irradiation. An analytical model based on the MC calculations that estimates in-and out-of-field neutron dose equivalent from proton Craniospinal irradiation (CSI) was also developed. Methods: The MCNPX MC code was used to simulate a Mevion S250 proton therapy system. The simulated proton depth doses and profiles for pristine and spread-out Bragg peaks were benchmarked against the measured data. Previous measurements using extended-range Bonner spheres were used to verify the calculated neutron spectra and dose equivalent. Using the benchmarked results as a reference condition, a correction-based analytical model was reconstructed by fitting the data to derive model parameters at 95% confidence interval. Sensitivity analysis of brass aperture opening, thickness of the Lucite (PMMA) range compensator, and modulation width was performed to obtain correction parameters for nonreference conditions. Results: For the neutron dose equivalent per therapeutic proton dose, the MCNPX calculated dose equivalent matched the measured values to within 8%. The benchmarked neutron dose equivalent at the isocenter was 41.2 and 20.8 mSv/Gy, for cranial and spinal fields, respectively. For in-and outof-field neutron dose calculations, the correction-based analytical model showed up to 17% discrepancy compared to the MC calculations. The correction factors may provide a conservative estimation of neutron dose, especially for depth ≤ 5 cm and regions underneath the brass aperture. Conclusion: The proposed analytical model can be used to estimate the contribution of the neutron dose to the overall CSI treatment dose. Moreover, the model can be employed to estimate the neutron dose to the implantable cardiac electronic devices.

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