Shielding verification and neutron dose evaluation of the Mevion S250 proton therapy unit

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.