SU‐E‐T‐296: Dose Delivered to Gantry‐Mounted Electronics in Proton Therapy

Abstract

Purpose: To evaluate the dose equivalent from scattered and secondary radiation fields present in and around a passive proton treatment nozzle, which may pose a threat to electronic equipment.Methods: Landauer Luxel dosimeters were used to evaluate the radiation field around one of the proton passive scattering nozzles at Loma Linda University proton treatment center. These detectors use optically stimulated luminescence technology in conjunction with CR‐39 to measure doses from X, gamma, proton, beta, fast and thermal neutron radiation. The dosimeters were stationed at various positions around the gantry pit and on racks on the gantry itself to evaluate the dose to electronics. Wax shielding was also employed on some detectors to evaluate the usefulness of this material as a dose moderator. To create the scattered and secondary radiation field in the gantry enclosure, a polystyrene phantom placed at isocenter and irradiated with 250 MeV protons to a dose of 1.3 kGy over 16 hours.Results: The measured overall dose equivalent ranged from 100–6000 mrem (0.08 – 4.62 mrem/Gy primary radiation), with proton/photon, thermal neutron, and fast neutron radiation individually evaluated. The distance of the detector/electronics from isocenter and neutron producing devices, such as the collimators and first and second scatters, had a bearing on the dose received. Interestingly, the addition of 1 inch thick wax shielding decreased the fast neutron omponent by almost 50% and the overall dose equivalent by 30–40%, yet it increased the thermal neutron dose by 100% and the photon/proton dose by 50%, as there as no B‐10 component to capture thermal neutrons Conclusions: The data obtained in this study will benefit future upgrades and facility designs by identifying mounting positions for electronics that minimize radiation dose.