Evaluation of a Conventionally Shielded Proton Treatment Room for FLASH Radiotherapy

Purpose FLASH radiotherapy (FLASH-RT) is the potential for a major breakthrough in cancer care, as preclinical results have shown significantly reduced toxicities to healthy tissues while maintaining excellent tumor control. However, FLASH conditions were not considered in the current proton facilities' shielding designs. The purpose of this study is to validate the adequacy of conventionally shielded proton rooms used for FLASH-RT. Methods Clinical FLASH irradiations typically take place in a few 100 ms, orders of magnitude shorter than the response time of the wide-energy neutron detector (WENDI-II). The nozzle beam current (representing the dose rate) dependence of the WENDI-II detector response was empirically determined to stabilize with a beam current of <= 10 nA at the measurement point with the highest dose rate. A large, predefined proton transmission FLASH plan (250 MeV, 7 x 20 cm(2), 8 Gy at isocenter) was commissioned as part of a FLASH clinical trial. For purpose of this study, that field was adjusted from 250 to 244 MeV, allowing a lower beam current of 10 nA to provide reliable detector response. Radiation surveys were performed for the proton beams with/without extra beam stopper (30 x 30 x 40-cm(3) solid water slabs) at 0 degrees, 90 degrees, 180 degrees, and 270 degrees gantry angles. Results Ambient doses were recorded at seven different locations. A 170-nA beam current, commonly used for clinical FLASH plans, was chosen to normalize the average ambient dose rate to FLASH conditions. Assuming 200-Gy/h workload (25 FLASH beams, 8 Gy/beam), annual occupational dose at controlled areas was calculated. For all gantry angles, <= 0.4 mSv/year is expected at treatment room door. The highest ambient dose, 2.46 mSv/year, similar to 5% of the maximum annual permissible occupational dose, was identified at the isocenter of the adjacent treatment room with 90 degrees gantry. Conclusion These survey results indicate that our conventionally shielded proton rotating gantry rooms result in acceptable occupational and public doses when the transmission FLASH beams delivered at four cardinal gantry angles based on 200-Gy/h workload assumption. These findings support that FLASH clinical trials in our conventionally shielded proton facilities can be safely implemented.