Purpose: State of art scanning proton beam accelerators can deliver transmission FLASH -RT with ultra-high dose rate ≥40Gy/s. However, transmission FLASH conditions were not considered in 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 take place in a few hundred milliseconds, orders of magnitude shorter than the response time of the wide energy neutron detector (WENDI-II). The nozzle beam current (dose rate) dependence of 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, 7×20 cm, 8Gy at isocenter) was commissioned as part of a FLASH clinical trial. For purpose of this study, that field was adjusted from 250 MeV 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 beam stopper at 0°, 90°, 180° and 270° gantry angles.
Results: Ambient doses were recorded at seven different locations. 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/hr workload (25 FLASH beams, 8Gy/beam), annual occupational dose at controlled areas was calculated. For all gantry angles, ≤ 40 mRem/yr is expected at treatment room door. The highest ambient dose, 246 mRem/yr, was identified at isocenter of the adjacent treatment room with 90° gantry.
Conclusion: These survey results indicate that the conventionally shielded proton rooms result in acceptable occupational and public doses when the transmission FLASH beams delivered at four cardinal gantry angles. These findings support that FLASH clinical trials in conventionally shielded proton facilities can be safely implemented.
Not Applicable / None Entered.