Radiation shielding and safety implications following linac conversion to an electron FLASH-RT unit

Purpose: Due to their finite range, electrons are typically ignored when calculating shielding requirements in megavoltage energy linear accelerator vaults. However, the assumption that 16 MeV electrons need not be considered does not hold when operated at FLASH-RT dose rates (similar to 200x clinical dose rate), where dose rate from bremsstrahlung photons is an order of magnitude higher than that from an 18 MV beam for which shielding was designed. We investigate the shielding and radiation protection impact of converting a Varian 21EX linac to FLASH-RT dose rates. Methods: We performed a radiation survey in all occupied areas using a Fluke Biomedical Inovision 451P survey meter and a Wide Energy Neutron Detection Instrument (Wendi)-2 FHT 762 neutron detector. The dose rate from activated linac components following a 1.8-min FLASH-RT delivery was also measured. Results: When operated at a gantry angle of 180 degrees such as during biology experiments, the 16 MeV FLASH-RT electrons deliver similar to 10 mu Sv/h in the controlled areas and 780 mu Sv/h in the uncontrolled areas, which is above the 20 mu Sv in any 1-h USNRC limit. However, to exceed 20 mu Sv, the unit must be operated continuously for 92 s, which corresponds in this bunker and FLASH-RT beam to a 3180 Gy workload at isocenter, which would be unfeasible to deliver within that timeframe due to experimental logistics. While beam steering and dosimetry activities can require workloads of that magnitude, during these activities, the gantry is positioned at 0 degrees and the dose rate in the uncontrolled area becomes undetectable. Likewise, neutron activation of linac components can reach 25 mu Sv/h near the isocenter following FLASH-RT delivery, but dissipates within minutes, and total doses within an hour are below 20 mu Sv. Conclusion: Bremsstrahlung photons created by a 16 MeV FLASH-RT electron beam resulted in consequential dose rates in controlled and uncontrolled areas, and from activated linac components in the vault. While our linac vault shielding proved sufficient, other investigators would be prudent to confirm the adequacy of their radiation safety program, particularly if operating in vaults designed for 6 MV.