Room: AAPM ePoster Library
Purpose: Electrons are generally assumed to contribute negligibly compared to photons when calculating linac vault shielding requirements. However, these assumptions quickly break down when operating at FLASH-RT dose rates. We investigate the implications of FLASH-RT dose rates on radiation safety and shielding following the conversion of a Varian C-series linac to deliver 16 MeV electrons at 200× clinical dose rates.
Methods: We investigated three issues pertaining to radiation safety: 1) instantaneous dose rates, 2) neutron production, and 3) dose from activated linac components in the vault itself. We performed a new radiation survey in uncontrolled and controlled areas for photon and neutrons. The radiation dose rate in the vault from activated components of the linac following a 2 minute operation using FLASH-RT dose rates was similarly measured at regular time intervals.
Results: The photon and neutron dose rates in controlled areas are approximately twice as high when operated in FLASH-RT mode compared to the 18 MV photons for which shielding was designed. However, in uncontrolled areas the dose rate reached 430 µSv/h from photons and 350 µSv/hr from neutrons, 70 times higher than the 18 MV dose rate. More importantly, the hotspot location was different from that for 18 MV, possibly outside the primary shielding area designed for photons, suggesting a different angular distribution of the bremsstrahlung photons. The activated linac head delivered a dose rate >10 µSv/hr in the vault itself immediately following operation, but this quickly fell below 1 µSv/hr after 10 minutes.
Conclusion: While our linac vault shielding proved sufficient, it is possible to exceed federal regulatory limits if FLASH is operated for relatively long periods of time (90s in our case). Other investigators would be prudent to confirm the adequacy of their radiation safety program, particularly if operating vaults only shielded for 6 MV photons.