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

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiation Safety

Background:

• Traditional shielding calculations for megavoltage linear accelerator vaults often neglect electrons due to their finite range.
• Operating at ultra-high dose rates, such as those used in FLASH radiation therapy (FLASH-RT), significantly increases dose rates from bremsstrahlung photons, potentially exceeding design limits.
• This necessitates a re-evaluation of shielding requirements and radiation protection measures.

Purpose of the Study:

• To investigate the shielding and radiation protection impact of converting a Varian 21EX linear accelerator to FLASH-RT dose rates.
• To assess the dose rates in controlled and uncontrolled areas resulting from 16 MeV electron beams at FLASH-RT conditions.
• To evaluate the contribution of neutron activation of linac components to the overall radiation dose.

Main Methods:

• Radiation surveys were conducted in occupied areas using a Fluke Biomedical survey meter and a Wide Energy Neutron Detection Instrument (Wendi)-2.
• Dose rates from activated linac components were measured after a 1.8-minute FLASH-RT delivery.
• Calculations considered dose rates from bremsstrahlung photons and neutron activation.

Main Results:

• At a gantry angle of 180°, 16 MeV FLASH-RT electrons delivered approximately 10 µSv/h in controlled areas and 780 µSv/h in uncontrolled areas, exceeding USNRC limits for continuous exposure.
• Exceeding the 20 µSv limit required a continuous operation of 92 seconds, corresponding to a high workload (3180 Gy) that is experimentally unfeasible for typical treatments.
• Neutron activation of linac components reached 25 µSv/h near the isocenter but dissipated quickly, with total doses within an hour remaining below 20 µSv.

Conclusions:

• Bremsstrahlung photons from 16 MeV FLASH-RT electron beams create significant dose rates in controlled and uncontrolled areas, as well as from activated linac components.
• The existing linac vault shielding was found to be sufficient in this study.
• Investigators planning to operate at FLASH-RT dose rates, especially in vaults designed for lower energies (e.g., 6 MV), should confirm the adequacy of their radiation safety programs.