Author: Alexandru Badalan, Peter Petric, I. Antoniu Popescu, Andrew Kyle Henderson Robertson 👨🔬
Affiliation: Bc Cancer, BC Cancer - Vancouver, BC Cancer 🌍
Purpose: Ultra-high dose rate (>40 Gy/s) or “FLASH” radiation therapy (RT) has the potential to improve cancer treatment through reduced radiation side effects. While electron FLASH beams can be produced on modified medical linacs, radiation surveys performed at our clinic during FLASH irradiations suggest that effects which are negligible for photon shielding may be non-negligible for electron FLASH-RT beams. This work investigates shielding considerations of using existing medical linac vaults for electron FLASH-RT beam production.
Methods: A 15 MeV electron FLASH beam was generated on a TrueBeam linac by running a 15 MV photon beam with the target and MU chamber retracted and an electron scattering foil in the beam path. The radiation shielding of the vault included steel primary shielding and concrete. Radiation surveys outside the vault were conducted using a calibrated Fluke 451P survey meter. These surveys were compared to Monte Carlo (MC) simulations using VirtualLinac and MCNP. This survey and MC simulation procedure was repeated with a conventional 15 MV photon beam for comparison, as was simulation of a concrete-only (steel-less) primary barrier.
Results: MCNP 15 MeV FLASH and conventional 15 MV photon simulations showed agreement within 50% or 7 µSv/hr of the survey results. The dose rate outside the vault was highest at points where the beam passed through the steel primary barrier. An MCNP simulation of a concrete primary barrier resulted in dose rates similar to a conventional 15 MV beam. This suggests that bremsstrahlung production in the steel primary barrier is a relevant shielding consideration for electron FLASH beams.
Conclusion: Primary barrier material choice is an important consideration for clinics looking to convert existing vaults to FLASH radiation therapy vaults. Further MC simulations will be conducted to determine what additional low-Z material could potentially mitigate such effects for a vault with a high-Z barrier.