Noninvasive Reversible Software-Based Electron FLASH Irradiation Configuration of a Linear Accelerator in Clinical...

Configuring clinical linear accelerators (linacs) for ultra-high-dose-rate (UHDR) electron experiments typically requires invasive hardware manipulation and/or irreversible modifications, limiting broader implementation. This work reports a reversible and noninvasive UHDR electron configuration of a clinical TrueBeam linac that enables switching between preclinical UHDR and conventional (CONV) clinical treatment modes through software settings, without accessing the linac interior.

Built-in service mode software was used to configure the UHDR mode with settings typical of a standard megavoltage photon beam. Using service mode, the photon target and monitor chambers were retracted. An unused low-energy electron scattering foil was loaded. An external AC current transformer for beam control and monitoring was mounted on the accessory tray, with an ionization chamber placed downstream in solid water to monitor exit dose. Dose profiles were measured for UHDR and CONV beams with radiochromic films for open field, in vivo, and in vitro setups. Dose per pulse was varied by adjusting the gun voltage and quantified. Day-to-day output variation was assessed to evaluate dose reproducibility.

Percent depth-dose measurements confirmed similar energy between UHDR (9.2 and 12.6 MeV) and CONV electron beams (8.4 and 11.7 MeV), with matching profiles throughout the typical thickness of a mouse or cell culture media. Maximum dose per pulse reached 1.5 Gy/pulse and 0.7 Gy/pulse for in vivo and in vitro setups at 64 and 82 cm source-to-surface distances, respectively. Field flatness and symmetry were maintained between UHDR and CONV, supporting organ-specific in vivo irradiation and a maximum of 5 × 5-cm² field for in vitro irradiation. Day-to-day output variation remained small, with both inter- and intra-animal coefficients of variation averaging <3% for FLASH and <1% for CONV.

Accurate and reproducible UHDR electron delivery was demonstrated without invasive hardware manipulation, enabling preclinical FLASH research on a clinical linac.