2985 - A Retractable Exit-Window Shield to Mitigate Unintended Electron FLASH Exposure from Modified LINACs

Purpose/Objective(s): Ultra-high dose rate (UHDR) electron FLASH therapy delivers dose on millisecond timescales, rendering conventional emergency stop buttons ineffective for stopping unintended beam delivery. If an inter-vault physical shield is placed at the LINAC exit window, then unintended operator and subject dose during a FLASH misfire will be reduced to levels that preserve sufficient human reaction time for beam termination using conventional controls, improving safety for preclinical and clinical trial FLASH.

Materials/Methods: A LINAC was modified to achieve isocentric electron FLASH with beam energy of 5.2 MeV (estimated from PDDs). A fully retractable 5 cm PMMA shield was mounted directly below the exit window to intercept the beam during unintended delivery. Film dosimetry quantified surface and depth dose to solid water phantoms representing a subject (in-field) and nearby operators (adjacent to the field) with and without the shield during a simulated mis-fire scenario. The simulation used a 4.2-s beam-on time and 40 Gy/s via time-based console control.

Results: The unshielded FLASH misfire was estimated to deliver a lethal 168 Gy (i.e., 40 Gy/s x 4.2 s) to the subject at the depth of maximum dose. With the exit-window shield in place, subject dose was reduced to 0.25 ± 0.15 Gy at the surface and 0.3 ± 0.17 Gy at dmax, corresponding to an approximate 99.8% reduction in depth dose. Out-of-field surface dose to nearby operators was significantly reduced at all measured distances (Table).

Conclusion: The retractable exit window shield converted a potentially severe FLASH misfire into a low dose event, preserving several seconds of reaction time for emergency beam termination using existing controls. This proof-of-concept study demonstrates a practical, independent safety layer to support safer FLASH research and early clinical trial translation when conventional emergency stop mechanisms are insufficient.