Pulse-width-specific phase space modeling and reference beam characterization for UHDR electron LINAC in FLASH-RT

Objective: FLASH radiotherapy requires ultra-high dose rates (>40 Gy/s), yet commercial treatment planning systems are currently unavailable. Monte Carlo (MC) simulation offers high transport accuracy, but full-head modeling for every setup is computationally prohibitive. This work establishes a methodology for generating pulse-width-specific phase-space files for the Mobetron UHDR (9 MeV), accounting for systematic beam quality shifts caused by RF waveguide loading across pulse widths of 1.2-4.0 μs. Approach: Using GAMOS 6.2.0 MC simulations, voxelized dose distributions were post-processed via an iterative script to refine source-level parameters against experimental targets. Mean energy was optimized by matching phantom-measured R50, while energy spread was refined using surface dose and build-up gradients. Relationships derived from a mid-range 6 cm applicator were applied across clinical apertures (2.5-10 cm) to test aperture-independence. The geometric mean of experimental pulse widths (2.28 μs) was evaluated as a single reference condition. Main Results: Phase space files were generated across pulse widths and apertures. For the A6I6 reference, mean energy decreased monotonically from 9.58±0.24 to 9.04±0.24 MeV with pulse width, best described by a logarithmic model (LOO-RMSE=0.033 MeV). Energy spread increased linearly from 0.173±0.020 to 0.287±0.020 MeV (LOO-RMSE=0.005 MeV). Cross-aperture validation confirmed these shifts occur upstream of collimation. Evaluating the 2.28 μs reference predicted 9.30 MeV mean energy and 0.215 MeV energy spread, yielding depth-dose deviations ≤1.3 mm across experimental extremes and satisfying AAPM TG-106 tolerances (≤2.0 mm). Significance: This work provides pulse-width-specific phase-space characterization for Mobetron FLASH commissioning. Cross-validated regression models enable beam parameter prediction within 1.2-4.0 μs, and the 2.28 μs reference phase space streamlines routine planning while maintaining clinical accuracy.