Pulse width-dependent Monte Carlo source modeling for ultra-high dose rate electron beams.

Objective.The emergence of ultra-high dose rate (UHDR) electron beams has highlighted the need for accurate Monte Carlo (MC) source models after recent measurements revealed pulse width (PW)-dependent discrepancies in dose profiles. This work aims to develop and validate a MC source model for conventional and FLASH modes using GATE 10, including PW dependence.Approach.Percentage depth dose (PDD) curves and lateral dose profiles were measured in water using a flashDiamond detector for PWs of 0.5-5s. These data were used to optimize the parametric source model throughminimization and gamma analysis. Validation was performed using a mouse collimator and PMMA slabs. Relative dose distributions and cumulative dose-volume histograms were computed in a CT-based voxelised mouse, with and without bolus.Main results.The optimized FLASH source model features a dual-peak energy spectrum with a PW-dependent energy component (7.0-7.8 MeV for 0.5-5s) and low-energy component probability. In conventional mode, a two-component spectrum (1.51.0 and 6.81.5 MeV) was used. Simulations showed good agreement with measurements (gamma-index 2 mm/2% anddifferences for,and). Differences in the mouse lung volume receiving at least 95% of the maximum dose (V95) were 1% across FLASH PWs, increasing to 5% and 12% in CONV without and with bolus, respectively. Bolus increased V95in FLASH by 17%, indicating improved target coverage.Significance.A GATE 10 MC-based source model of the ElectronFLASH LINAC was developed for both UHDR (PW-dependent) and conventional modes. Despite PW-dependent energy variations, no significant dosimetric differences were observed between PWs in FLASH in the preclinical mouse model. The model provides a reliable tool for optimization of preclinical irradiation setups for FLASH biological studies in the absence of a treatment planning system.