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

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-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 pulse width (PW) dependence.

Percentage depth dose (PDD) curves and lateral dose profiles were measured in water using a flashDiamond detector for PWs of 1 -5 µs. These data were used to optimized the parametric source model through χ 2 minimization and gamma analysis. Validation was performed using a mouse collimator and PMMA slabs. Relative dose distributions and cumulative dose-volume histograms (cDVHs) were computed in a CT-based voxelised mouse, with and without bolus.

The optimized FLASH source model features a dual-peak energy spectrum with a PW-dependent energy component (7.5-7.8 MeV for 1-5 µs) and low-energy component probability. In conventional mode, a two-component spectrum (1.5 ± 1.0 and 6.8 ± 1.5 MeV) was used. Simulations showed good agreement with measurements (gamma-index 2 mm/2% and < 5% differences for R 50 , R 90 and R p ). Differences in the mouse lung volume receiving at least 95% of the maximum dose (V95 lung ) were 1% across FLASH PWs, increasing to 5% and 12 % in CONV without and with bolus, respectively. Bolus increased V95 lung in FLASH by 17%, indicating improved target coverage.

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 (TPS).