Kevin J. Willy | Searcharxiv

arXiv2026

Pulse-Width-Specific Phase Space Informed Universal Beam Modeling for UHDR electron LINAC in FLASH-RT

Commercial treatment planning systems for electron FLASH radiotherapy are unavailable, and the dosimetric precision required for ultra-high dose rate delivery makes Monte Carlo (MC) simulation the gold standard approach. This work establishes a methodology for generating pulse-width-specific phase space (PHSP) files for the Mobetron UHDR system (9 MeV), accounting for systematic beam quality shifts caused by RF waveguide loading across pulse widths of 1.2-4.0 microsecond. Using GAMOS 6.2.0, source parameters were iteratively refined against experimental targets: mean energy was optimized by matching phantom-measured R50 in the fall-off region, while energy spread was refined using surface dose and build-up gradients. Relationships derived from a mid-range 6 cm aperture were applied across all clinical configurations (2.5-10 cm) to test the aperture-independence of beam loading effects. Mean energy decreased exponentially from 9.58 to 9.04 MeV (R^2=0.99) with increasing pulse width, while energy spread increased quadratically (R^2=0.99), with a strong negative correlation (r=-0.98). Cross-aperture validation confirmed that energy shifts are independent of downstream collimation. The geometric mean pulse width (2.28 microsecond) was evaluated as a universal clinical reference, yielding 9.32 MeV mean energy. Across experimental extremes, R50 deviations were within 1.3 mm and critical depth-dose parameters remained within 2.0 mm, meeting AAPM TG-106 tolerances. Validated regression models enable beam parameter prediction at arbitrary pulse widths, and the universal reference reduces computational burden by 75% while maintaining clinical accuracy.

physics.med-ph↗

arXiv2026

Mitigation of Boundary Sampling Artifacts in Phase Space Generation for Electron FLASH Radiotherapy

Applicator-specific phase space (PHSP) files recorded at the aperture exit reduce Monte Carlo dose calculation time by 30-50% for electron FLASH radiotherapy. However, positioning PHSP scoring planes coincident with the applicator-air interface introduces boundary sampling artifacts. This study characterizes these artifacts in Geant4-based simulations and demonstrates their mitigation. PHSP files were generated using GAMOS 6.2.0 for a 9 MeV Mobetron UHDR model across twelve clinical aperture configurations (2.5-10 cm diameter). Three scoring plane positions were evaluated relative to the physical aperture exit: coincident with the interface, 0.1 mm downstream, and 1 mm downstream. Scoring at the exact interface produced proximal R50 shifts of up to 2.2 mm and Distance-to-Agreement (DTA) values of 4-6 mm, exceeding clinical acceptance criteria. Artifact severity scaled inversely with aperture diameter, with the smallest configurations most severely affected. A 0.1 mm offset partially restored the primary electron energy spectrum but failed to recover the bremsstrahlung tail. A 1 mm offset fully resolved all artifacts, achieving mean DTA values within 2.0 mm, equivalent to or better than linac-exit references. These artifacts arise from the degenerate behavior of the fUseSafety step-limitation algorithm when safety equals zero at exact material boundaries, producing incomplete secondary electron equilibration and suppressed bremsstrahlung production. Angular distribution analysis revealed a near-forward particle pileup in the 0 mm PHSP and a deficit of large-angle secondaries recovered by the 1 mm offset. A 1 mm downstream offset fully mitigates these artifacts while introducing negligible perturbation to primary beam characteristics. This requirement applies to any Geant4-based framework (including TOPAS and GATE) scoring PHSP files at material exit surfaces.

physics.med-ph↗