Characterization and Mitigation of Boundary Sampling Artifacts in Electron Phase Space Generation for Clinical FLASH-RT Deployment
Abstract
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.
Related
- Foreground Characterization and Mitigation in the Observations of the CD/EoR with the SKA
- When Error Mitigation Makes Things Worse: Budget-Aware Evaluation, Extrapolation Failure, and the Calibration Trust Boundary
- Parasitic Interference in Heterodyne Interferometers: Modeling, Characterization, and Mitigation
- Numerical modelling of the poto-poto by coupling of finite elements and boundary elements for acoustic characterization
- Characterization of YAl₃(BO₃)₄ nonlinear properties for second harmonic generation in the green and UV ranges
BibTeX
@misc{carballeira2026characterization,
title = {{Characterization and Mitigation of Boundary Sampling Artifacts in Electron Phase Space Generation for Clinical FLASH-RT Deployment}},
author = {Rafael Carballeira and Rongxiao Zhang and Kevin J. Willy and Hayley Cash and David J. Gladstone},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2605.02605},
eprint = {2605.02605},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2605.02605},
note = {CC BY-NC-ND 4.0},
}
RIS
TY - UNPB
TI - Characterization and Mitigation of Boundary Sampling Artifacts in Electron Phase Space Generation for Clinical FLASH-RT Deployment
AU - Rafael Carballeira
AU - Rongxiao Zhang
AU - Kevin J. Willy
AU - Hayley Cash
AU - David J. Gladstone
PY - 2026
DA - 2026/09/24
PB - arXiv
DO - 10.48550/arxiv.2605.02605
UR - https://arxiv.org/abs/2605.02605
AB - 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.
ER -