Monte Carlo and film dosimetry study of collimator effects on penumbra and out-of-field dose for very high-energy electrons

Journal article

Objective. Very high-energy electrons (VHEEs) offer deep penetration, low scattering, and the potential for ultra-high dose rate delivery, making them promising candidates for future radiotherapy. However, the collimation of VHEE beams to achieve sharp beam penumbra remains poorly characterized. This study experimentally and computationally investigates how collimator material, thickness, and beam characteristics affect penumbra and out-of-field dose for VHEEs and establishes an initial foundation for the design of clinically feasible VHEE collimators. Approach. Tungsten, lead, and brass 5 mm diameter collimators were evaluated using film dosimetry with a 200 MeV electron beam delivered at the CERN Linear Electron Accelerator for Research and validated through Monte Carlo (MC) simulations. Experimental measurements of penumbra and out-of-field dose were compared with simulations that systematically varied material (tungsten, lead, brass), thickness (20–80 mm), and beam energy (150–250 MeV). Additional sensitivity tests quantified the impact of beam instability on field shaping. Main results. For measurements in air, penumbrae increased linearly with distance from the collimator and was smallest for tungsten. Out-of-field dose decreased with increasing thickness, falling below 0.5% for a 40 mm thick tungsten collimator. Brass exhibited the highest out-of-field dose (up to 4.8%) and broadest penumbra. MC models reproduced experimental trends within 5% for penumbrae but underestimated out-of-field dose, particularly for brass. The simulations indicated that VHEE beam divergence, beam size and collimator misalignment strongly influence beam penumbra and out-of-field dose. Significance. The presented work demonstrates that collimator material and geometry play a critical role in defining VHEE beam quality. Tungsten provided optimal attenuation and sharpness compared to brass and lead. These results establish quantitative benchmarks for VHEE collimator design and emphasize the importance of beam stability.

Authors: Fischer, J; Gilardi, A; Malyzhenkov, A; Korysko, P; Hart, A

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: IOP Publishing
• Journal: Physics in Medicine & Biology
• Volume: 71
• Issue: 8
• Pages: 085015
• Article number: 085015
• Publication date: 2026-04-24
• Acceptance date: 2026-04-14
• DOI: 10.1088/1361-6560/ae6018
• EISSN: 1361-6560
• ISSN: 0031-9155
• Language: English
• Keywords: film dosimetry; Monte Carlo simulations; collimation; radiotherapy; very-high energy electrons