Anthropomorphic phantom dosimetry and validation of 150–200 MeV very high-energy electron radiotherapy at CERN

Abstract Treatment-planning studies indicate potential dosimetric advantages of very high-energy electron (VHEE) radiotherapy for deep-seated tumours, but the dose models on which these studies rely have few experimental benchmarks above 100 MeV. Here, we report radiochromic film dosimetry of 150 and 200 MeV beams at CERN’s CLEAR facility and validate a Geant4-based GATE Monte Carlo (MC) framework. In air and water, simulations reproduced scattered-beam spot-size and percentage depth-dose trends at both energies; at 200 MeV, agreement between simulations and measurements was generally within ± 1 mm for spot size and 2–3% for percentage depth dose. Single- and seven-spot dose distributions were also measured at 200 MeV in a heterogeneous anthropomorphic head-and-neck phantom with water-equivalent and low-density foam inserts. Globally normalised 1%/1 mm gamma analysis of simulated and film-measured phantom dose distributions with a 20% low-dose threshold yielded pass rates of 98.7–99.9% for all four single-spot and both sagittal-plane seven-spot comparisons, and 81.9% and 97.0% for the two posterior-oblique seven-spot comparisons. During the phantom irradiations, estimated instantaneous per-spot dose rates exceeded 10⁸ Gy s⁻¹, providing physical dosimetry under ultra-high dose rate (UHDR) conditions relevant to future VHEE-FLASH studies. These data and the validated framework can inform VHEE treatment planning and accelerator design.