Treatment planning comparison of focused very high energy electron and volumetric modulated arc therapy

Very high energy electron (VHEE) radiotherapy gained interest owing to technical advances and its potential for FLASH radiotherapy. Magnetically focused VHEE (fVHEE) beams showed promises in preliminary investigations in water phantoms. However, inverse treatment planning for fVHEE remains unexplored in clinically motivated scenarios. This study presented first inverse-optimized fVHEE treatment planning, permitting comparison to the current clinical benchmark, volumetric modulated arc therapy (VMAT), independent of FLASH considerations.

Seven cases across five sites (brain, head and neck, lung, prostate, and femoral head) were investigated. fVHEE plans were generated using Monte Carlo-based beamlet dose calculations and in-house inverse optimization. Plans employed 250 MeV electrons focused via idealized magnetic lenses. Plan quality was compared regarding target coverage and organ-at-risk (OAR) sparing.

fVHEE achieved equivalent or improved target coverage in four sites, with V_95% increasing by up to 4.5% in lung cases. Notable OAR sparing occurred, including D_2% reductions of 0.2-7.6 Gy for the spinal canal and esophagus versus VMAT. The femoral head case's D_2% to the rectum/bladder decreased by 1.3 Gy/2.6 Gy. Compared to VMAT, fVHEE enabled dose deposition via limited beam angles, reducing OAR dose at selected angles and to contralateral structures in lateralized tumors. Prostate cases showed less benefit due to target-OAR proximity in multiple directions, limiting directional selectivity.

First results on inverse-optimized fVHEE planning are encouraging, particularly for lateralized targets or directionally isolated OARs. fVHEE demonstrates potential for selective dose sculpting, further comparison to VHEE is required to isolate fVHEE-specific benefits.