Comparative treatment planning of very high-energy electrons and photon volumetric modulated arc therapy: Optimising energy and beam parameters

Background: Very High-Energy Electron (VHEE) beams offer potential advantages over current clinical radiotherapy modalities due to their precise dose targeting and minimal peripheral dose spread, which is ideal for treating deep-seated tumours. To aid the development of clinical VHEE machines, this study adressed the need to identify optimum VHEE beam characteristics for tumours across various anatomical sites.<br /> Materials and methods: VHEE treatment planning employed matRad, an open-source treatment planning system, by adapting its proton pencil beam scanning implementation. VHEE beam characteristics were generated using TOPAS Monte Carlo simulations. A total of 820 plans were retrospectively created and analysed across 10 pelvic and 12 thoracic cases and compared against clinical photon VMAT plans to identify the most optimal VHEE beam configuration and energy requirement.<br /> Results: VHEE plans outperformed photon VMAT in sparing organs-at-risk (OARs) while maintaining or improving target coverage. While 150 MeV served as the threshold for effectively treating deep-seated sites, 200 MeV was identified as a more optimal energy in the pelvis for achieving the best balance of penetration and sparing abutting OARs. Lower energies (70-110 MeV) also benefitted mid-to-superficial disease in the lung cohort. Typically, VHEE plans required 3-5 fields, and resulted in notable dose reductions to OARs across treatment sites, including: 22.5% reduction in rectal Dmean; 13.8% decrease in bladder Dmean; 8.2% reduction in heart Dmean; and a 24.4% decrease in lung V20Gy.<br /> Conclusion: The study reinforces VHEE's potential in clinical settings, emphasising the need for varied energy ranges to enhance treatment flexibility and effectiveness.