ESTRO 2025 - Abstract Book

S2732

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern University Hospital, and University of Bern, Bern, Switzerland

Purpose/Objective: Very high energy electron radiotherapy (VHEE-RT) has gained research interest, particularly for its potential in FLASH therapy and targeting deep-seated tumors. Since VHEE beams are affected by magnetic fields, one proposed VHEE RT opportunity is to position magnetic lenses upfront of the patient to focus the beam, reducing entrance and exit doses compared to conventional VHEE beams 1,2 . To date, studies of focused-beam VHEE-RT (fVHEE-RT) have been limited to technical investigations of 1D-profiles in water. We aimed to develop inversely optimized fVHEE-RT treatment planning for clinically motivated cases to evaluate its potential dosimetric benefits compared to state-of the-art VMAT. Material/Methods: Treatment planning was performed for two clinically motivated lung cases and one brain case. For each case, 3-4 individual beam angles were defined manually, followed by a regular “scanning-grid” spot-placement, motivated by proton therapy pencil beam scanning, across the PTV (1 mm spacing, 1 mm FWHM in the focal spot). The magnetic lens itself was not explicitly simulated. Instead, 250 MeV fVHEE beams with a pre-defined 5 cm magnetic lens radius and variable focal lengths from the chosen directions were calculated with Monte Carlo dose calculation (EGSnrc). These beams served as input for our in-house inverse optimization framework that determined fluence map weights to meet case-specific objectives. Dose distributions and DVHs were compared to VMAT plans. Results: The dose distributions for lung cancer case 1 are shown in Figure 1, and the corresponding DVHs are in Figure 2. The fVHEE plan reduced D mean compared to VMAT for the heart, esophagus, trachea, spinal canal, and lungs. D max decreased for the esophagus, heart, and trachea, but increased for the lungs, spinal canal, and the liver. In lung case 2, the spinal canal D max is reduced by -8.3% by omitting beam directions in spinal cord proximity, while other dose parameters were similar. For the brain case, fVHEE and VMAT provided comparable plan quality.