ESTRO 2021 Abstract Book

S353

ESTRO 2021

reductions in healthy tissue doses. VHEE beams can be generated with laser accelerators. These accelerators allow the equipment to be smaller than currently possible, and make it possible to fit them in a standard radiotherapy treatment bunker. This makes VHEE a potential middle ground between photon and proton therapy in relation to equipment costs and treatment quality. Here we used automated multi-criterial optimization to systematically evaluate the performance of VHEE beams in comparison to photon VMAT for treatment of prostate cancer. Materials and Methods For 10 prostate cancer patients, an in-house developed multi-criterial optimizer, configured for generating treatment plans with clinically desired trade-offs, was used to automatically generate VHEE and VMAT plans. For VHEE, the algorithm optimized patient-specific electron pencil beam intensities. For each patient, the VHEE pencil beam dose distributions were pre-calculated using the TOPAS Monte Carlo package, and imported into MATLAB for plan optimization. VHEE plans were compared to VMAT. Moreover, the dependencies of VHEE plan quality on beam energy (200, 300 or 400 MeV), and on the number of applied equi-angular VHEE beams (9, 18 or 36) were investigated. All plans were normalized to the same PTV median dose (i.e., D 50% = 60Gy). Differences in PTV doses and healthy tissue sparing were quantified. Results PTV dose parameters for VHEE and VMAT were similar, but VHEE plans with energies of 300 or 400 MeV and 18 or 36 beams largely outperformed VMAT for rectum, anus, bladder and patient doses (Table 1 and Figure 1a). Differences between VMAT and VHEE increased with increasing beam energy (Table 1) and beam number (Figure 1a). Rectum V 50Gy was reduced from 8.8±4.5% for VMAT to 8.4±4.4% (200 MeV, p =0.014), 7.5±4.0% (300 MeV, p =0.002), and 7.2±3.9% (400 MeV, p =0.002) for VHEE plans with 18 equi-angular beams (Table 1). Anus D mean reduced from 11.6±4.8 Gy with VMAT to 7.0±4.0 Gy ( p =0.002) with 400 MeV/18-beam VHEE . Bladder D mean went down from 18.9±11.4 Gy for VMAT to 14.8±9.7 Gy ( p =0.002) for 400 MeV/18-beam VHEE. Fig. 1b shows dose distributions for an example patient.