ESTRO 2025 - Abstract Book

S2701

Physics - Dose calculation algorithms

ESTRO 2025

3392

Digital Poster A Monte Carlo based tracking framework to investigate very high energy electron (VHEE) interactions Chengchen Zhu 1 , Claire M Costantini 1,2 , Florian Amstutz 1 , Werner Volken 1 , Jan Unkelbach 2 , Marco FM Stampanoni 3 , Michael K Fix 1 , Peter Manser 1 1 Division of Medical Radiation Physics and Department of Radiation Oncology, Inselspital, Bern, Switzerland. 2 Department of Physics, University of Zürich, Zürich, Switzerland. 3 Institute for Biomedical Engineering, ETH Zürich and PSI, Villigen, Switzerland Purpose/Objective: Very high energy electrons (VHEE), with energies ranging between 50-250 MeV, hold promise in radiotherapy owing to their potential to reach deep-seated tumors and spare healthy tissue. This study aims to develop a versatile framework for tracking VHEE interactions in matter, facilitating the analysis of associated interaction types and providing potential insights to enhance VHEE dose calculation efficiency. Material/Methods: The framework is developed based on "EgsAdvancedApplication" with EGSnrc 1 for particle transport, excluding Rayleigh scattering and using the impulse approximation for Compton scattering. Electrons, positrons, and photons are transported down to 100 keV (kinetic energy). Simulations start with a user-defined input file detailing the phantom geometry, source, and run control parameters. In our framework, users can selectively enable/disable specific physical interactions (default EGSnrc settings used), including Møller scattering, Bremsstrahlung production, pair production, Compton scattering, photoelectric effect, annihilation, and Bhabha scattering. We simulated a 200 MeV monoenergetic VHEE pencil beam, impinging perpendicularly on a water/lung/bone phantom (50 x 50 x 100 cm 3 ) with 0.2 x 0.2 x 0.2 cm 3 voxel resolution. Sequential simulations were performed, each time disabling one interaction while maintaining previous exclusion. Each simulation considered one million particle histories. Particle tracks, interactions counts, integrated depth dose curves, remaining dose contributions (compared to the simulation with all interactions enabled), and computation times were recorded. Results: Figure 1 shows the particle tracks from a primary electron history in water, lung, and bone. Logged events per particle history (PPH) in water, lung, and bone included Møller scattering (740, 300, 823 PPH), Bremsstrahlung (21, 9.3, 33 PPH), Compton scattering (57, 5.6, 111 PPH), pair production (1.3, 0.2, 2.6 PPH), and photoelectric effect (0.2, 0.02, 4.1 PPH). Positrons generated underwent Bhabha scattering (79, 11, 142 PPH) and annihilation events (1.2, 0.2, 2.6 PPH). The greatest dose contribution came from the secondary electrons generated from Bremsstrahlung events that underwent Møller scattering, as photons are indirectly ionizing (Figure 2). Dose contributions from photoelectric effect, annihilation, and related subsequent interactions were <1% in water. Computation times ranged between 0.3-389.6 min (Intel Xeon, 2 GHz with 24 CPUs).