Characterization of Titanium Effects on Very High Energy Electron (VHEE) Beams

Very High Energy Electron (VHEE) therapy is gaining attention as an innovative approach for treating deep-seated tumors, combining favorable dose deposition, sharp lateral penumbra, and compatibility with ultra-high dose rate (FLASH) delivery. A critical aspect of clinical translation is understanding how beamline components and high-density materials, such as patient implants or collimators, influence beam quality and secondary radiation production. In this work, we performed a comprehensive Monte Carlo investigation of titanium effects on collimated and magnetically focused VHEE beams using TOPAS/Geant4. A 10 × 10 cm², 150 MeV electron beam was directed into a water phantom containing a titanium insert at 5 cm depth. Phase-space planes were scored downstream of the insert to quantify electron, photon, positron, and neutron yields, energy spectra, angular distributions, and spatial fluence maps. Percentage depth–dose (PDD) curves were also evaluated. Titanium produced negligible energy degradation of primary electrons (<0.5% mean loss) but induced modest angular broadening, >70% photon yield enhancement via bremsstrahlung, and nearly two orders of magnitude increase in neutron production through photonuclear reactions near the giant dipole resonance. Focused beams demonstrated improved fluence recovery and maintained distal dose conformity. These results inform material selection, shielding design, and treatment planning for future VHEE radiotherapy systems.