A Proposed Beamline Optics for Focused Very High Energy Electron Radiotherapy

Very High Energy Electron (VHEE) beams, in the energy range of 150 to 250 meV, have sufficient penetration depth, representing a promising alternative for treating deep-seated tumors. However, a VHEE beam generates quasi-uniform dose distribution along its path, resulting in excessive exposure of healthy tissue surrounding the tumor. This paper proposes a scheme to optimize the dose distribution by changing the focused beam energy and focusing strength. Based on beam optics simulation codes MAD-X and COSY, a beamline is designed to transport and focus VHEE beams to the tumor using two triplets of quadrupole magnets. The influence of energy dispersion on the focusing effects is evaluated theoretically and numerically. Furthermore, the Monte Carlo simulation code FLUKA is implemented to calculate the dose distributions of different focused beams, demonstrating the feasibility of this scheme. Percentage on-axis depth-dose, 2D dose profiles, surface and exit doses and shift of the peak dose depth are studied in detail.