Optimizing focused very-high-energy electron beams for radiation therapy based on Monte Carlo simulation.

A TOPAS-based optimization program has been developed to precisely concentrate the dose of focused very-high-energy electron (VHEE) beams on deep-seated targets. This is accomplished by optimizing the magnetic gradients, positions, and number of quadrupole magnets within TOPAS. Using only three quadrupole magnets, the program focuses 250 MeV VHEE beams to achieve a maximum dose position deeper than 17 cm, while maintaining entrance and exit doses within 25% and limiting the lateral dimensions to ≤ 1 cm at the maximum dose location. The linear relationship between the magnetic gradient of the last quadrupole magnet and the maximum dose position enables dose location adjustments through gradient variation. Multiple positions were validated in TOPAS with errors within 1%. The spread-out electron peak (SOEP) is achieved by combining two VHEE beams with different maximum dose positions using the differential evolution method, covering a target depth of 12-17 cm and attaining a dose flatness better than 99%. This pioneering program imposes constraints on entrance dose, exit dose, maximum dose position, and the lateral dimensions of dose deposition at the maximum dose position within phantom. This program may be a promising tool in the applications of focused VHEE in highly conformal treatment plans based on TOPAS.