VHEE-SSB Grid: Spot-scanning beam enabled spatially fractionated and FLASH radiotherapy with very high energy electron beams

Objectives. This study aims to demonstrate the feasibility of combining spatially fractionated radiotherapy (SFRT) and FLASH radiotherapy (FLASH-RT) techniques using VHEE through Monte Carlo simulations and to develop a simulation platform for this purpose. Approach. The simulation platform was built in TOPAS based on the beam parameters of the Wuhan Advanced Light Source linear accelerator. Quadrupole magnets are used to focus very high energy electron beams to control the longitudinal depth of the maximum dose. Lateral scanning irradiation is achieved by combining deflecting magnets. The spread-out electron peak (SOEP) depth adjustment, and spot scanning were achieved through magnetic field adjustments. The dose distribution of VHEE in the water phantom was simulated, and the three-dimensional dose distribution, Dose Volume Histogram (DVH), and peak-to-valley ratio were analyzed. Main Results. Based on the fitted relationships, Monte Carlo simulations with 106 particles were conducted in TOPAS to verify the positions of maximum dose, yielding millimeter errors. The SOEP flatness between 12 - 17 cm depths exceeded 98.7%. Spatially fractionated radiotherapy simulated at four target points showed a peak-to-valley dose ratio (D10%/D90%) of 11.026, where D10% is the minimum dose within the highest 10% dose voxels, and D90% is the minimum dose received by 90% of the target volume. A volume fraction of 4.41% within the PTV received ≥90% of the maximum dose. The dose rate in the PTV reached up to 54.25 Gy/s under 200 nA electron beam irradiation. Significance. Take the example of beam parameters of the linear accelerator at Wuhan Advanced Light Source, the simulation platform is validated for conducting research on combined SFRT and FLASH-RT for deep tumor radiation treatment. This also demonstrates the feasibility of combining SFRT and FLASH-RT using VHEE.