Macro Monte Carlo dose calculation for very high energy electron (VHEE) radiotherapy

Abstract Background Very high energy electron (VHEE) radiotherapy has gained growing interest owing to its potential to reach deep‐seated targets and induce FLASH effect. Dose calculations can be performed using analytical or Monte Carlo (MC) methods. Analytical approaches enable rapid dose computation but suffer from limited accuracy in heterogeneous media, whereas MC methods provide high accuracy at the expense of substantial computational cost. Macro Monte Carlo (MMC) is a local‐to‐global method designed to improve dose calculation efficiency compared to general‐purpose MC methods. In MMC, particle transport is based on precalculated transport data generated with general‐purpose MC simulations on specific geometries, which is subsequently used to model particle transport over macroscopic steps within the absorber, avoiding computationally expensive microscopic tracking. MMC made it to a standard electron dose calculation engine in a commercial treatment planning system. However, to date, MMC has not been investigated for electron energies above 25 MeV. Purpose To develop and validate an MMC framework for VHEE radiotherapy that improves dose calculation efficiency while preserving accuracy compared to general‐purpose MC methods for electron energies up to 250 MeV. Methods Local simulations were performed using EGSnrc with monoenergetic electron pencil beams incident perpendicularly on spherical geometries (0.2‐25 MeV) with radii of 0.5‐3 mm, and slab geometries (25‐250 MeV) of 2 mm thickness, composed of various materials. Physical quantities including energy loss, lateral displacement, and angular distributions of primary and secondary particles were scored and stored in a database. This database was subsequently used to transport electrons step‐by‐step in the global simulations, employing slab‐based transport at energies ≥25 MeV and switching to spherical geometries for electron energies