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

backgroundVery 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.

methodsLocal 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 <25 MeV to account for increased scattering. Energy deposition was scored in a 3D dose grid. MMC dose calculations were validated against EGSnrc for monoenergetic VHEE beams (50-250 MeV) incident on homogeneous and heterogeneous slab phantoms, using pencil beams, parallel spot beams with 1 mm radius, and parallel beams with a field size of 5 × 5 cm

resultsIntegrated depth dose curves and lateral dose profiles agreed within 2% of the maximum dose for all cases considered. For homogeneous and heterogeneous phantoms, MMC dose distributions yielded gamma passing rates above 97% (2%/1 mm) and 99% (2%/2 mm), respectively, compared to EGSnrc. For patient CT datasets, gamma passing rates exceeded 94% (2%/1 mm) and 97% (2%/2 mm). Overall, MMC achieved up to a 27-fold improvement in dose calculation efficiency compared to EGSnrc.

conclusionsAn MMC framework for VHEE dose calculation was successfully developed and validated for electron energies up to 250 MeV. The method demonstrated good agreement with EGSnrc while providing up to an order-of-magnitude improvement in dose calculation efficiency for the studied cases.