Monte Carlo modeling of a commercial machine and experimental setup for FLASH-minibeam irradiations with electrons

Ultra-high dose rate (UHDR/FLASH) irradiations, along with particle minibeam therapy (PMBT) are both emerging as promising alternatives to current radiotherapy techniques thanks to their improved healthy tissue sparing and similar tumor control.

Monte Carlo (MC) modeling of a commercial machine delivering 5-7 MeV electrons at UHDR. This model was used afterward to compare measurements against simulations for an experimental setup combining both FLASH and PMBT modalities.

We modeled the main accelerator elements with TOPAS3.8/Geant4.10.07.p03, optimized the electron source parameters, and subsequently benchmarked this geometry against measurements. Minibeam experiments were performed by delivering 7 MeV electrons at UHDR on three different 65-mm thick brass collimators as manufactured for protons with a 400-µm slit width: single slit, 5 slits with a center-to-center (CTC) distance of 4 mm and 9 slits with CTC of 2 mm. Finally, complementary simulations were run by changing critical PMBT collimator parameters to assess their specific impact on peak-to-valley dose ratio (PVDR) as well as on the Bremsstrahlung photon contribution to the total dose.

Percentage depth dose (PDD) distributions and lateral dose profiles showed a good agreement between simulations and measurements, with a maximum discrepancy of less than 4%. With the PMBT collimators in place, discrepancies between simulated and measured dose profiles, lateral and in-depth in peaks and valleys, were within 3%. High PVDR between 5 and 26 were observed until 4 mm in the phantom. During the experiments, a mean dose rate of 167 Gy/s and an instantaneous dose rate of 1.2 × 10⁵ Gy/s were obtained for the FLASH-minibeam setup. PMBT collimator parameters need to be optimized to maximize PVDR while limiting Bremsstrahlung photon contribution to the total dose.

The validation of the MC model and the configuration of an electron FLASH-minibeam setup were successfully completed, paving the way for future radiobiological investigations.