Diffusing alpha‐emitters radiation therapy dosimetry: Geant4 multi‐stage Monte Carlo modeling of sources

Background Diffusing alpha‐emitters Radiation Therapy (DaRT) is a brachytherapy treatment modality that utilizes the diffusing progeny of to treat solid tumors with therapeutic alpha radiation. The treatment is very complex, with comprehensive dosimetry, micro‐ and nanodosimetry, and detector studies requiring temporal modelling of the entire decay chain, diffusion, energy‐transfer physics, and the implementation of boundary conditions for both desorbed and source‐bound nuclides. Purpose To present and validate a 3D Monte Carlo (MC) model that combines the full decay chain, radionuclide diffusion, and particle energy‐transfer physics, to enable temporal DaRT dosimetry for both source‐bound and diffusing nuclides. Methods Using the Geant4 toolkit, a multi‐stage Monte Carlo model (MSMCM) was developed to combine radioactive decay, Brownian motion, and particle energy‐transfer physics into a single framework. Using this framework we performed three simulations, with different boundary condition variants, for point, single, and multi‐source model configurations, with the dose distributions validated against published analytical models. The MSMCM's potential application for use in multi‐source in‐vivo detector analyses was also assessed. Results The MSMCM was successfully able to produce spatial and temporal dose distributions for all source types: point, single, and multi‐source. Additionally, for the single source model, we produced full‐spectrum dosimetry, dose‐buildup, and dose‐rate curves over clinically relevant timeframes. Comparing the MSMCMs benchmark models, we found that the measured dose depositions of point‐ and radial‐source scenarios were within 5%. We also found that boundary conditions had a marked impact on the axial depth‐dose for realistic source geometries. Conclusions The MSMCM provides a flexible particle‐by‐particle solution to modelling DaRT, producing spatial and temporal dosimetry consistent with current analytical models. Given the MSMCM's capacity to model the complete decay chain, diffusion, track‐level energy depositions, and complex boundaries in a single framework, it is suitable for micro‐ and nanodosimetry, multi‐source in‐vivo dosimetry, and detector analysis studies.