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

Diffusing alpha-emitters Radiation Therapy (DaRT) is a brachytherapy treatment modality that utilizes the diffusing progeny of ${}^{224}Ra$ 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 ${}^{224}Ra$ decay chain, diffusion, energy-transfer physics, and the implementation of boundary conditions for both desorbed and source-bound nuclides.

To present and validate a 3D Monte Carlo (MC) model that combines the full ${}^{224}Ra$ decay chain, radionuclide diffusion, and particle energy-transfer physics, to enable temporal DaRT dosimetry for both source-bound and diffusing nuclides.

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.

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.

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 ${}^{224}Ra$ 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.