Geant4 simulation of the enhanced in-vivo ²²⁰Rn spread in DaRT

Recent in-vivo experimental measurements of DaRT seeds have shown that the spread of ²²⁰Rn away from the DaRT seed is greater than expected. This has implications for accurate treatment planning.

Geant4 and Geant4-DNA have been used to simulate the additional spread of ²²⁰Rn and study the impact on particle and RBE (relative biological effectiveness) weighted dose distribution from the whole DaRT decay chain. A percentage of ²²⁰Rn is assumed to migrate with a higher effective diffusion constant due to transport in the blood flow in the surrounding vasculature. The DNA damage was simulated for the fit to the measurement showing the greatest additional spread.

Good agreement was found between the simulation and experimental in-vivo results. The additional spread of ²²⁰Rn increases the number of α-particles from these decays distally from the DaRT seed. However, the distribution of α-particles emitted from later stages of the decay chain is not as significantly affected. There is an increase in DSB yield and the RBE weighted dose extends further from the DaRT seed with the additional spread. The distance of the 1 Gy RBE weighted isodose from the DaRT seed surface moves from 4.4 mm without the additional spread to 5.7 mm including it.

The increase DSB induction distally from the DaRT seed has the potential to increase treatment efficacy. However, this additional spread cannot easily be predicted. Understanding this uncertainty is important for the dose calculations in treatment planning for tumours and normal tissues.