A method to implement inter-track interactions in Monte Carlo simulations with TOPAS-nBio and their influence on simulated radical yields following water radiolysis

Abstract

Objective: In FLASH radiotherapy (dose rates ≥ 40 Gy/s), a reduced normal tissue toxicity has been observed, while maintaining the same tumor control compared to conventional radiotherapy (dose rates ≤ 0.03 Gy/s). This protecting effect could not be fully explained yet. One assumption is that interactions between the chemicals of different primary ionizing particles, so-called inter-track interactions, trigger this outcome. In this work, we included inter-track interactions in Monte Carlo track structure simulations and investigated the yield of chemicals (G-value) produced by ionizing particles. 
Approach: For the simulations, we used the Monte Carlo toolkit TOPAS, in which inter-track interactions cannot be implemented without further effort. Thus, we developed a method enabling the simultaneous simulation of N original histories in one event allowing chemical species to interact with each other. To investigate the effect of inter-track interactions we analyzed the G-value of different chemicals using various radiation sources. We used electrons with an energy of 60 eV in different spatial arrangements as well as a 10 MeV and 100 MeV proton source. For electrons we set N between 1 and 60, for protons between 1 and 100.
Main results: In all simulations, the total G-value decreases with increasing N. In detail, the G-value for ●OH, H3O and eaq decreases with increasing N, whereas the G-value of OH- , H2O2 and H2 increases slightly. The reason is that with increasing N, the concentration of chemical radicals increases allowing for more chemical reactions between the radicals resulting in a change of the dynamics of the chemical stage.
Signifcance: Inter-track interactions resulting in a variation of the yield of chemical species, may be a factor explaining the FLASH effect. To verify this hypothesis, further simulations are necessary in order to evaluate the impact of varying G-values on the yield of DNA damages.

Comparison of two methods simulating inter-track interactions using the radiobiological Monte Carlo toolkit TOPAS-nBio

“…While the G-value decreases for most molecule types ( • OH, H 3 O, OH − , H 2 O 2 and e aq 9 ) with N, for some molecules (recombined H 2 O, H 2 ) it increases. In comparison to our previous work (Derksen et al 2023) the trends of individual molecules with N in this study differ slightly because on the one hand, a different source with a different LET was used and on the other hand, a different list of chemical reactions was included and the SBS approach was used instead of the IRT approach in this study. For a detailed analysis of G-values in dependence of N, we refer to Derksen et al (2023).…”

“…In comparison to our previous work (Derksen et al 2023) the trends of individual molecules with N in this study differ slightly because on the one hand, a different source with a different LET was used and on the other hand, a different list of chemical reactions was included and the SBS approach was used instead of the IRT approach in this study. For a detailed analysis of G-values in dependence of N, we refer to Derksen et al (2023). Regarding the summarized G-value of all molecule types, G all , the G-values agree very well with a maximum difference of 0.2%.…”

“…This method was developed in a preliminary work (Derksen et al 2023) to investigate inter-track interactions with TOPAS-nBio 1.0 using the SBS approach. However, it is also possible to apply this method using the IRT model available in TOPAS-nBio 2.0.…”

“…This approach allows to directly set the number N of primary particles whose secondary particles can interact with each other. For details of this method, we refer to Derksen et al (2023). In the following, this method is referred to as the phsp method.…”

“…However, in TOPAS-nBio 1.0, there is no feature included allowing for the modelling of inter-track interactions. Thus, in a preliminary work (Derksen et al 2023), we developed a method enabling inter-track interactions and investigated their effect on the dynamics of the chemical stage following radiation using TOPAS-nBio 1.0.…”