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

• Published in: Physics in medicine & biology Vol. 68; no. 13; pp. 135017 - 135035
• Main Authors: Derksen, Larissa, Flatten, Veronika, Engenhart-Cabillic, Rita, Zink, Klemens, Baumann, Kilian-Simon
• Format: Journal Article
• Language: English
• Published: England IOP Publishing 07.07.2023
• Subjects: Computer Simulation; FLASH radiotherapy; GEANT4-DNA; inter-track interaction; Linear Energy Transfer; Monte Carlo Method; Monte Carlo simulation; Protons; radiation chemistry; radiolysis; TOPAS-nBio; Water - chemistry; radiation chemistry; FLASH radiotherapy; GEANT4-DNA; TOPAS-nBio; Monte Carlo simulation; inter-track interaction; radiolysis
• ISSN: 0031-9155; 1361-6560
• DOI: 10.1088/1361-6560/acdc7d

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 ( -value) produced by ionizing particles. 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 original histories in one event allowing chemical species to interact with each other. To investigate the effect of inter-track interactions we analyzed the -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 between 1 and 60, for protons between 1 and 100. In all simulations, the total -value decreases with increasing . In detail, the -value for OH , H O and e decreases with increasing , whereas the -value of OH , H O and H increases slightly. The reason is that with increasing , 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. 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 -values on the yield of DNA damages.