Validation and investigation of reactive species yields of Geant4‐ DNA chemistry models

International audiencePurpose: Indirect biological damage due to reactive species produced in water radiolysis reactions isresponsible for the majority of biological effect for low linear energy transfer (LET) radiation. Modelingwater radiolysis and the subsequent interactions of reactive species, as well as track structures, isessential to model radiobiology on the microscale. Recently, chemistry models have been developedfor Geant4-DNA to be used in combination with the comprehensive existing physics models. In thecurrent work, the first detailed, independent, in silico validation of all species yields with publishedexperimental observations and comparison with other radiobiological simulations is presented. Additionally,the effect of LET of protons and heavier ions on reactive species yield in the model wasexamined, as well as the completeness of the chemical reactions following the radiolysis within thetime after physical interactions simulated in the model.Methods: Yields over time of reactive species were simulated for water radiolysis by incident electrons,protons, alpha particles, and ions with various LETs using Geant4 and RITRACKS simulationtools. Water dissociation and recombination was simulated using Geant4 to determine the completenessof chemical reactions at the end of the simulation. Yield validation was performed by comparingyields simulated using Geant4 with experimental observations and other simulations. Validation wasperformed for all species for low LET radiation and the solvated electron and hydroxyl radical forhigh LET ions.Results: It was found that the Geant4-DNA chemistry yields were generally in good agreement withexperimental observations and other simulations. However, the Geant4-DNA yields for the hydroxylradical and hydrogen peroxide at the end of the chemistry stage were found to be respectively considerablyhigher and lower than the experimentally observed yields. Increasing the LET of incidenthadrons increased the yield of secondary species and decreased the yield of primary species. Theeffect of LET on the yield of the hydroxyl radical at 100 ns simulated with Geant4 was in good agreementwith experimental measurements. Additionally, by the end of the simulation only 40% of dissociatedwater molecules had been recombined and the rate of recombination was slowing.Conclusions: The yields simulated using Geant4 are within reasonable agreement with experimentalobservations. Higher LET radiation corresponds with increased yields of secondary species anddecreased yields of primary species. These trends combined with the LET having similar effects onthe 100 ns hydroxyl radical yield for Geant4 and experimental measurements indicate that Geant4accurately models the effect of LET on radiolysis yields. The limited recombination within the modeledchemistry stage and the slowing rate of recombination at the end of the stage indicate potentia