Ionizing radiation relevant for therapy (photons, electrons, protons, carbon and helium ions) elicits localized damage in biological targets, which may ultimately lead to cell death. Two general pathways to induce harm can be distinguished: direct interaction with biomolecules, and indirect action through the intermediate production of reactive chemical species stemming from water radiolysis. The relative contribution of the latter pathway to the overall damage depends on the specific radiation type, but it can be significant, with a contribution of up to 90% being observed for a high energy proton beam. Monte Carlo simulations have been employed as a method to study and predict how the chemical yields change as a function of radiation quality, energy, LET, and environment conditions (e.g., oxygenation).

At GSI we have developed an algorithm, TRAX [1,2], which, together with its integrations TRAX-CHEM [3,4] and TRAX-CHEMxt [5], is capable of accurately predicting outcomes of various stages of radiation damage: physical, physico-chemical, chemical, and biochemical. Our goal is to refine and improve the algorithm, in order to simulate a more realistic and variegated cell-like environment, together with more complex treatment modalities. The introduction of biological targets is expected to facilitate the comprehension of the exact pathways behind the indirect damage induction. Furthermore, it will enable the testing and disentangling of hypotheses concerning more complex, dose-rate dependent delivery methods. A potential application can be the study of the FLASH effect [6, 7], where the base mechanism is still unknown, but it is deemed to be connected to phenomena such as oxygen depletion, radical-radical recombination, and intertrack interactions.