PROBING INTERTRACK EFFECTS AT ULTRA-HIGH DOSE RATES OF PARTICLE BEAMS WITH A NEW IMPLEMENTATION OF SIMULTANEOUS MULTIPLE TRACKS IN TRAX-CHEM

Among the most investigated hypotheses for a radiobiological explanation of the FLASH effect in radiotherapy, intertrack recombination between particle tracks arriving in a close spatiotemporal regime has been suggested. In the present work we scrutinize these conditions for different particle types and energy, defining the limits of spatiotemporal proximity where a non-negligible effect could be realized. The TRAX-CHEM Monte Carlo code [Boscolo et al., 2018; IJMS, 2020] underwent upgrades to investigate intertrack effects on radical species evolution. The enhancements allowed the propagation and combination of diverse chemical histories, introducing features to examine chemical track evolution, considering species from different primary particles at various temporal and spatial coordinates (Figure 1). The study evaluated proton, helium, and carbon ion tracks, exploring spatial separations up to 1 μm and temporal separations up to 1 μs. The analysis included the relative yield modification for all produced radical species, such as H2O2 and OH.. The obtained outcomes were cross-validated with similar simulations in PARTRAC [Kreipl et al., 2009]. Exploring the intertrack phenomenon by manipulating the initial spatiotemporal positions of two primaries has allowed to assess the main features the process, revealing that while adjusting the spatial gap is the dominant factor influencing the chemical progression of the track, studying temporal separation yields intriguing and somewhat unexpected results. Although changes in temporal separation lead to only marginal alterations in radical yield at the microsecond scale, particularly noticeable for small spatial distances (<10nm), the nuances observed in this aspect add an interesting dimension to the conventional understanding of intertrack effects. Conversely, adjusting the spatial gap exhibits clear dominance in shaping the track's chemical evolution. Further examination over extended temporal distances uncovers significant impacts on chemical species, notably illustrated in Figure 2, with a noteworthy reduction in H2O2 molecules, especially in the case of high Linear Energy Transfer (LET) particles. While spatial separation of 1 μm is in general sufficient for a negligible yield modification for all the evaluated radicals, intertrack effects are not negligible even at high temporal distances (1 μs) and a substantial effect of H2O2 reduction is apparent at high LET for high temporal distances.