The Effect of Non-Ionizing Excitations On the Diffusion of Ion Species and Inter-Track Correlations in FLASH Ultra-High Dose Rate Radiotherapy
R Abolfath1, A Baikalov2*, S Bartzsch3, N Afshordi4, R Mohan1, (1) UT MD Anderson Cancer Center, Houston, TX, (2) Technical University of Munich, Neuherberg, BV, DE, (3) Klinikum Rechts Der Isar Der Tum, (4) University of Waterloo, Waterloo, Canada
Presentations
PO-GePV-T-154 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
ePoster Forums
Purpose: We present a microscopic mechanism that accounts for the outward burst of “cold”ion species (IS) in a high-energy particle track due to coupling with ”hot” non-ion species (NIS), mediated by a quantized field of acoustic phonons.
Methods: We perform MC simulations of ionizing radiation track structures in water and calculate the temperature profiles to enforce non-equilibrium boundary conditions in MD and derive a stochastic coarse-grained Langevin equation to describe the irreversible flow of thermal energy pumping from NIS to IS.
Results: We demonstrate the coexistence of “hot” NIS with “cold” IS in the radiation track. NIS, concentrated within nano-scale volumes wrapping around IS, is the main source of intensive heat waves and the outward burst of IS. By comparing the transport of IS coupled to NIS with identicalconfigurations of non-interacting IS in thermal equilibrium at room temperature, we demonstratethat the energy gain of IS due to the surrounding hot nanoscopic volumes of NIS significantlyincreases their effective diffusion constants. Comparing the average track separation and the timescale calculated for a deposited dose of 10 Gy and a dose rate of 40 Gy/s, typical values used inFLASH ultra-high dose rate (UHDR) experiments, we find that the sudden expansion of tracksand ballistic transport proposed in this work strengthens the hypothesis of inter-track correlationsrecently introduced to interpret mitigation of the biological responses at the FLASH-UHDR
Conclusion: The predicted higher diffusion constants suggest higher inter-track chemical reaction rates, as well as lower intra-track reaction rates. This study explains why research groups relying on the current Monte Carlo frameworks have reported negligible inter-track overlaps, simply because of underestimation of the diffusion constants. We recommend incorporation of the IS-NIS coupling and heat exchange in all MC codes to enable these tool-kits to appropriately model reaction-diffusion rates at FLASH-UHDR.
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