ESTRO 2025 - Abstract Book

S2691

Physics - Dose calculation algorithms

ESTRO 2025

2. Spenkelink GB, Huijskens SC, Zindler JD, de Goede M, van der Star WJ, van Egmond J, Petoukhova AL. Comparative assessment and QA measurement array validation of Monte Carlo and Collapsed Cone dose algorithms for small fields and clinical treatment plans. J Appl Clin Med Phys. 2024 Sep 17:e14522. doi: 10.1002/acm2.14522.

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Proffered Paper Inter-track interactions significantly impact the yields of radicals in ultra-high dose rate water irradiation: results from a Monte Carlo study Gavin M Pikes 1,2,3 , Martin A Ebert 2,1,4 , Pejman R Farzad 1,5 1 Physics, University of Western Australia, Perth, Australia. 2 Radiation Oncology, Sir Charles Gairdner Hospital, Perth, Australia. 3 Computational Oncology, Centre for Advanced Technologies in Cancer Research, Perth, Australia. 4 Clinical Engagement, Centre for Advanced Technologies in Cancer Research, Perth, Australia. 5 Research, Centre for Advanced Technologies in Cancer Research, Perth, Australia Purpose/Objective: FLASH radiotherapy can have the unexpected effect of reducing normal tissue complications for a given tumour control probability when using ultra-high dose rate (UHDR) radiation of . However, the radiobiological mechanisms behind the FLASH effect remain unclear, requiring further study for optimized clinical use. We performed Monte Carlo (MC) simulations of UHDR protons to investigate inter-track interactions and their effects on radical production and consumption, which are significant only at UHDRs. Material/Methods: The MC code TOPAS-nBio, which integrates CERN’s Geant4 code, was utilised to simulate biological radiation damage at the sub-cellular level. A custom FLASH chemistry module was employed to evaluate the chemical yields of radiolysis species under UHDR irradiation, with and without inter-track interactions. These interactions were assessed by shifting from an independent reaction times approach to a step-by-step calculation. Simulations were conducted on an aerated water phantom containing 0.25 mM/L dissolved oxygen concentration, irradiated with protons ranging from 10 MeV to 300 MeV, allowing UHDRs up to . The production and consumption of radiolysis species was tracked over time to evaluate the impact of inter-track interactions. Results: Significant alterations in the G-values (production/consumption of radicals per 100 eV) of all species under UHDR proton irradiation were observed when inter-track interactions were included in the radiolysis simulations. By the end of the chemical stage (10 μs), an 85% reduction in hydroxyl radical production, the most damaging species to DNA, was observed, decreasing from 2.6 molecules/100 eV without inter-track interactions to 0.4 molecules/100 eV with them. Hydrolysed electrons showed a similar reduction, from 2.7 to 0.3 molecules/100 eV. Comparable changes in magnitude were observed for other radicals, with some species showing increased production when inter-track interactions were included. The energy dependence of G-values for hydroxyl radicals and hydrolysed electrons was almost eliminated, although this effect was not seen in three of the six radicals tracked. Inter-track interactions also substantially reduced the G-value dependence on the number of primary particles for three radicals. Conclusion: The G-values of radicals in UHDR water irradiation were found to be substantially influenced by inter-track interactions. These effects should be further explored in future studies investigating the mechanisms underlying the FLASH effect.

Keywords: FLASH, Monte Carlo, Radiation chemistry