Abstract: High dose rate irradiation has been demonstrated to improve the sparing of normal tissues while maintaining an equivalent tumor response. However, the fundamental biological mechanism of such an effect is still not understood. Mechanistic modeling of radiation chemistry based on the Monte Carlo track-structure (MCTS) has the potential to assist in understanding the physico-chemical effects occurring after high dose irradiation. However, current MCTS models rely on liquid water calculations which compromises the interpretation of results when extrapolated to a biological environment. This talk discusses the key aspects of MCTS modeling of radiation chemistry used in radiobiology applications. Two simplistic models for estimating the oxygen depletion under FLASH and conventional dose rate irradiation with fast electrons are given. One model considering reactions of free radicals with oxygen, the other considering reactions of free radicals with DNA, RNA, proteins, amino acids, free nucleotides, and oxygen. The exercise aims to demonstrate the difficulties of extrapolating pure liquid water calculations to a biological environment. Finally, the status of a hybrid MCTS-analytical approach developed on top of the TOPAS-nBio MCTS toolkit is presented.

About the speaker: Dr. Ramos-Méndez is a PY2 Medical Physics Resident at the Department of Radiation Oncology at the University of California San Francisco (UCSF). He received his PhD degree from Benemérita Universidad Autónoma de Puebla, México, in 2013. From 2014 to 2021 he held a postdoctoral position and an Assistant Researcher position at UCSF. His research interests focused on the development of Monte Carlo simulation frameworks. Dr. Ramos-Méndez is one of the core developers of the TOPAS and TOPAS-nBio frameworks, and member of the Geant4 and Geant4-DNA Collaborations that facilitate Monte Carlo simulations for radiation therapy research and to investigate the connection between physics and biology at the cell and sub-cellular scale.