Ultra-High Dose-Rates, the FLASH Effect, and Hydrogen Peroxide Yields: Do Experiments and Simulations Really Disagree?

TL;DR

This paper investigates discrepancies between experimental measurements and simulations of hydrogen peroxide yields in water irradiated at ultra-high dose rates, aiming to clarify the underlying causes and improve mechanistic understanding of the FLASH effect.

Contribution

It provides a comprehensive review of experimental and simulation studies on water radiolysis at UHDR, identifying factors causing discrepancies and proposing testable models to resolve them.

Findings

UHDR modifies radical interactions beyond diffusion control

Experimental parameters significantly influence H2O2 yields

Step-by-step Monte Carlo approaches help clarify discrepancies

Abstract

Radiation chemistry of model systems irradiated with ultra-high dose-rates (UHDR) is key to obtain a mechanistic understanding of the sparing of healthy tissue, which is called the FLASH effect. It is envisioned to be used for efficient treatment of cancer by FLASH radiotherapy. However, it seems that even the most simple model systems, water irradiated with varying dose-rates (DR), pose a challenge. This became evident, as differences within measured and predicted hydrogen peroxide (H2O2) yields (g-values) for exposure of liquid samples to conventional DR and UHDR were reported. Many of the recently reported values contradict older experiments and current Monte-Carlo simulations(MCS). In the present work, we aim to identify possible reasons of these discrepancies and propose ways to overcome this issue. Hereby a short review of recent and classical literature concerning experimental and simulational studies is performed. The studies cover different radiation sources, from gamma rays, high-energy electrons, heavy particles (protons and ions) with low and high linear energy transfer (LET), and samples of hypoxic & oxygenated water, with cosolutes such as bovine-serum albumine (BSA). Results are for additional experimental parameters, such as solvent, sample container and analysis methods used to determine the respective g-values of H2O2. Similarly the parameter of the MCS by the step-by-step approach, or the independent-reaction time (IRT) method are discussed. Here, UHDR induced modification of the radical-radical interaction and dynamics, not governed by diffusion processes, may cause problems. Approaches to test these different models are highlighted to allow progress: by making the step from a purely descriptive discourse of the effects observed, towards testable models, which should clarify the reasons of how and why such a disagreement came to light in the first place.