ESTRO 2025 - Abstract Book

S3566

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTRO 2025

Purpose/Objective: Monte Carlo (MC) simulations are critical for accurately modeling radiation-induced chemical processes, including water radiolysis. This study validates MC-predicted radical yields per 100 eV deposited energy (G-values) by comparing them with experimental data from modified Fricke solution irradiations. It examines how sodium nitrate (NaNO ₃ ) modifies radical dynamics and influences ferric ions (Fe³ ⁺ ) G-values. Material/Methods: The Fricke solution serves as a chemical dosimeter where irradiation induces the oxidation of ferrous ions (Fe² ⁺ ) to Fe³ ⁺ , measurable by changes in optical density (OD) at 304 nm. NaNO ₃ was added at different concentrations to the Fricke solutions to investigate its scavenging effect on solvated electrons (e ⁻ aq ), essential for initiating reduction reactions and generating radicals [1, 2]. The Fricke solution was irradiated with 80 MeV/u protons at different dose levels and the G-value of Fe 3+ was measured using a spectrometer. MC simulations using the TOPAS-nBio framework were employed to model both the oxidation of Fe² ⁺ to Fe³ ⁺ and the scavenging reactions of NaNO ₃ , simulating the resulting changes in G-values. Results: Figure 1 shows the G-values of Fe³ ⁺ in the modified Fricke solution with varying NaNO ₃ concentrations. The experimental and simulated G-values of Fe³ ⁺ strongly agree, with deviations ranging from 0.1% to 4.8%, confirming the validity of the MC simulations. A reduction in e ⁻ aq decreases the production of hydrogen peroxide (H ₂ O ₂ ) Consequently, in the presence of NaNO ₃ , Fe³ ⁺ yields were lower due to the scavenging-induced reduction in H ₂ O ₂ availability. This reduction in H ₂ O ₂ , in turn, limits the oxidation of Fe² ⁺ to Fe³ ⁺ , as predicted by both the experimental and simulation results. While the production of H ₂ O ₂ can also result from hydroxyl radical (•OH) recombination, the lower availability of e ⁻ aq slightly enhances H ₂ O ₂ formation by reducing competition between e ⁻ aq and •OH. However, this effect is overshadowed by the overall reduction in radical generation, resulting in decreased Fe³ ⁺ yields.

Conclusion: Simulations revealed that NaNO ₃ scavenges e ⁻ aq , shifting the balance toward oxidizing species, which reduces H ₂ O ₂ production and limits Fe³ ⁺ yields. This study demonstrates the importance of scavenger dynamics in MC simulations to accurately predict radical yields. Next, further validation will involve exploring additional scavengers, such as copper sulfate and methanol. Investigating these interactions will provide deeper insights into the complex dynamics of radical behavior and product formation in water radiolysis, enhancing the chemical phase validation of MC simulations.