TOPAS-nBio simulation of the impact of ultrahigh dose rate and oxygen concentration on the Fenton reaction using the Fricke solution

Background: Preclinical studies have shown that FLASH radiotherapy (FLASH-RT), delivering radiation in ultrahigh dose rates (UHDR), presents reduced healthy tissue toxicity, while maintaining an iso-effective tumor response compared to conventional radiotherapy (CONV-RT). This combined biological benefit was termed as “FLASH effect.” The mechanisms responsible for this effect remain unclear, however, it has been hypothesized that the Fenton reaction which produces (Formula presented.) can inflict substantial damage to biomolecules and thus might play a role in the “FLASH effect.”. Purpose: We propose to investigate the influence of radiation delivery parameters including pulse width, dose, dose rate, and initial oxygen concentration on the activation of the Fenton reaction using the Fricke solution. Methods and materials: TOPAS-nBio version 4.0 was used to simulate the radiolysis of the Fricke solution. A cubic water phantom (3 µm side) was irradiated laterally by 300 MeV protons. Irradiation was delivered in pulses of tracks with absorbed doses from 1–40 Gy, or in independent tracks. For ultrahigh dose rate (UHDR), we varied the pulse widths between 10 ns and 10 µs, corresponding to instantaneous dose rates of 10⁹–10⁵ Gy/s. For conventional dose rate (CONV), both the independent history approach (mimicking ⁶⁰Co) and multi-pulse irradiations at different frequencies were simulated, yielding a mean dose rate of 0.28 Gy/s. Irradiation times reached between 78 ns-85 s. Oxygen concentrations in the equivalent condition of hypoxic and normoxic tissues (1%-21%) were considered. The G-value for oxidant ions G(Fe³⁺) and ΔG-value of Fenton reaction (H₂O₂ + Fe²⁺→ Fe³⁺ + (Formula presented.) + (Formula presented.)) were scored. The simulations ended after G(Fe³⁺) achieved steady-state, and calculated yields were compared with published data. Results: For CONV, G(Fe³⁺) agreed with ICRU-report 34 data within (0.97 ± 0.1)%. For UHDR, G(Fe³⁺) agreed with ICRU data within (1.24 ± 0.1) % and (0.92 ± 0.1) % for 5 and 10 Gy, respectively. Notably, UHDR at 10 Gy reduced the occurrence of Fenton reactions by 1% and 11.5% at initial oxygen of 21% pO₂ and 1% pO₂, respectively. In consequence, UHDR decreased G(Fe³⁺) by 1.8% and 12.5% at these oxygen levels. Additionally, increasing the dose per pulse to 40 Gy further reduced the G(Fe³⁺) by 40% at 1%pO₂. This effect became negligible for long pulse durations ≥10 µs and low mean dose rate (0.28 Gy/s), highlighting the importance of instantaneous dose rates. Precisely, the decrease was driven by intertrack effects present in UHDR pulses and their impact on the scavenging effect that oxygen had over hydrogen radicals. Conclusions: The observed reduction in G(Fe³⁺) is primarily driven by inter-track effects, which limit the availability of radicals participating in Fenton chemistry under UHDR conditions. This effect is most pronounced at low initial oxygen concentrations and minimal at higher oxygen concentrations. The effect is further amplified at higher per-pulse doses (25–40 Gy) but vanishes at long pulse durations and low mean dose rates. This emphasizes the important role of the initial oxygen concentration in UHDR and its influence on the latest activation of Fenton reaction, a mechanism that may contribute to elucidating the FLASH effect.