Analysis of hydrogen peroxide production in pure water: Ultrahigh versus conventional dose-rate irradiation and mechanistic insights

Abstract

Background: Ultrahigh dose-rate radiation (UHDR) produces less hydrogen peroxide (H₂O₂) in pure water, as suggested by some experimental studies, and is used as an argument for the validity of the theory that FLASH spares the normal tissue due to less reactive oxygen species (ROS) production. In contrast, most Monte Carlo simulation studies suggest the opposite.

Purpose: We aim to unveil the effect of UHDR on H₂O₂ production in pure water and its underlying mechanism, to serve as a benchmark for Monte Carlo simulation. We hypothesized that the reaction of solvated electrons ( $e_{\mathrm{aq}}^{-}$ ) removing hydroxyl radicals (•OH), the precursor of H₂O₂, is the reason why UHDR leads to a lower G-value (molecules/100 eV) for H₂O₂ (G[H₂O₂]), because: 1, the third-order reaction between $e_{\mathrm{aq}}^{-}$ and •OH is more sensitive to increased instantaneous ROS concentration by UHDR than a two-order reaction of •OH self-reaction producing H₂O₂; 2, $e_{\mathrm{aq}}^{-}$ has two times higher diffusion coefficient and higher reaction rate constant than that of •OH, which means $e_{\mathrm{aq}}^{-}$ would dominate the competition for •OH and benefit more from the inter-track effect of UHDR. Meanwhile, we also experimentally verify the theory of long-lived radicals causing lower G(H₂O₂) in conventional irradiation, which is mentioned in some simulation studies.

Methods and materials: H₂O₂ was measured by Amplex UltraRed assay. 430.1 MeV/u carbon ions (50 and 0.1 Gy/s), 9 MeV electrons (600 and 0.62 Gy/s), and 200 kV x-ray tube (10 and 0.1 Gy/s) were employed. For three kinds of water (real hypoxic: 1% O₂; hypoxic: 1% O₂ and 5% CO₂; and normoxic: 21% O₂), unbubbled and bubbled samples with N₂O, the scavenger of $e_{\mathrm{aq}}^{-}$ , were irradiated by carbon ions and electrons with conventional and UHDR at different absolute dose levels. Normoxic water dissolved with sodium nitrate (NaNO₃), another scavenger of $e_{\mathrm{aq}}^{-}$ , and bubbled with N₂O was irradiated by x-ray to verify the results of low-LET electron beam.

Results: UHDR leads to a lower G(H₂O₂) than conventional irradiation. O₂ and CO₂ can both increase G(H₂O₂). N₂O increases G(H₂O₂) of both UHDR and conventional irradiation and eliminates the difference between them for carbon ions. However, N₂O decreases G(H₂O₂) in electron conventional irradiation but increases G(H₂O₂) in the case of UHDR, ending up with no dose-rate dependency of G(H₂O₂). Three-spilled carbon UHDR does not have a lower G(H₂O₂) than one-spilled UHDR. However, the electron beam shows a lower G(H₂O₂) for three-spilled UHDR than for one-spilled UHDR. Normoxic water with N₂O or NaNO₃ can both eliminate the dose rate dependency of H₂O₂ production for x-ray.

Conclusions: UHDR has a lower G(H₂O₂) than the conventional irradiation for both high LET carbon and low LET electron and x-ray beams. Both scavengers for $e_{\mathrm{aq}}^{-}$ , N₂O and NaNO₃, eliminate the dose-rate dependency of G(H₂O₂), which suggests $e_{\mathrm{aq}}^{-}$ is the reason for decreased G(H₂O₂) for UHDR. Three-spilled UHDR versus one-spilled UHDR indicates that the assumption of residual radicals reducing G(H₂O₂) of conventional irradiation may only be valid for low LET electron beam.

Keywords: hydrogen peroxide; solvated electron; ultrahigh dose rate; water radiolysis.