KEY POINTS
- Purified water was irradiated with 67.5 MeV protons at mean dose rates ranging from approximately 0.2 Gy/s to 6 kGy/s under aerated and deaerated conditions. Hydrogen peroxide was measured 15 minutes after irradiation and compared with Geant4-DNA simulations.
- Under aerated conditions, experimental hydrogen peroxide yield decreased from 2.13 ± 0.05 to 1.43 ± 0.07 molecules per 100 eV between 0.21 and 6384 Gy/s, corresponding to an approximately 28% reduction.
- Under deaerated conditions, experimental yield decreased from 1.52 ± 0.04 to 1.14 ± 0.06 molecules per 100 eVacross the tested dose-rate range, with reductions approaching 32% at approximately 5.4 kGy/s.
- Geant4-DNA reproduced the dose-rate-dependent trend. Under aerated conditions, simulated yield decreased from 2.20 at 0.27 Gy/s to 1.69 at 114 Gy/s; under 0.4% oxygen, it decreased from 1.58 at 0.26 Gy/s to 1.22 at 42 Gy/s.
- Simulations indicated that dissolved carbon dioxide affected hydrogen peroxide production mainly under low-oxygen conditions, reducing the simulated yield by 20.4% and improving agreement with experiment. The study used pure water rather than biological media.
CLINICAL TAKEAWAY
The results strengthen the evidence that ultrahigh dose-rate proton irradiation alters water radiolysis through dose-rate-dependent radical recombination and oxygen-sensitive chemistry. However, hydrogen peroxide production in purified water is a mechanistic surrogate, not proof of normal-tissue sparing, so the findings remain preclinical and cannot directly guide FLASH treatment protocols.