Development of a Primary Absorbed Dose-to-Water Standard for the Flash Regime: Radiation Chemistry Framework
Abstract
Purpose
To establish a benchmarked radiochemical framework under conventional irradiation and present preliminary results on extending radiochemical simulations to ultra-high-dose-rate (UHDR) beams. FLASH radiotherapy employs UHDR radiation beams that challenge the assumptions underlying primary absorbed-dose standards based on water calorimetry. While radiochemical heat-defect corrections are well established for conventional dose rates, their validity under FLASH conditions remains unverified.
Methods
A numerical model of water radiolysis was developed by solving fourteen coupled nonlinear ordinary differential equations describing the temporal evolution of chemical species. The model includes fifty elementary reactions with radiation-induced source terms based on established chemical yields. Three irradiation protocols were implemented, each delivering 3 Gy once every 10 minutes: (i) over an exposure period of 120 s (cobalt-60), (ii) over 120 ms, and (iii) over 1 µs. Stiff kinetics were solved using an adaptive backward differentiation formula integrator. Heat-defect corrections were computed from reaction enthalpies, and hydrogen peroxide was tracked as a stable chemical observable.
Results
For cobalt-60 photon irradiation with conventional dose rates, the model reproduces steady-state radiochemical behaviour for absorbed dose-to-water determination in a primary standards dosimetry laboratory (PSDL). Predicted hydrogen peroxide concentrations agree with benchmark simulations within numerical precision. The heat-defect correction was determined to be 2.04 ± 0.30 %, consistent with a PSDL implementation (2.15 ± 0.30 %). The reported uncertainty reflects approximations in radiochemical kinetics, thermochemical data, and unmodelled impurity effects, and is quoted at a 68% confidence level. Under high–dose-rate conditions, temporal compression of dose delivery modifies the heat-defect correction by up to 2 %, which is substantial for PSDLs. Hydrogen and oxygen atom conservation was satisfied within numerical tolerance.
Conclusion
This work extends a validated radiochemical framework toward FLASH irradiation. By linking water radiochemistry, heat-defect corrections, and stable chemical observables, the framework supports development of primary absorbed dose-to-water standards for FLASH radiotherapy.