Purpose: To investigate the variations in relationship between reactive species generated and dose with consumption of oxygen which differ between conventional and ultra-high (FLASH) dose rates in protein-rich environments that mimic in vitro and in vivo situations.
Methods: Two assays were developed to investigate hydrogen peroxide production and pO2 reduction as function of delivered radiation dose at both conventional and FLASH dose rates. The assays consist of deionized water with bovine serum albumin (BSA), with Amplex Red for H₂O₂ detection or Oxyphor for pO₂ detection. Hydrogen peroxide production and pO₂ levels were quantified using spectrofluorometry immediately following sample irradiation. Samples were irradiated at clinically relevant doses of 0-10 Gy in increments of 1 Gy. Ultra-high and conventional electron beam dose rates were achieved using a modified clinical Varian Trilogy linear accelerator.
Results: Examination of fluorescence response versus dose indicates that hydrogen peroxide production is linear with delivered dose at both conventional and FLASH dose rates. FLASH dose rate produced hydrogen peroxide as a significantly lower rate than conventional dose rate. Furthermore, we expect that an examination of pO₂ level versus dose indicates that there is radiation-induced oxygen depletion for both conventional and FLASH, and FLASH more rapidly depletes oxygen within the sample.
Conclusion: Oxygen consumption occurs during peroxide radical formation, and while this is a very small change, it is measurable during FLASH radiotherapy, and might be indicative of radiochemical damage that leads to radiobiological damage. The relationship between reactive molecule production and oxygen consumption provides a window into what is occurring in FLASH versus comparable dose at conventional dose rates. It appears that there is a loss of reactive species in FLASH which is correlated to a reduction in oxygen consumption.
Radiation Effects, Dose Response, Radiobiology