Quantification and local density estimation of radiation-induced reactive oxygen species (ROS) were described focusing on our recent and related studies. Charged particle radiation, i.e. heavy-ion beams, are currently utilized for medical treatment. Differences in ROS generation properties between photon and charged particle radiation may lead to differences in the quality of radiation. Radiation-induced generation of ROS in water was quantified using several different approaches to electron paramagnetic resonance (EPR) techniques. Two different densities of localized hydroxyl radical (•OH) generation, i.e. milli-molar and molar levels, were described. Yields of sparse •OH decreased with increasing linear energy transfer (LET), the yield total •OH was not affected by LET. In the high-density, molar level, •OH environment, •OH can react and directly make hydrogen peroxide (H2O2), and then possible to form a high-density H2O2 cluster. The amount of total oxidation reactions caused by oxidative ROS, such as •OH and hydroperoxyl radial (HO2•), was decreased with increasing LET. Possibilities of the sequential reactions were discussed based on the initial localized density at the generated site. Water-induced ROS have been well investigated. However, little is known about radiation-induced free radical generation in lipidic conditions. Radio-chemistry to understand the sequential radio-biological effects is still under development.
Keywords: Carbon-ion beam; hydrogen peroxide; hydroxyl radical; ionizing radiation; reactive oxygen species; redox reaction.