LET dependent hydroxyl radical generation in water by heavy-ion beam irradiation

Nano-geometry of hydroxyl radical ( • OH) generation in water caused by heavy-ion beam irradiation was evaluated using a fluorescent probe, disodium terephthalate (TPA-Na). Since • OH is recognized as a primary product in biological effects of radiation, elimination of • OH may be important to regulate the side effect of radiation therapy. Recently, using the electron paramagnetic resonance (EPR) spin-trapping technique, we reported that the two different types of • OH generations by ionizing radiation. In this study, linear energy transfer (LET) dependence on the density of localized • OH generation was evaluated using TPA-Na. The irradiation of heavy-ion beams was performed at the Heavy-Ion Medical Accelerator in Chiba (HIMAC, National Institute of Radiological Sciences, Japan). Several concentrations (0.5-208 mM) of TPA-Na in aqueous solutions were irradiated with 32 Gy of a heavy-ion beam (C400, Si490 or Fe500) at several different LET. The amount of • OH adduct of TPA-Na (hTPA) thus generated was measured by fluorophotometer (Ex. 310 nm/Em. 425 nm). The relationship between molecular density of TPA-Na and hTPA generated was analyzed. The amount of hTPA generated by low-LET heavy-ion beam was increased in proportion to the molecular density of TPA-Na and reached a plateau through an unclear inflexion point. In the case of high-LET heavy-ion beam, however, another linear trend that passed through the origin was again observed when TPA-Na density was extremely high (TPA-Na conc.: 156-208 mM). The linear increasing trend observed at high TPA-Na density suggested an extremely dense • OH generation.