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

Abstract

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.