Hydroxyl Radical Generation by Ionizing Radiation

High linear energy transfer (LET) carbon ion beam cancer therapy using the Heavy-Ion Medical Accelerator in Chiba (HIMAC) (National Institute of Radiological Sciences, Chiba, Japan) began in 1994. It is effective against hypoxic tumor because irradiaton with high values of LET has a lower oxygen effect compare to low LET radiations. ROS generation of high LET beams, however, can not negridge when clinical dose become higher in the future low flactionated protcol.When ionizing radiation is irradiated to living organisms, various reactive oxygen species (ROS) such as hydroxyl radical (OH), superoxide (O2-), and hydrogen peroxide (H2O2) are produced through radiolysis of water. Approximately 70% of the cell damage is caused by indirect reactions through ROS, and 70-80% of these indirect effects of radiation are caused by OH, which is the most active species of ROS. In this experiment, distribution of OH was investigated.100 mM Phosphate buffer including 0.05 mM DTPA was wormed upto 80 DegC, and 3.5% gelatin was desolved to the phosphate buffer. The gelatin solution was cooled down to room temperature, then a spin trapping agent, DMPO, was added to make several concentration (3.3 to 195 mM). The gelatin solution was poured in 50 mL plastic flask, and caked at 4 DegC for 2 hr in a reflidgerater. 290 MeV carbon mono beam was irradiated to the caked gelatin sample using HIMAC. Irradiation was performed by applying several LET (20 to 180 keV/um) at the surface of the gelatin. The dose at the surface of the gelatin sample was 32 Gy. An aliquot (~3.5 uL) of gelatin from the irradiated surface was sampled into glass capillary after irradiation. Three capillary samples were obtained from each flask. The capillary samples were measured by X-band EPR 30 min after irradiation. The same gelatin samples containing several different concentration of DMPO were irradiated by 32 Gy of X-ray (80 keV/um) and measured by X-band EPR.EPR signal of DMPO-OH was obtained in irradiated sample. Amount of DMPO-OH generated by carbon-ion beam was markedly lower than that by X-ray. For carbon-ion beam, amount of DMPO-OH decreased with increasing LET. The EPR signal intensity of DMPO-OH in carbon-beam-irradiated sample increased with concentration of DMPO added in the sample; however, amount of DMPO-OH in X-ray-irradiated sample reached plateau around 25 mM DMPO.第16回日本学術振興会先端研究拠点事業セミナ