@article{oai:repo.qst.go.jp:00045438,
 author = {Hirayama, Ryoichi and Ito, Atsushi and Tomita, Masanori and Tsukada, Teruyo and Yatagai, Fumio and Noguchi, Miho and Matsumoto, Yoshitaka and Kase, Yuuki and Ando, Koichi and Okayasu, Ryuichi and Furusawa, Yoshiya and Hirayama, Ryoichi and Ito, Atsushi and Yatagai, Fumio and Matsumoto, Yoshitaka and Kase, Yuuki and Ando, Koichi and Okayasu, Ryuichi and Furusawa, Yoshiya},
 issue = {2},
 journal = {Radiation Research},
 month = {Feb},
 note = {Biological effects of radiation originate principally in damages to DNA. DNA damages by X-rays as well as heavy ions are induced by a combination of direct and indirect actions. The contribution of indirect action in cell killing can be estimated from the maximum degree of protection by dimethylsulfoxide (DMSO) which suppresses indirect action without affecting the direct one. Exponentially growing Chinese hamster V79 cells were exposed to high LET radiations of 20 to 2106 keV/micrometer in the presence or absence of DMSO and their colony survival was determined. The contribution of indirect action in cell killing decreased with the increase in the LET. However, the contribution did not reach to zero even at very high LETs and was estimated to be 32% at an LET of 2106 keV/micrometer. Therefore, even though the radiochemically estimated G value of OH radicals was nearly zero at an LET of 1000 keV/micrometer, indirect action by OH radicals contributed to a substantial fraction of biological effects of high LET radiations. A RBE determined at a survival level of 10% increased with LET, reaching a maximum value of 2.88 at 200 keV/micrometer, and decreased thereafter. When the RBE was estimated separately for direct action (RBED) and indirect action (RBEI), both of them exhibited a similar LET dependency as the RBE, peaking at 200 keV/micrometer. However, the peak value was much higher for RBED (5.99) than RBEI (1.89). Thus, direct action contributes more to the high RBE of high LET radiations than indirect action does.},
 pages = {212--218},
 title = {Contributions of Direct and Indirect Actions in Cell Killing by High-LET Radiations},
 volume = {171},
 year = {2009}
}