title: Contribution of indirect action to radiation-induced mammalian cell inactivation: dependence on photon energy and heavy-ion LET.
abstract: The contribution of indirect action mediated by OH radicals to cell inactivation by ionizing radiations was evaluated for photons over the energy range from 12.4 keV to 1.25 MeV and for heavy ions over the linear energy transfer (LET) range from 20 keV/microm to 440 keV/microm by applying competition kinetics analysis using the OH radical scavenger DMSO. The maximum level of protection provided by DMSO (the protectable fraction) decreased with decreasing photon energy down to 63% at 12.4 keV. For heavy ions, a protectable fraction of 65% was found for an LET of around 200 keV/microm; above that LET, the value stayed the same. The reaction rate of OH radicals with intracellular molecules responsible for cell inactivation was nearly constant for photon inactivation, while for the heavy ions, the rate increased with increasing LET, suggesting a reaction with the densely produced OH radicals by high-LET ions. Using the protectable fraction, the cell killing was separated into two components, one due to indirect action and the other due to direct action. The inactivation efficiency for indirect action was greater than that for direct action over the photon energy range and the ion LET range tested. A significant contribution of direct action was also found for the increased RBE in the low photon energy region.
authors.affiliation: Course of Energy Engineering, Department of Applied Science, School of Engineering, Tokai University, Hiratsuka-shi, Kanagawa 259-1292, Japan. aeito@keyaki.cc.u-tokai.ac.jp
authors.forename: Atsushi
authors.initials: A
authors.lastname: Ito
authors.forename: Hisako
authors.initials: H
authors.lastname: Nakano
authors.forename: Yohsuke
authors.initials: Y
authors.lastname: Kusano
authors.forename: Ryoichi
authors.initials: R
authors.lastname: Hirayama
authors.forename: Yoshiya
authors.initials: Y
authors.lastname: Furusawa
authors.forename: Chieko
authors.initials: C
authors.lastname: Murayama
authors.forename: Tomoyuki
authors.initials: T
authors.lastname: Mori
authors.forename: Yosuke
authors.initials: Y
authors.lastname: Katsumura
authors.forename: Kunio
authors.initials: K
authors.lastname: Shinohara
mesh_terms: D002470:Cell Survival / Q000187:drug effects* / Q000528:radiation effects*; D003198:Computer Simulation; D004121:Dimethyl Sulfoxide / Q000008:administration & dosage*; D004305:Dose-Response Relationship, Drug; D004307:Dose-Response Relationship, Radiation; D018922:HL-60 Cells; D020450:Heavy Ions; D006801:Humans; D006878:Hydroxides / Q000378:metabolism; D018499:Linear Energy Transfer; D008954:Models, Biological; D017785:Photons; D011829:Radiation Dosage; D011837:Radiation-Protective Agents / Q000008:administration & dosage
chemical_list: D006878:Hydroxides; D011837:Radiation-Protective Agents; C031356:hydroxide ion; D004121:Dimethyl Sulfoxide