title: Relative biological effectiveness for photons: implication of complex DNA double-strand breaks as critical lesions.
abstract: Current knowledge in radiobiology ascribes the adverse biological effects of ionizing radiation primarily to the induction of DNA double-strand breaks (DSBs), which is supposed to be potentially lethal and may be converted to lethal damage due to misrepair. Soft and ultrasoft x-rays have been found to bear elevated biological effectiveness for cell killing compared with conventional x-rays or60Co γ-rays. This phenomenon is qualitatively interpreted as the increased level of DSB induction for low energy photons, however, a thorough quantitative reasoning is lacking. Here, we systematically compared the relative biological effectiveness (RBE) with relative DSB induction for photons from several hundreds of eV up to MeV. Although there is an approximate two-fold increase in the yields of DSB for low energy photons found in our calculation and a large number of experimental measurements, it is far from enough to account for the three- to four-fold increase in RBE. Further theoretical investigations show that DSB complexity (additional single-strand breaks and base damage within 10 base pairs) increases notably for low energy photons, which largely reconciles the discrepancy between RBE and DSB induction. Our theoretical results are in line with accumulating experimental evidence that complex DSBs are refractory to repair machinery and may contribute predominantly to the formation of lethal damage.
authors.affiliation: State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing 100871, People's Republic of China.
authors.forename: Ying
authors.initials: Y
authors.lastname: Liang
authors.forename: Qibin
authors.initials: Q
authors.lastname: Fu
authors.forename: Xudong
authors.initials: X
authors.lastname: Wang
authors.forename: Feng
authors.initials: F
authors.lastname: Liu
authors.forename: Gen
authors.initials: G
authors.lastname: Yang
authors.forename: Chunxiong
authors.initials: C
authors.lastname: Luo
authors.forename: Qi
authors.initials: Q
authors.lastname: Ouyang
authors.forename: Yugang
authors.initials: Y
authors.lastname: Wang
mesh_terms: D003198:Computer Simulation; D053903:DNA Breaks, Double-Stranded / Q000528:radiation effects*; D004307:Dose-Response Relationship, Radiation; D005720:Gamma Rays; D006801:Humans; D008954:Models, Biological; D017785:Photons; D011839:Radiation, Ionizing; D012062:Relative Biological Effectiveness; D014965:X-Rays