Dependence of the yield of DNA double-strand breaks in Chinese hamster V79-4 cells on the photon energy of ultrasoft X rays

Induction of DNA DSBs by low-LET radiations reflects clustered damage produ ced predominantly by low-energy, secondary electron "track ends". Cell inac tivation and induction of DSBs and their rejoining, assayed using pulsed-fi eld gel electrophoresis, were determined in Chinese hamster V79-4 cells irr adiated as a monolayer with characteristic carbon K-shell (C-K) (0.28 keV), aluminum K-shell (Al-K) (1.49 keV), and titanium K-shell (Ti,) (4.55 keV) ultrasoft X rays under aerobic and anaerobic conditions, Relative to Co-60 gamma rays, the relative biological effectiveness (RBE) for cell inactivati on at 10% survival and for induction of DSBs increases as the photon energy of the ultrasoft X rays decreases. The RBE values for cell inactivation an d for induction of DSBs by C-K ultrasoft X rays are 2.8 +/- 0.3 and 2.7 +/- 0.3, respectively, and by Ti-K ultrasoft X rays are 1.5 +/- 0.1 and 1.4 +/ - 0.1, respectively. Oxygen enhancement ratios (OERs) of similar to2 for ce ll inactivation and induction of DSBs by ultrasoft X rays are independent o f the photon energy. The time scale for rejoining of DNA DSBs is similar fo r both ultrasoft X rays and Co-60 gamma rays. From the size distribution of small DNA fragments down to 0.48 kbp, we concluded that DSBs are induced r andomly by C-K and Al-K ultrasoft X rays. Therefore, ultrasoft X rays are m ore efficient per unit dose than gamma radiation at inducing DNA DSBs, the yield of which increases with decreasing photon energy. (C) 2001 by Radiati on Research Society.