Effects of hypersensitivity and induced radioresistance in CHO-K1 cells following exposure to gamma-rays and accelerated carbon ions

The study presents results of investigations on chromosome aberrations (CA) yield in Chinese hamster ovary cells after exposure to g-rays and accelerated carbon ions (455 MeV/amu) at doses less than 1 Gy in the pristine Bragg curve plateau and behind the Bragg peak, where normal tissues are situated in the course of radiotherapy. Initial parts of dose curves for total CA and terminal deletions frequencies differed from linear-quadratic dependence and the region of induced radioresistance were observed at 0.1-0.6 Gy for g-rays. The similar curve shapes were detected for carbon ions, but plateau region was shorter (0.15-0.35 Gy). Despite the uniform shape of the dose curves for the CA yield, a definite dependence of the cytogenetic effect on the linear energy transfer (LET) is observed. The CA frequency increased with LET rising in the order: g-irradiation (0.2 keV/μm), carbon ions at the Bragg curve plateau (10-12 keV/μm) and at the «tail» of Bragg peak (25-27 keV/μm). Outside this range, the yield of chromosome aberrations also enhanced with increasing LET in the same order. The results obtained confirm that the hypersensitivity and induced radioresistance phenomena are characteristic for low level exposure to low-LET and middle-LET radiations when chromosome aberration test is used.