Clonogenic assay. A and B described the cell survival condition after irradiation. X-ray irradiation was performed as planed in 6 Gy/fraction and 2 Gy/fraction with total dose 60 Gy. An appropriate number of cells in different generations were seed in 6-well plates. Each group was irradiated at the dose of 0 Gy, 1 Gy, 2 Gy, 4 Gy, 6 Gy, 8 Gy, 10 Gy, respectively. After incubation for 14 days, the colonies were fixed and stained by crystal violet. The coloniesâ€‰>â€‰50 cells were scored as viable colonies. The data was analyzed by single-hit multi-target model and survival curve of each group was described by Graphpad prism 5.0 software.