Abstract
The purpose of this study is to clear a time dependency of radiation-induced bystander cell-killing effect. Thus, we irradiated normal human fibroblasts with carbon- and neon-ion microbeam (LET=103 keV/μm and 380 keV/μm, respectively), carbon-ion broad beam (108 keV/μm) and γ-rays (0.2 keV/μm). Survival rate of bystander cells was measured after 6-24 h co-culture with irradiated cells. The ratio of irradiated and bystander cells was <0.0005:1 in microbeam irradiation and 0.5:1 in broad-beam and γ-ray irradiation, respectively. In microbeam-irradiated samples, the survival rate of bystander cells did not change at 6 h but decreased to about 85% of control at 24 h. In 0.13-Gy broad beam and 0.5-Gy γ-ray irradiated samples, the survival rate of bystander cells decreased to 80 to 85% of control at 6 h or later. From these results, it is found that bystander cell-killing effect is delayed when irradiated cells are extremely less than bystander cells.
