Radiation-induced bystander cell-killing effect depends on time after irradiation

Yokota, Yuichiro; Funayama, Tomoo; Muto, Yasuko; Ikeda, Hiroko; Kobayashi, Yasuhiko

In this study, we investigated a time dependency of bystander effect. In experiments, we irradiated normal human fibroblasts with carbon-ion microbeam (LET=103 keV/m, 10 particles/irradiated site), carbon-ion broad beam (0.13 Gy) and Co-60 -rays (0.2 keV/m, 0.5 Gy). In broad-beam and -ray irradiation, irradiated cells and non-irradiated cells were co-cultured with porous membrane. The ratio of irradiated cells and bystander cells was 1:20,000 in microbeam irradiation and 1:2 in broad-beam and -ray irradiation, respectively. In microbeam-irradiated samples, the survival rate of bystander cells did not change at 6 h but decreased to about 80% at 24 h. In broad-beam and -ray irradiated samples, the survival rate of bystander cells decreased to 80 to 90% at 6 h or later. From these results, it was found that bystander effect is dependent on time after irradiation and number of irradiated cells.