Tanaka, K., Kohda, A., Satoh, K., Toyokawa, T., Ichinohe, K., Ohtaki, M. and Oghiso, Y. Dose-Rate Effectiveness for Unstable-Type Chromosome Aberrations Detected in Mice after Continuous Irradiation with Low-Dose-Rate γ Rays. Radiat. Res. 171, 290–301 (2009).

Chronological changes in the chromosome aberration rates of splenocytes from specific-pathogen-free (SPF) mice after continuous and long-term exposure to low-dose-rate γ rays were studied. Incidences of dicentrics plus centric rings (Dic Rc), detected by conventional Giemsa staining, and dicentric chromosomes, detected by fluorescence in situ hybridization (Dic by FISH) using a centromere probe, showed an essentially linear increase up to a total accumulated dose of 8000 mGy after irradiation for about 400 days at a low dose rate of 20 mGy/day. For comparison, acute high-dose-rate and medium-dose-rate irradiation were performed. The values of the α coefficients in the linear regression lines for these unstable-type aberrations decreased as the dose rates were lowered from medium dose rates (200 and 400 mGy/day) to low dose rates (1 and 20 mGy/day). The dose and dose-rate effectiveness factor (DDREF), estimated by the ratio of calculated incidences using the best-fit regression lines at a high dose rate (890 mGy/min) and low dose rate (20 mGy/day), was 4.5 for Dic by FISH and 5.2 for Dic Rc, respectively, at the same dose of 100 mGy, while different DDREFs were obtained for different accumulated doses. This is the first study to provide information regarding the effects of long-term exposure to low-dose-rate radiation on chromosomes.