Abstract
Chromosome aberration rates of splenocytes from specific pathogen-free (SPF) mice after continuous and long-term exposure to low-dose-rates (LDRs; 1 and 20 mGy/day) gamma-rays were compared to those following high-dose- rate (HDR; 890 mGy/min) and medium-dose-rates (MDRs; 200 and 400 mGy/day)-gamma rays. Incidences of dicentric plus centric rings (Dic plus Rc) detected by conventional Giemsa staining, and dicentric chromosomes (Dic by FISH) detected by fluorescence in situ hybridization (FISH) using a centromere probe, increased almost linearly up to a total accumulated dose of 8000 mGy following irradiation for about 400 days at a LDR of 20 mGy/day. The values of regression coefficient in the linear regression lines for these unstable-type aberrations, obtained by multiple liner regression analysis adjusted for age-related differences, decreased as the dose rate was lowered from MDR to LDR, suggesting clear dose-rate effects in the range of 400-fold different dose rates (1-400 mGy/day), and also discrepancy with the current formula based on DNA repair to estimate dose and dose-rate effectiveness factor (DDREF). Therefore, DDREF estimated by the ratio of calculated incidences using the best-fit regression lines at HDR and LDR (20 mGy/day), was 4.5 for Dic by FISH and 5.2 for Dic plus Rc, respectively, at the same dose of 100 mGy, while different DDREF values were obtained for each of the aberrations and accumulated doses. These findings provide for the first time important information about the biological effects of long-term exposure to low-dose-rate radiation on chromosomes. This study was performed under contract with the Aomori Prefectural Government, Japan.
