Abstract
Objectives: The present study was performed to investigate whether dose and dose-rate effects are observed on dicentric chromosome aberrations in splenocytes of mice exposed to wide ranges of doses (100~8,000mGy) at different dose-rates, i.e. low (LDRs: 1 mGy/22h/day and 20 mGy/22h/day:0.91 mGy/h) to medium dose-rates (MDRs:200 and 400 mGy/22h/day:18.2 mGy/h). Methods: SPF C3H female mice were continuously exposed to gamma-rays with LDRs and MDRs for different periods from the age of 56 days to a maximum of 615 days. Lymphocyte chromosome spreads were prepared from splenocytes from these mice. Results: Incidences of dicentrics (Dics) plus centric rings detected by conventional Giemsa staining and those of Dics detected by FISH increased almost linearly up to a total accumulated dose of 8000 mGy at LDR (20 mGy/22h/day). The values of linear term for linear or linear quadratic regression lines for these aberrations were obtained by multiple linear regression analysis adjusted for age-related differences. The values were significantly decreased as the dose rate was lowered from MDRs to LDRs, suggesting that chromosome aberrations are varied at dose rates ranging 1 to 400 mGy/22h/day, which is discrepant from the recommended formula by ICRP to estimate dose and dose-rate effectiveness factor (DDREF). DDREF of 4.5 for Dics was obtained by comparing chromosome aberration rates at the same total dose of 100 mGy, at high-dose-rate (HDR:890 mGy/min) and LDR (20 mGy/22h/day). DDREF was varied from 4.5 to17.8 as accumulated doses were increased from 100 to1000 mGy. Further, ratio of values of linear terms of the LDR to HDR was 3.0. Conclusions: DDREF for Dic decreased as dose-rates were lowered in lower dose range. These results are useful information for risk assessment of low-dose radiation effects in human. This study was performed under the contract with the Aomori Prefectural Government, Japan.
