Abstract
Whether or not dose rate effects is observed in low dose rate region less than 60 mGy- 600 mGy/h has not been well studied. C3H mice of 8 weeks age were chronically exposed to up to 700 days with 20 mGy/day (approximately 1mGy/h) and 500 days with 1mGy/day (approximately 0.05mGy/h). Chronological change of chromosome aberration rates in spleen cells were observed along with accumulated doses in these two low dose rates. Unexpectedly, unstable-type aberrations were increased in two phase manner within 0-2Gy and 4-14Gy in 20mGy/day exposure, and also they were slightly increased up to 0.5Gy in 1mGy/day exposure, although such an unstable type of aberrations has been considered to increase up to some dose range and they are saturated after that. Chromosome aberration rates in 20mGy/day and 1mGy/day were compared at the same total doses at 0.5Gy and 0.25Gy, which were shown as 2.0 vs. 0.53, and 1.0 vs. 0.47 respectively. In the calculation, chromosome aberration rates increased by age were subtracted from the observed aberration rates. Dose-rate effect was observed in the low dose rates region. An increase of unstable type of aberrations along with the accumulated doses may be caused from balance between cell growth and apoptosis, long lived lymphocytes or chromosomal instability. These results will be important to understand the mechanisms of low dose rate radiation –induced cancer development and risk assessment. The present study was supported by grant-in aid from Aomori Prefecture.
