Abstract
Dose-dependant increase of chromosome aberrations was found in the SPF C3H/HeN mice continuously irradiated with γ-rays at a low-dose-rate (LDR) of 20 mGy/22 h/day (0.91 mGy/h) from the age of 8 weeks up to a maximum of about 400 days. Translocations and dicentric chromosomes detected by M-FISH were increased almost in linear up to a total accumulated dose of 8000 mGy. Clones were appeared at 4000 mGy, and rapidly increased over 6000 mGy. Dose and dose-rate effectiveness factor (DDREF) was 2.6-4.1, varied for types of chromosome aberrations, by comparison of aberration rates at the same dose of 500 mGy between HDR (890 mGy/min) and continuous LDR (20 mGy/22 h/day) γ-irradiations.
Furthermore, mice were kept in SPF condition for 200 days or more, after continuous LDR irradiation for 200 days (4,000 mGy), it was examined whether or not chromosome aberration was reduced. Both frequencies of translocations and dicentric chromosomes were slowly decreased at 150 days after irradiation, but not down to those levels of age-matched non-irradiated control mice. A cause of slow reduction of dicentric chromosomes after irradiation can not only be explained by life span of splenic lymphocytes, but also by turn-over of splenic lymphocytes, in which more number of immature lymphocytes recruited into spleens from bone marrows or other tissues. These results suggest that mechanism for the effects of LDR irradiation on splenic lymphocytes could be different from that for HDR irradiation. The lymphocytes with dicentric chromosomes persisting for more than 150 days after irradiation without rapid reduction might be related to dicentric chromosomes increasing almost in linear depending on accumulated doses during the continuous LDR irradiation. These will be important information for radiation risk assessment of LDR radiation exposure. This study was performed under contact with Aomori Prefecture Government, Japan.
