Abstract
The reversed dose rate effects of mutation induction are often reported for fission neutron radiation. Our former investigation revealed that the reversed dose rate effects might be caused by hypersensitivity of G2/M cells for mutation induction by high LET radiation. To clarify whether this phenomenon could occur in any high LET radiations, a novel hyper-sensitive mutation detection system has been developed. The system uses hamster fibroblast cells carrying a normal human X-chromosome, and we found that the mutation frequencies obtained from our system is 100-fold higher than that from conventional system using an internal Hprt gene. This suggests that the system appears to be able to detect a wide spectrum of mutations, even mutations that affect expression of any important genes in neighbor of the Hprt. Using the system, we analyzed low dose-rate effects of heavy particle beam (carbon 290 MeV/u beam) with different LETs, Because a lower limit of radiation dose for mutation induction with the system was found to be around 0.1 Gy, this enabled us to perform cell irradiation at lower dose rate about 0.84 mGy/min. Apparent reversed-dose rate effect was observed for higher LET (66 keV/µm) but not for lower LET (13 keV/µm). The results suggest that the reversed dose-rate effects occur dependent on radiation LETs.
