Abstract
To investigate the mechanisms of the suppression of incidence of methylcholanthrene(MC)-induced fibrosarcoma by low dose-rate irradiation, I examined the rejective effect against tumor cells in irradiated and non-irradiated mice using TD50 (tumor dose 50) assay. C57BL/6N(B6) mice were irradiated with 137-Cs gamma-rays at 0.4-1.2mGy/hr, X-rays at 2.4-30Gy/hr, or 60-Co gamma-rays at 96mGy/hr until total dose reached 250mGy. Then, various number of tumor cells prepared from MC-induced fibrosarcoma were injected to estimate TD50 values. An increased TD50 value, by a factor of 3-5, was observed only in the irradiated mice with 0.4-1.2mGy/hr. But the values in irradiated mice with 250mGy at 96mGy/hr and 2.4-30Gy/hr don't increase compared to the value in non-irradiated mice. This result suggests that the enhancement of the rejective effect against tumor cells is induced by low dose-rate irradiation, about 1mGy/hr, and a specific response of low dose-rate irradiation .
