Abstract
We reported the suppressive effect of low dose-rate irradiation on tumorigenesis in methylcholanthrene (MC)-injected mice. To elucidate the mechanism underlying the tumor suppression we examined the tumor cell rejection, using a TD50 (tumor dose 50) assay, in irradiated mice. Female C57BL/6N mice were irradiated with 137Cs γ-rays at 0.4, 0.7, or 1.2 mGy/hr for 1-8 weeks. Then appropriate numbers of tumor cells prepared from a MC-induced tumor were injected into groins and axillae of mice to estimate TD50 value. Mice received a total dose of 250 mGy, regardless of dose rate, showed maximum TD50 value (3-5 fold compared to the non-irradiated controls). The increase in TD50 was not observed at total dose of 50, 150, 400, 750, or 1200 mGy. These results indicated that the ability of tumor cell rejection was increased at a certain condition and that the low dose-rate irradiation never decreased the tumor rejection even at as much as 1200mGy.
