Stimulation of anti-tumor effect by low-dose irradiation. Pt. 2. The prolongation of life span in AKR mice

To elucidate the antileukemic effect of low-dose X-irradiation, we studied the influence of periodical low-dose X-irradiation on survival and tumor incidence of thymus using AKR mice. The findings of the experiments were as follows; (1) The median survival time of control AKR mice was 283±3 days. It of irradiation group of 15 cGy/week and 30cGy was 309±14 days and 316±10 days respectively. The life span was significantly prolonged (p < 0.05 and p < 0.01 respectively by Wilcoxon test) by periodical low-dose X-irradiation in term of breeding. (2) The incidence of thymus tumor which is observed remarkably in control AKR mice was 48.8%. It of irradiation group of 15 cGy/week and 30 cGy/week was 40% and 20% respectively. Inversely, the non-tumor incidence of tymus in control AKR mice was 19.5%. It of irradiation group of 15 cGy/week and 30 cGy/week was 32.5% and 51.4% respectively. The thymic tumor incidence was significantly decreased (p < 0.01 by chi-square test) in irradiation group of 30 cGy/week. (3) The incidence of thymic lymphoma as a death cause in control AKR mice was 80.4%. It of irradiation group of 15 cGy/week and 30cGy/week was 67.5% and 48.6% respectively. The incidence of thymic lymphoma was significantly decreased (p < 0.05 by chi-square test) in irradiation group of 30 cGy/week. (author)