Suppressive effect of long-term low-dose rate gamma-irradiation on chemical carcinogenesis in mice

Female ICR mice, 6 weeks old, 35 in each group, were exposed to gamma-rays from a Cs-137 source in the long-term low-dose rate irradiation facility at the Central Research Institute of Electric Power Industry (CRIEPI). The dose rate was 2.6 (A), 0.96 (B), or 0.30 mGy/h (C). Thirty-five days later, the mice were injected in the groin with 0.5 mg of methylcholanthrene (MC) dissolved in olive oil and irradiation was continued. Cumulative tumor incidences after 216 days following MC injection were 89% in group A, 76% in group B, and 94% in group C. The one in the non-irradiated control group was 94%. The difference in the tumor incidence between the control and position B was statistically significant, indicating the suppressive effect of the low-dose rate irradiation on the process of MC-induced carcinogenesis with an optimum dose rate around 1 mGy/h. (C) 2002 Elsevier Science B.V All rights reserved.