Relative biological effectiveness of Cf-252 with teratogenicity as index

Teratogenicity of Cf-252, having the neutron-to-gamma ratio similar to that of Hiroshima type atomic bombs, was examined with rat fetuses. The whole body of rats were irradiated in a single fraction with graded doses of Cf-252 (10 - 180 cGy) on gestation Days 7, 8, 9, 10, and 11 days. On gestation Day 18, their fetuses were examined for the external body and organ anomaly. Co-60 gamma-ray served as control. The highest incidence of anomaly was observed in implanted fetuses (85.7 %) and survival fetuses (100 %) when fetuses were irradiated with 80 cGy of Cf-252 on gestation Day 8. Relative biological effectiveness (RBE) was calculated to be 3.1 to 3.3. Anomaly in the heart and major blood vessels was a common manifestation in both Cf-252 and Co-60 groups. Regarding LD50 for fetuses as an index, RBE was found to be 2.3 in the group exposed on gestation Day 8 and 3.2 in the group exposed on gestation Day 9. These results suggest a strong teratogenicity of Cf-252. (Namekawa, K.)