Effect of dose-rate and age at irradiation on cancer development in Sprague-Dawley rats exposed to gamma rays

The probability that rats develop tumors following a 3 Gy exposure to gamma rays from cobalt 60 was observed to depend on age at exposure and on dose rate. Dose rate reduction by a factor of 60 decreased the carcinogenic incidence (mainly carcinomas) by a factor of about 5. Lifetime excess of malignant neoplasia decreased by a factor of about 10 in 9 month-old rats as compared to animals irradiated in utero. The 3-month age group developed slightly fewer cancers than the group irradiated in utero, and tumor location was different. The higher incidence of tumors following in utero exposure was dependent to the high sensitivity of brain and gonads during organogenesis. (author)