Effect of high 131I doses to the thyroid gland on tumorigenicity of 90Sr and 90Y in mice.

The incidence of tumors was studied in mice injected with 90Sr only or with 90Sr in combination with high amounts of 131I. The high 131I-dose to the thyroid gland was necrotizing to the glandular tissue and the main aim of the investigation was the possible effects of the thyroidal destruction on the formation of bone tumors. After correction for competing mortality, no significant difference in the frequency of bone tumors could be found between 90Sr-treated and (90Sr + 131I)-treated mice. The incidence rate of bone tumors, however, was higher in mice with radiogenically destroyed glands than in those with intact glands. The limitations of using the concept of 'actuarial tumor incidence' in correction for competing mortality in animal experiments are discussed. Large numbers of lymphatic tumors were found in all animal groups. The frequencies of such tumors were independent of the radiation doses but their incidence rates were shortened in a dose dependent manner. Other, directly or indirectly radiation induced tumors were observed.