Thyroid Tumors Following 131 I or Localized X Irradiation to the Thyroid and Pituitary Glands in Rats

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Abstract
Six-week-old female Long-Evans rats (3000) were randomly assigned to 10 equal treatment groups. Three groups were injected i.p. with 0.48, 1.9 and 5.4 .mu.Ci of Na 131I yielding mean thyroid doses of 80, 330 and 850 rad, respectively. Three groups were irradiated with 94, 410 and 1060 rad from localized X ray to the thyroid. One group was irradiated with 410 rad to the pituitary, and another group was given 410 rad to both the thyroid and the pituitary with localized X rays. The remaining 2 groups of animals were used as separate sham-irradiated controls for the 2 types of radiation. All the surviving animals were killed 2 yr later. The proportion of animals with thyroid carcinoma is similar for 131I and X irradiation within the dose range of 0-1000 rad. The thyroid carcinoma dose-response functions fitted by the least-squares method are nearly proportional to the square root of the thyroid dose. Thyroid carcinoma induction appears to be independent of the dose rates resulting from the radiations used. A localized X-ray dose of 410 rad to the pituitary, whether the dose was administered concomitantly with thyroid irradiation or without thyroid irradiation, did not modify the risk of thyroid tumor.