ye a ace eae Bye De -10- When the head and neck are exposed to X-rays, up to 5 percent of exposed rats may develop thyroid cancer. Parabiont rats develop cancer of many organs readily when one of the pair is given 1000 rads of X-ray, but cancer of the thyroid is rare, Whole-body doses of X-rays that readily induce leukemia in most strains of mice very rarely produce thyroid cancer. Jodine-131 given to mice in large doses (delivering thousands of rads to the thyroid) will heavily damage the thyroid without causing cancer there, perhaps because of the many thyroid cells destroyed. However, this may produce adenomas or cancers of the pituitary gland, which is not itself significantly irradiated but is assumed to be stimulated to abnormal activity and hyperplasia by the absence of normal feedback from a functionally impaired thyroid. The animal data are inadequate to permit firm conclusions, but available information suggests that cancers of the thyroid are not easily induced by radiation and that radiation from iodine-131, largely restricted to the thyroid, is an even less efficient carcinogen in laboratory animals than are X-rays. CONCLUSIONS 1. Therapeutic doses of X-rays to the thyroid region of children have been followed after some years by the development of thyroid neoplasms. Whereas the percent of cases of malignant neoplasms is small, the proportion of persons irradiated who develop nodular thyroid disease can be extremely high. The incidence of radiationinduced thyroid disease is strongly dose dependent above 100 rads (thyroid dose). The shape of the response curve below 100 rads is unknown. 2. X-rays are probably as effective if not more so than iodine-131 in producing thyroid lesions for equal, average absorbed doses delivered to the gland at similar rates. An apparent greater effectiveness ot X-ray irradiation may be due to the higher dose rate used, 3. Whereas it was formerly believed that the induction of thyroid tumors was enhanced by irradiation of tissues other than the thyroid itself, it now seems possible to explain variability in tumor induction in children on the basis of whether or not the gland was in the primary X-ray beam. 4. Radioactive iodine in amounts sufficient to deliver several hundred rads to the thyroid of the infant or young child has been shown to produce a high incidence of thyroid nodules. Radioactive iodine has been shown to be carcinogenic in some animals. No case of thyroid cancer clearly ascribable to it has been reported in man. DOE ARCHIVES