Te eeaes geee ar eTae ag = oh eT vee TD > When the head and neck are exposed to X-rays, up to 5 perceyt of exposed rats may develop thyroid cancer. Parabiont rats develop cancer qf many organs readily when one of the pair is given 1000 rads of X-ray, Jbut cancer of the thyroid is rare, Whole-body doses of X-rays that readily induce leukemia in of mice very rarely produce thyroid cancer. Iodine-131 given to large doses (delivering thousands of rads to the thyroid) will h damage the thyroid without causing cancer there, perhaps because many thyroid cells destroyed. However, this may produce adenoma of the pituitary gland, which is not itself significantly irradi assumed to be stimulated to abnormal activity and hyperplasia by of normal feedback from a functionally impaired thyroid. or cancers ed but is he absence The animal data are inadequate to permit firm conclusions, b able information suggests that cancers of the thyroid are not eas by radiation and that radiation from iodine-131, largely restrict thyroid, is an even less efficient carcinogen in laboratory anima are X-rays. CONCLUSIONS 1. Therapeutic doses of X-rays to the thyroid region of children been followed after some years by the development of thyroid plasms. Whereas the percent of cases of malignant neoplasms small, the proportion of persons irradiated who develop nodul thyroid disease can be extremely high. The incidence of radi induced thyroid disease is strongly dose dependent above 100 (thyroid dose). The shape of the response curve below 100 ra unknown. 2. X-rays are probably as effective if not more so than iodine-1l producing thyroid lesions for equal, average absorbed doses d livered to the gland at similar rates. An apparent greater e tiveness ot X-ray irradiation may be due to the higher dose r used. 3. Whereas it was formerly believed that the induction of thyroi tumors was enhanced by irradiation of tissues other than the thyroid itself, it now seems possible to explain variability tumor induction in children on the basis of whether or not th gland was in the primary X-ray beam. 4. Radioactive iodine in amounts sufficient to deliver several hindred rads to the thyroid of the infant or young child has been sh to produce a high incidence of thyroid nodules. Radioactive iodjme has been shown to be carcinogenic in some animals. No case o thyroid cancer clearly ascribable to it has been reported in dan. DOE ARCHIVES (2