30 cases (5.6% incidence) thus represents a considerable increase over the expected number( <0.01, x? test). Among the 157 Utirik people about 0.14 cases would be expected, and 1 case is reported. It seemsless likely that this single case would involve radiation etiology in view of the low dose received by this group andthefact that no nodular lesions had been noted among the children. Based on thepresent incidence of thyroid malignancyin the high exposure Rongelap group,the risk of this malignancy developing — per 108 persons per rad per year — is 3 cases for the children exposed at <(10 years of age, 10 cases for the older people, and 5.6 cases for the group as a whole. The risk in the Marshallese children is not inconsistent with that reported by others.34 Radioiodine exposure has been generally considered less effective than x-radiation in producing thyroid lesions, primarily because few thyroid tumors have been noted following radioiodine therapy.76 It seems likely, however, that the scarcity ofsuchfindings is related to the high doses ofradiation used (5000 to 10,000 rads or more in thetreatment of hyperthyroidism and 50,000 rads or more for ablation of the gland to ameliorate symptoms in certain diseases). Such doses probably are so destructive that they preclude proliferative activity and malignant transformation. The increasing incidence of hypothyroidism without tumorformation, years after treatment of hyperthyroid patients with radioiodines,illustrates this point. Tumorformation in animals is not always a dose dependent phenomenon.*5 Shellabarger et al.36 showedthat the incidence of breast tumors in rats reached a maximum atabout 400 rads andfell off with higher doses. Lindsay etal.29 reported that in rats doses of 131] >>200 to 400 pCi wereless carcinogenic than lower doses. Mark and Bustadreported similar findings in sheep.37 Although the dose to the thyroid gland in the Marshallese was generally considerably below the dose of 131I used for therapy of hyperthyroidism, the doses received by someof the children were probably above the optimum carcinogenic range andtherefore the true risk per effective rad may begreaterin this group. The paradoxicalfinding of greaterrisk in the older group appears to be in line with this 5006237 reasoning. The two stunted Marshallese bovs who showed almost complete atrophyof the thyroid gland with no evidence of nodular development are probably comparable with cases of hypo- thyroidism developing in patients years after radioiodine therapy. The thyroid exposures of the Rongelapese wereslightly different from those of patients treated with 1311 because their thyroids were not hyperplastic when exposed,at least part of the radioiodineisotopes were of shorter half-life than '3!T, and their exposure was complicated by gammaradiation. Somefactors secondary to radiation exposure might have enhanced the developmentof thyroid lestons in the Marshallese. Iodine deficiency or goitrogensin the diet did not appear to be among these. However,the physiological stresses ofpuberty and pregnancy mayhaveplayed role in the de- velopmentofthe lesions. For instance, the development of 10 cases of nodular goiter in the children during or near the time of puberty might indicate that this stress may have enhanced nodular development. In 3 females who later developed thyroid nodules the demand of multiple pregnancies may have beenrelated. Since the latent period between exposure and nodule discovery varied only between 10 and 13 years, it may be argued that developmentof thyroid nodules was independent ofthese stresses. On the other handit is noteworthy that all the women who developed malignant thyroid lesions had had muliple pregnancies. The occurrenceof thyroid nodules in the Marshallese was only slightly higher in females than | in males (ratio of 1.3 to 1.0). This is similar to the nearly equalsex distribution reported by Toyooka et al.33 for thyroid nodules developing in persons irradiated over the neck region in infancy. However, in the case of carcinoma of the thyroid the expected female preponderance occurred.?& The insidious developmentof growth retarda- & nfl SDB aye me ee Le . Based onthe incidence of carcinoma of the thyroid amongthe 17,000 Marshallese reported by the Trust Territory, the expected incidence in the Rongelap high exposure group would be 0.056 cases over the 15-year period. The finding of 3 tion in some of the Marshallese children before i clinical evidence of thyroid abnormality or defi- § ciency was recognized demonstrates the apparent L sensitivity of growth and developmentprocesses to borderline or subclinical thyroid deficiency. All § possible steps are being taken to help the children §& adhereto the present thyroid treatment schedule & so that they will achievesatisfactory growth and % maturation. 4 Careful medical surveillance of these exposed people, including the Utirik, will be essential in future years. The latent period for the develop- §