ful review ofthe dose calculations Pincus et al'' and Hempelmann’* indicated that considerable expoin their studies of populations who sures from radioactive iodine absorphad been exposed to therapeutic tion had probably occurred particx-irradiation of the neck region at a ularly in the children. The exposure young age. However, when comof the Rongelap people was not parison is made on a risk-per-rad comparable to exposure of populabasis the incidence is quite comtions from fallout from reactor acparable with 51 cases per 10° percidents where radionuclides are sons per rad per year for the Marchiefly absorbed from contaminated shallese and 24 cases for one group milk obtained from cattle grazing and 64 for a second group 10° peron contaminated pastures. In the sons per rad per year calculated by Marshall Islands there are nocattle Pincus et al and Hempelmann. and no local milk supply. (Mother’s This comparison seems to indicate milk may have contributed to the similar effectiveness per rad of radioactive iodine absorption in two x-radiation compared with per-rad children who were reported to have dose from radioactive iodine expobeen nursing at the time of the sure. accident). But there was heavy conThe three malignant lesions of tamination of food and water sup- © the thyroid reported here in the plies on Rongelap and a relative heavily exposed Rongelap people abundance of radioactive iodines in appear to be the first such cases the fallout. The dose to the thyroid clearly associated with radioactive glands was greater by a factor of 2 iodine exposure with the exception in adults and a factor of about 7 in of the one possible malignant thychildren over that to other organs roid lesion reported by Sheline et of the body. al.° Numerous animal studies have Based on the Trust Territory demonstrated the role of radiation records of incidence of carcinoma in the etiology of thyroid neoof the thyroid among the 17,000 Marshallese, the expected incidence plasms.** In the human being the in the original 64 Rongelap people development of thyroid nodules and cancer form x-irradiation,’ particin the high-exposure group would ularly when the radiation of the be 0.056 cases over the 15-year gland occurs in infancy and childperiod. The finding of three cases hood''''* is well documented. Develthus represents a considerable increase over the expected numberof opment of such lesions from radioactive iodines has also been seen in cases (P > 0.01, chi square test). animals but has been Jess frequentAmong the 157 Utirik people about ly observed in human beings. Eight 0.14 cases would be expected with 1 patients with nodular goiter have case reported. It seems less likely been reported by Sheline et al® in that this single isolated case among their follow-up study of 250 pathe Utirik people would involve | tients treated for hyperthyroidism. radiation etiology in view of the Six of these patients were irradiated low dose received by this group and before 20 years of age and four bethe fact that no nodular lesions had fore 10 years of age. One patient been noted among the children. showed a possible invasion of the Based on the present incidence of thyroid capsule. thyroid malignancy in the highThe int¢idence of thyroid nodularexposure Rongelap group (based on ity in the exposed Marshallese the number of exposed people now ‘based on the number of exposed living}, the risk for development of people. now living) is considerably this malignancy per 10° persons per higher than has been reported by rad per year is three cases for the jaye. Oct 12, 1970 © Vol 214, No 2 children exposed at less than 10 years of age, ten 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.'® It has been generally believed - that radioactive iodine exposure was less effective than x-radiation in producing thyroid lesions based primarily on the fact that few thyroid tumors had been noted following radioactive iodine therapy.® It seems likely, however, that the scarcity of such findings is relative to the high doses of radiation used (5 to 10,000 rad or more in the treatment of hyperthyroidism and 50,000 rad or more for ablation of the gland to ameliorate symp- toms in cerfain. diseases). Such doses probably are so destructive that they preclude proliferative activity and malignant transformation in such damaged glands. The increasing incidence of hypothyroidism without tumor formation, years after treatment of hyperthyroid patients with radioactive iodines illustrates this point.’* It has been shown that tumor formation in animals is not always a dose- dependent phenomenon.'® Shellabarger et al'® showed that breast tumors in rats reached a maximum incidence at about 400 rad and the occurrence of neoplasms fell off with higher doses. Lindsey and Chaikoff* reported that doses of '*'] in rats in excess of 200 to 400 microcuries were less carcinogenic than lower doses. Marks and Bustad report similar findings in sheep.'7 Though the dose to the thyroid gland in the Marshallese was generally considerably below the dose of '"I used for therapy of hyperthyroidism it is likely that the doses received by some of these children were in excess of the optimum carcinogenic range and, therefore, the true risk per effective rad may be greater in this group. The paradoxical finding of greater risk Thyroid Neoplasta—Conard et al 323