ConarD Effects of Ionizing Radiations 490 It appears that the 3 cases of thyroid cancer in the Rongelap people are the first to be clearly associated with radioiodine exposure with the possible exception of a single case reported by SHELINE etal. [28]. The incidence of thyroid cancer on a per rad basis in the Rongelap people (5.6 cases/10® people/rad/year) is not inconsistent with the incidence reported by others [17] following X-ray exposure of children. It does appear from these findings that radioiodines may be as carcinogenic as X-rays. Per- haps the reason for lack of development of thyroid malignancy in patients treated with radioiodineis related to the high dosesofirradiation used which might so damagethe glands as to preclude proliferative activity and malignant transformation. . . The incidence of benign nodules in the Marshallese is higher than pre- viously reported, but when considered on a risk/rad basis, the incidenceof 51 cases/10® persons/rad/year is not too different from that reported by Pincus et al. [25}.° The data on the Marshallese are too meagre to draw any valid conclusions about dose responserelationship associated with radioiodine exposure of the thyroid. Based on his Ann Arbor series and the Marshallese data, HEMPELMANN[17] postulated a linear dose response with no threshold (at least above 20 rads) for radiation-induced thyroid nodularity. There have been 3 additional cases of cancer (of the female genital tract) in the exposed Marshallese and though no definite malignancy has been re- ported in the unexposed comparison population, correlation of those malignancies with radiation exposureis uncertain [9]. In spite of a high incidence of ‘y-burns’ of the skin in the Marshallese there has been no evidence of late chronic radiation dermatitis or malignant transformation of the burned areas. Some scarring and pigmentaberrations are still evident. Since skin cancer usually has a latent period of 20 or some years, it may be too soon to expect such changesin the Marshallese [9]. 3 The question was asked this morning concerning the relative radiosensitivity of the thyroid gland of the child compared with the adult gland from radioiodines. From the Marshallese data, we could reason that the increased dose due to the smaller size of the gland was responsible for the higher incidence of nodularity. On the other hand since the infant thyroid grows from about 1-20 g at maturity requiring 4—5 cell divisions, there is a greater opportunity for expression of radiation-induced mutational changes and neoplastic transformation in the developing gland. In the adult glandcell division is seldom observed. Thoughit is not possible at this time to define a tolerance range for either the adult's or child’s thyroid from these data it seems prudent to use caution in the use of radioiodines, particularly in children where the additional possibility of growth and development retardation exists, * F \7 ji 1 SOS Te egg TOD OTTTe ty , ~ 1