62 receiving 1000 radsfalls on a line corresponding to 1000 rads of x-radiation (Figure 44), or it might indicate that the radioiodine exposure was about 10 times as effective as 1311 alone would have been (see Cole!37). Thus the two boys with atrophy of the thyroid may have had doses to the gland com- i 40.0 i 35.0 - 1 30.0 F | 23,0: 20.0 + 15.0 10.05 30 s 0 : Fetal age, in weeks | | Child | At birth | parable with 4000 to 6000 rads or more of xirradiation. as genic effect in rats and dogs is equal on a rad basis to that of x rays. Walinder et al. have reported similar findings.'34.135 Dunning,!36 in estimating thyroid dose from absorption of radioiodines in fallout, considers the shorter-lived isotopes to be 4 times as energetic as 1311. The more energetic beta radiation with ahigher dose rate and longer range gives a more uniform tissue distribution of radiation than is produced by 1411. This might explain why the incidence of thyroid lesions in the children Adult Infant Figure 48. Thyroidal uptake of !3!I (% per gram) versus age. (From Evanset al.143) 3. The Developmentof Thyroid Abnormalities From In Utero Exposure The development of thyroid adenomas in one of the Marshallese boys exposed zn utero is ofinterest. Four children were exposed in utero: in the higher radiation group, one at the endofthe secondtrimester and two at the end ofthefirst trimester; in the lower dose Rongelap group, one at the end of the second trimester. Only the boy in the higher dose group and exposed in the second trimester has developed thyroid lesions. Several cases have been reported of children who had myxedema following in utero exposure during treatmentof the mother with large doses of radioiodine. 138-141 However, this Marshallese boy ap- pears to be the first reported case of thyroid adenomas developing presumably from in utero exposure to radioiodines. Zn utero thyroid dose calculations are not possible. since insufficient data are available on fetal thyroid uptake of radioiodines from the motherat various stages of gestation.142 Figure 48 showsthyroid function of the humanfetus. The glandis not thoughtto begin to function and to accumulate iodine until about the 12th week of gestation.144.145 At the time of exposure of the Marshallese boy (about 22 to 24 weeks) the thyroid should have been actively functioning. He received 175 rads of gammaradiation but the dose from radioiodines is uncertain. In view of the high uptake of radioiodines per gram bythefetal thyroid at 22 weeks, the thyroid dose might be expected to be high. However, since it appears from our data that longer latent periods are associated with lower doses of radiation to the thyroid, it is likely that the thyroid in this boy exposed in utero received a lower dose than that in Marshallese children exposed at a young age. Also, this boy and the others exposed in utero have shown no impairment in growth and development. It is of interest that the motherofthis boy has not developed any thyroid abnormalities and appears to be euthyroid. The two children exposed at the endofthefirst trimester probably had nonfunctioning primordial thyroid glands at that time so that the glands received only the gamma exposure and were thereforeless likely to develop thyroid abnormalities. None of the four children exposed in utero has shown microcephaly or mental retardation, which have been noted in someof the children exposed in utero in Japan. V. Neoplasia Amongthelate effects of radiation, the enhancement of the development of neoplasia has been well documented. In irradiated animals, malignancies not only may be induced at anearlier age but appear in increased numbers. In the Japanese atom bombsurvivors, leukemia and thyroid neoplasia have shown the mostclear-cut association with radiation exposure, though more recently