62 genic effect in rats and dogs is equal on a rad basis radiation with a higher dose rate and longer range gives a more uniform tissue distribution of radia- tion than is produced by 1311. This might explain whythe incidenceof thyroid lesions in the children receiving 1000 rads falls 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 151] alone would have been (see Cole!37), Thus the two boys with atrophy of the thyroid may have had doses to the gland com- 40.0 + - 33.0 F 4 30.0r 25.0b 20.0 , 15.0 10.0 5.06 ce 0 3 10 15 20 25 30 35 40 $ | Child Fetal age, in weeks At birth| Adult parable with 4000 to 6000 rads or more of xirradiation. 3. The Development of Thyroid Abnormalities From in Utero Exposure The development of thyroid adenomas in one of the Marshallese boys exposed in utero is of interest. Four children were exposed in utero: in the higher radiation group, one at the endofthe second trimester and twoat the end ofthe first 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 ia utero exposure during treatment of the mother with large doses of radioiodine.!38-141 However, this Marshallese boy appears to be the first reported case of thyroid adenomas developing presumably from in utero exposure to radioicdines. In utero thyroid dose calculations are not possible, since insufficient data are available on fetal thyroid uptake of radioiodines from the mother at various stages of gestation. !42 Figure 48 showsthyroid function of the humanfetus. The gland is 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 gamma radiation but the dose from radioiodines is uncertain: In view of the high uptake of radioiodines per gram by thefetal thyroid at 22 weeks, the thyroid dose might be expected to be high. However, since it appears from our data that I m bbdbikdeow ald to that of x rays. Walinder et al. have reported similar findings.134.135 Dunning,!96 in estimating thyroid dose from absorption of radioiodines in fallout, considers the shorter-lived isotopes to be + times as energetic as '51[. The more energetic beta Iniant Figure +8. Thyroidal uptake of 1211 (% per gram) versus age. (From Evans et al.1#3) longer latent periods are associated with lower doses of radiation to the thyroid,itis 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 motherof this boy has not developed any thyroid abnormalities and appears to be euthyroid. The two children exposed at the end ofthefirst trimester probably had nonfunctioning primordial thv- roid glands at that timeso that the glands received only the gammaexposure and werethereforeless likely to develop thyroid abnormalities. None of the four children exposed in utero has shown microcephaly or mental retardation, which have been noted in some of the children exposedin utero in Japan. V. Neoplasia Amongthelate effects of radiation, the enhancement of the development of neoplasia has been weil documented. In irradiated animals, malignancies not only may be inducedat an earlier 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