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