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