74
myelogenous form, since his past hemograms
showed fairly consistent depression of neutrophil
counts comparedwith those of other exposed boys
of the same age.
Theinability to demonstrate clear-cut agingeffects in the exposed groupin spite of repeated attempts with a variety oftests is in accord with the
generally negative results of similar attempts in
the much larger exposed Japanese populations.
The absenceof radiation-inducedcataractsis
not unexpected since the dose to the lens was probably below threshold and neutrons (known to
have a higher RBE for cataract induction than
gammaradiation) were not involved in the Mar-
shallese exposure.
E. THYROID EFFECTS
The high incidence of thyroid neoplasms(in 27
of 86 exposed Rongelap people, including 3 with
carcinomas) and the developmentin somechildren of hypothyroidism and growth retardation
provideclear evidencefor the seriousness of thyroid
injury due to radioiodine absorbed from fallout associated with atomic detonations. Had notcareful
medical studies in the population been instituted
soon after the exposure and continuedtothepres-
ent date, the extent of thyroid effects might not
have been discovered.
Thyroid exposureis likely to be greater when
individuals are exposed within 100 to 150 miles of
the bomb if detonated near the ground. In such
situations, if lethal exposure to penetrating radiation does not occur, the principal hazard appears
to be the late developmentof thyroid tumors and
leukemia. Since thelatter is ultimately fatal, it is
moreserious than thyroid malignancies, most of
which are well differentiated and have an excellent prognosis. The relative incidence of thyroid
malignancies from radiation appears to be about
the sameas that of leukemia on a “per rad”basis.177
Theincidence of thyroid cancer is considerably
higher than the mortality from it. Of 40 cases
amongthe exposed Japanese, 34 wereliving in
1973, and only one death had beenattributed to
this cause. No evidenceof thyroid dysfunction had
been notedatthe timeof diagnosis. In the Marshallese reduced function was not found in two of
the cancer cases exposed as adults, but wasin the
case exposed as a child.
The high incidence of thyroid effects in children
exposed at < 10 years of age was no doubtrelated
to the higher dose to the child’s thyroid because of
its smaller size. The rapid growth of the glands
during childhood probably increased the chances
for neoplastic changes. The growth retardation
in someofthese children was thoughtto be related
to reduced thyroid function resulting in lower hormonelevels. This deficiency was not recognized
during the early years because offalsely high PBI
levels resulting from unusually high iodoprotein
levels, which turned out to occur generally in the
Marshallese and are now being furtherstudied.
Mostof the thyroid glands of the exposed people
undergoing surgery contained multiple nodules or
areas of adenomatous change. Many microscopic
areas, although considered benign, were composed
of discrete areas of atypical cells, suggestive in
somecases of malignant potential.
The risk of developing benign and malignant
neoplasms in the Rongelap people appeared to be
about the sameas that noted in people exposed to
x-radiation. Clinical experience with 131] sug-
gested that the risk would have beenless, but the
higher energy of the short-lived isotopes of iodine
(particularly 1371, 1331, and 1351), resulting in
higher dose rate and more uniform exposure ot
the thyroid,is thought to have been the important
factor in increasing the numberof thyroid abnor-
malities above that expected from similar doses
from 151f alone. It is not unreasonable to speculate
that tumorincidence in the Marshallese would
have been considerably smaller if only 1311 had
been involved in the exposure. The lesser amount
of short-lived iodine isotopes in the Utirik exposures (because of the later arrival of the fallout)
may have been an important factor in reducing
the dose effect to their thyroids, but the numberof
people involved is too small for any firm conclusion to be drawn.
Treatmentof the exposed Rongelap people with
thyroid hormone has been of benefit in enhancing
growth and developmentin the growth-retarded
children and in maintaining a normal metabolic
state in the operatedcases. It is not certain whether
it has prevented the development of thyroid
nodules.
The documentation of these thyroid effects has
importancenot only for the people involved but
also for the advancementof medical knowledge
and for planning with regard to Civil Defense and
remotely possible future accidents involvingrelease of radioiodines(e.g., from a nuclear power
plant). The Marshallese accident represents ex-