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Thyroid dose (rads)
Figure 44. Incidence of benign thyroid nodules.
partly related to the smaller contribution to the
dose from short-lived isotopes of iodine. Nevertheless the Ann Arbor children in Hempelmann’s
study!07 had an increased incidence of thyroid tumors following a mean dose of only 30 rads to the
gland (Figure 44), and more recently Modanet
al.429 and Harley et al.!!2 have reported anincreased incidence of thyroid tumors in children
who had received about 6.5 rads to the thyroid
gland during x-ray treatmentof the scalp for fungus infection. Modan etal. state that ‘one would
strongly suspect a mini-epidemic of thyroid adenoma” in the group studied. In considering the
risk data of Hempelmann’s and Modan’s groups,
the large element ofJewish people (who haveincreased susceptibility to thyroid tumors) should
be kept in mind. Also, Hempelmann’s wereirradiated as infants and therefore represent only a narrow age range.
1. Latent Period
In Figure 45 the developmentof thyroid abnormalities is plotted according to radiation dose and
umeafter exposure; the latent period appears to
be longer with lower doses. Figures 46 and 47 show
the relationship between dose and age at developmentof thyroid lesions. These data indicate that
the radiation-inducedlesions occurat earlier ages
in the exposed Rongelap people than in the Utirik
or unexposed groups. In the latter groups almost
all the thyroid nodularities develop in the older
people. Recent data of Hempelmannetal.!99 indicate that the latent period may be > 30 years for
0
o
po
5
a
10
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Time after exposure (years)
20
4
Figure 45. Time of appearance of thyroid lesions in
Marshallese. Exposed at age <10: o, benign: +. malignant. Exposed at age > 10: @, benign; *, malignant.
developmentof radiation-induced thyroid tumors.
Thyroid cancer has been reported as long as 40
years after radiation exposure.125
Correlation with sex showedthatslightly more
exposed Rongelap females (18 of 45) developed
thyroid lesions than did males (11 of 41). All three
cases of cancerof the thyroid were in females.
The thyroid gland is generally thought to be
moresensitive to radiation during childhood .!79.171
Pochin!!8 estimates a percent incidence per 100
rads of 0.5 to 1.0 for adults and 1.3 for children.
Certainly the largest numberof benign lesions occurred in the Marshallese children. This is probably due mainly to the smaller size of the thyroid
glandsresulting in larger doses per gram of gland.
Doniach?”° theorizes that thyroid tumors are produced byaninitiating factor (radiation) plus a
promoting factor (TSH) which increases mitosis
and enhances the expression of a possible malignant clone. In addition, in children the growth
factor may be important, since at maturity the thv-
roid weighs about 20 times as much as atbirth.
These factors would increase the chancesfor malignant transformation. The lower incidence of
cancerof the thyroid in children than in adults
may be related tc the lack of dose dependence of
the carcinogenic effect of radiation at high doses.
Several investigators have shownthat high doses
of radiation (both x rays and radioiodine) are as-
sociated with a lower incidence of thyroid malignancy than lowerdoses, possibly because the high
doses produce enough cell destruction to preclude
malignant transformation.85-120.121.126,127 This
may be the case with Marshallese children. It has
been notedthat following treatment of hyperthyroidism with large doses of radioiodine, although