Health Risks of Ionizing Radiation: - Clark University

dence of this disease includes elevated levels of
circulating autoantibodies, molecules that are produced
to destroy thyroid cells or thyroid hormone.
One of the more common forms of autoimmune
thyroid disease is known as Hashimoto’s thyroiditis.
Children around Chernobyl have demonstrated
evidence of autoimmune thyroid disease including
elevated levels of autoantibodies (Pacini et al. 1998,
Vykhovanets et al. 1997, Vermiglio et al. 1999) and
elevated levels of thyroid-stimulating hormone, a
sign that the pituitary gland is compensating for thyroid
damage (Quastel et al. 1997, Vykhovanets et al.
1997). Since the follow-up of this cohort has still
been relatively short it has been suggested that these
observations may be an early stage in the development
of hypothyroidism (Pacini et al. 1999). Goldsmith
et al. (1999) report a significant correlation
between body burden of cesium-137, a very rough
proxy for 131I dose, and hypothyroidism in boys
around Chernobyl; the relationship was not significant
among girls. Ivanov et al. (2000) reported on
the incidence of endocrine and metabolic disease, a
category that includes thyroid disease, in Chernobyl
cleanup workers who had average doses of ~100
mGy. They found an ERR of 0.58/Gy (0.3-0.87).
Thyroid disease has also been diagnosed in 28% of
the Mayak Children’s Cohort, residents of Ozyorsk,
Russia who were exposed as children to routine releases
of 131I from the Mayak complex.
B.5 Discussion
It is clear that radiation exposure can lead to thyroid
cancer. The more detailed story involves a discussion
of the relative abilities of internal 131I and external
gamma rays and x-rays to cause damage and
also involves a discussion of age.
Different forms of radiation are often compared
to gamma radiation or x-radiation by observing how
much damage is caused by the same dose of each
kind of radiation. For example, researchers might
expose one group of rats to 1 Gy of x-rays, expose
another group of rats to enough 131I to give a 1-
185 Appendix B
Gy thyroid dose, and then count the thyroid cancers
that evolve. If there were only half as many cancers
in the iodine-exposed group then the relative biological
effectiveness (RBE) of 131I would be 0.5.
If there were twice as many cancers in the iodineexposed
group then the RBE would be 2. Typically
beta emitters like 131I are assumed to have an RBE
of one or slightly less, indicating that they are about
as destructive as gamma radiation or x-radiation.
The choice of RBE can be very important when
risks are being estimated, as we see in the case of the
estimated thyroid cancer impact of Nevada Test Site
fallout. The National Cancer Institute has estimated
that approximately 50,000 thyroid cancer cases may
eventually develop because of the test site, and this
number is fairly uncertain (95% CI 11,300-212,000)
(NCI 2001). This was based on the assumption that
131I has an RBE of 0.66. However, Land (1997)
noted that there is also evidence for an RBE of 1, and
that this is more likely to describe events at lower
doses. A review of the 1997 NCI report (NAS 1999)
decided that an RBE of 1 was just as defensible as
an RBE of 0.66, and that, based on the Chernobyl
experience, the RBE is not likely to be any lower
than 0.66. If an RBE of 1 is assumed then the number
of thyroid cancer cases attributable to Nevada
Test Site fallout is 75,000 (17,000-324,000).
Children appear to be more susceptible to radiation-induced
thyroid cancer than adults, as shown in
Table B-1. Based on the available data we can conclude
that the ERR of thyroid cancer after childhood
radiation exposure is in the area of 5-10 per Gy. Most
studies have observed a linear dose-response relationship
without evidence of a threshold; expressed
in various ways we can assume that a child experiencing
a thyroid dose of 0.01 Gy would have an
ERR of 0.1, a relative risk of 1.1, or a 10% increase
in thyroid cancer risk. Risks in adults are much more
uncertain than risks in children, and although there
may be some thyroid cancer risk following radiation
exposure, it appears to be relatively low (Ron
2003).