In most cases of thyroid cancer, there is no known cause. People most at risk for thyroid cancer are those who were exposed to radiation or radioactive iodine from nuclear fallout. When the healthy thyroid gland is exposed to radioactive iodine, the cells of the thyroid gland can develop breaks in their chromosomes, which can cause cancer. Several studies have conﬁ rmed high rates of thyroid cancer in certain areas or populations. One study, published in 1997 by the National Cancer Institute, looked at health risks of radioactive fallout released at the Nevada Test Site from 1951 through 1958. The study concluded that people living in the midwestern regions of North America were more at risk for thyroid cancer, particularly if they were children during the testing. Reports of high rates of thyroid cancer are also coming in from Hanford, Washington, where residents were exposed to fallout from the Hanford nuclear facility, which produced plutonium for nuclear weapons from 1944 through 1957. In the Marshall Islands in the South Paciﬁc, as a result of atomic bomb testing at Bikini Atoll in 1954, thyroid cancer occurs 100 times more frequently than in the unexposed population.
The nuclear accident at the Chernobyl atomic power station on April 26, 1986, exposed millions of people with healthy thyroid glands to excessive levels of radioactive iodine. People living within a 30-kilometer radius of the accident inhaled the radioactive iodine, while people living outside the 30-kilometer radius ingested the radioactive iodine. For reasons not quite understood, potassium iodide tablets (thyroid blocking agents) weren’t distributed to the public by the appropriate government agencies except in Poland. In Belarus, Russia, and the Ukraine, there currently appears to be a 100-fold increase in the incidence of thyroid cancer in children and an increase of thyroid cancer in adults.
For example, one study out of the Ukraine found that between 1981 and 1985, the number of new cases of thyroid cancer in children aged zero to fourteen totaled 25. But between 1986 and 1994, the number of new cases of thyroid cancer in this age group totaled 210, with peak periods in 1992 and 1993.
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In other cases, radiation to the head and neck area from external beam radiation therapy, or high-dose x-rays, during childhood or adolescence can cause thyroid cancer to develop later in life.