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A-bomb survivors face greater risk of thyroid cancer

Survivors of the two atomic bombs dropped on Japan at the end of World War II are now at greater risk of developing certain thyroid diseases, including tumors and cysts, according to a study.

The risk increases the younger the survivor was at the time of exposure to the bombs' radiation.

The thyroid gland sits under the voice box and produces hormones to regulate growth and metabolism. Diseases there are a useful way for scientists to study the effects of radiation on the body. Thyroid cancers were the first solid tumors to increase in frequency among atomic bomb survivors, for example.

Misa Imaizumi, of the Radiation Effects Research Foundation, studied more than 4,000 survivors of the 1945 atomic bombs on Hiroshima and Nagasaki between 2000 and 2003 to look for thyroid problems. The Japanese researchers found thyroid diseases in almost 45 percent of the participants, with the risk of developing problems related directly to the level of exposure.

"The present study revealed that 55 to 58 years after radiation exposure, a significant linear dose-response relationship existed in the prevalence of not only malignant thyroid tumors but also benign thyroid nodules, and that the relationship was significantly higher in those exposed at younger ages," the researchers wrote in the Journal of the American Medical Association.

"On the other hand, autoimmune thyroid diseases were not found to be significantly associated with radiation exposure in this study," they wrote.

"Careful examination of the thyroid is still important long after radiation exposure, especially for people exposed at younger ages," they wrote.

John Boice, of Vanderbilt University school of medicine, said in an accompanying article: "The study of atomic bomb survivors remains the single most important study of radiation effects in humans, but the exposure was brief, lasting less than a second."

He added it was "remarkable that a biological effect from a single brief environmental exposure nearly 60 years in the past is still present and can be detected."

Survivors aged under 10 at the time of the bombs were most at risk of developing thyroid problems, but there was no significant increase in risk (beyond the radiation exposure itself) for those exposed after the age of 20.

For those who were children in 1945, the increased risk seemingly lasts for life.

"The radiosensitivity of the young thyroid gland is high and most likely relates to subsequent proliferative activity of the gland during puberty and growth, but the reasons for the absence of risk following adult exposures are not entirely clear," Boice said.
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