Survivors of the two atomic bombs in Japan 60 years ago who had a higher exposure to radiation now have a greater incidence of certain thyroid diseases, including tumors and cysts, and that risk increases with being younger at the time of exposure, according to a study in the March 1 issue of JAMA.
Thyroid disease has become an important target with which to study the effects of radiation. Many studies have reported that the risk for malignant and benign thyroid nodules increased with external and internal radiation exposure in people exposed at young ages, although few studies have followed exposed populations for long periods, such as more than 50 years, according to background information in the article.
Misa Imaizumi, M.D., of the Radiation Effects Research Foundation, Nagasaki and Hiroshima, Japan, and colleagues conducted a comprehensive thyroid disease survey between 2000 and 2003 to diagnose thyroid nodules (malignant and benign) and autoimmune thyroid diseases in survivors of the 1945 atomic bombs in Hiroshima and Nagasaki. The researchers determined the radiation dose-response relationships for these thyroid diseases. The study included 4,091 participants (average age, 70).
Thyroid diseases were identified in 44.8 percent of the total participants. In 3,185 participants, excluding persons exposed in utero, not in the city at the time of the atomic bombings, or with unknown radiation dose, the prevalence of all solid nodules, malignant tumors, benign nodules, and cysts was 14.6 percent, 2.2 percent, 4.9 percent, and 7.7 percent, respectively. The prevalence of all solid nodules, malignant tumors, benign nodules and cysts was significantly associated with thyroid radiation dose. The researchers estimate that 28 percent of all solid nodules, 37 percent of malignant tumors, 31 percent of benign nodules and 25 percent of cysts were associated with radiation exposure. No significant dose-response relationship was observed for positive antithyroid antibodies, antithyroid antibody-positive hypothyroidism, or Graves disease (an inflammatory disorder of the thyroid gland).
The interaction of age at exposure with dose was significant for the prevalence of all solid nodules, benign nodules, and other solid nodules, showing that the dose effects were significantly higher in those exposed when young. "... participants with age at exposure of younger than 20 years show significant dose-response relationships, while those with age at exposure of 20 years or older showed no significant dose response," the authors write.
"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. On the other hand, autoimmune thyroid diseases were not found to be significantly associated with radiation exposure in this study. Careful examination of the thyroid is still important long after radiation exposure, especially for people exposed at younger ages," the researchers conclude.
Editorial: Thyroid Disease 60 Years After Hiroshima and 20 Years After Chernobyl
In an accompanying editorial, John D. Boice, Jr., Sc.D., of the International Epidemiology Institute, Rockville, Md., and Vanderbilt University School of Medicine, Nashville, Tenn., comments on the study on radiation dose and thyroid disease.
"The risk following exposures [to radiation] in childhood apparently lasts for life, although it appears that the risk declines many years after exposure. 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. The risk of thyroid cancer seems to be enhanced if diets deficient in stable iodine result in chronic thyroid stimulation."
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