Highlights of the American Thyroid Association Symposium - Thyroid Health and the Environment: Threats and Effects

March 24, 2006; Washington, DC

Kenneth D. Burman, MD

Disclosures

December 12, 2006

In This Article

Effects of Radiation on the Thyroid

There has been significant interest in improving our understanding of radiation on thyroid gland tumorogenesis.[11] It is known that radiation exposure increases the risk for thyroid cancer.[11,12] This radiation could result from an atomic bomb, medical studies or therapies, or environmental or occupational exposure. Leukemia risk has been found to be markedly increased in atomic bomb survivors, and thyroid cancer was found to have an overall risk ratio of 1:2.[11] However, the risk ratio was 9:5 if the individual was between 0 and 9 years of age at the time of exposure, and 3:0 if the individual was between the ages of 10 and 19 years.[11] The risk attributable to radiation exposure in individuals younger than 20 years of age at the time of exposure is about 50%.[11,13,14,15]

Medical uses of external radiation have also been found to be problematic. The estimated risk for thyroid cancer is between 2.5 and 32 for patients who received external radiation for medical indications, usually as children.[11,16,17,18] The medical indications at the time included enlarged thymus, tinea infections, enlarged tonsils, and cutaneous hemangiomas. Ron[11] and Ron and colleagues[19] pooled these studies and estimated that the thyroid cancer risk is delayed, and seems to be greatest between 5 and 29 years after the medical use of external radiation -- and then decreases thereafter. Given its widespread use, it is important to determine the potential adverse effects of 131-I when used for diagnostic studies or therapy.[16] In a Swedish study that assessed diagnostic doses of 131-I, there was not a higher risk for thyroid cancer, except in the group of subjects who received greater than 1 Gy.[11,20,21] In studies assessing the potential adverse effects of 131-I therapy for hyperthyroidism, the relative risk of subsequently developing thyroid cancer was increased, but these data are difficult to interpret because the actual number of thyroid cancer cases was relatively small; of course, these patients had underlying thyroid disease, which may have predisposed them to additional thyroid abnormalities.

Our understanding of the possible effects of environmental radiation exposure is derived mainly from the Hanford Thyroid Disease Study[22] and the Chernobyl accident.[23,24,25] Approximately 740,000 Ci (2.73 × 1016 Bq) of iodine 131 were released to the atmosphere from the Hanford Nuclear Site in Washington State from 1944 to 1957, following which people living in the surrounding area were studied for the development of thyroid disease. Thyroid cancer occurred in less than 2% of the exposed individuals, and thyroid nodules occurred in about 8% to 11%, but there was not an apparent dose-response effect with either entity. The investigators concluded that there was no significant risk of developing thyroid cancer, benign thyroid nodules, carcinoma at any other site, autoimmune thyroiditis, or hypothyroidism.

The Chernobyl accident occurred on April 26, 1986. A case-control study (26 cases and 52 controls) showed an overall risk of developing thyroid cancer of 1.7, with a correlation between estimated dose exposure and effect.[11] Several investigators studied 276 cases and 1300 controls and found a relative risk of 5.5 for the development of thyroid cancer.[11,23,24,26,27] This risk was increased in iodine-deficient geographic areas and reduced in areas that received potassium iodide dietary supplementation.[11] Ron[11] and Jacob and colleagues[28] found a relative risk for thyroid cancer of about 2.5. Thyroid cancer that occurred following Chernobyl was more frequent in individuals who were younger than 10 years of age and was more likely to have extrathyroidal extension and the presence of lymph node metastases at the time of diagnosis. It is interesting to note that Chernobyl survivors also were more likely to have an unusual variant of papillary thyroid cancer called the solid/follicular variant; 74% of Chernobyl survivors had this subtype, compared with 35% of patients with spontaneous thyroid cancer unrelated to Chernobyl.[11,29,30,31,32,33]

In summary, radiation exposure from the Chernobyl accident did increase the risk of developing thyroid cancer, especially in children. This relative risk seems higher in women compared with men, and is higher in iodine-deficient areas. Regardless of this increased risk, the number of reported thyroid cancer-related risks is relatively low.

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