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Long-Term, Increased Risk of Cancer Death After Common Hyperthyroidism Treatment

​NCI Study: Radioactive Iodine Associated With Solid Tumors

Patients who received radioactive iodine (RAI) treatment for hyperthyroidism show an association between treatment dose and long-term risk of death from solid cancers, including breast cancer. The findings of the study were published on July 1, 2019, in JAMA Internal Medicine.

A team led by the National Cancer Institute identified a clear dose–response relationship between RAI and long-term risk of death from solid cancer, including breast cancer, in a large cohort study of patients treated for hyperthyroidism. The researchers estimate that for every 1,000 patients treated with a standard range of doses, about 20 to 30 additional deaths from solid cancer would occur because of radiation exposure.

Since the 1940s, RAI has been used widely in the U.S. to treat hyperthyroidism, and is one of three commonly used treatments for the condition. The other two are anti-thyroid drugs, which are increasingly popular, and surgical treatment, which is used less often.

The new findings are from a long-term follow-up study of people with hyperthyroidism (mainly Graves’ disease) who were treated with radiation between 1946 and 1964, the Cooperative Thyrotoxicosis Therapy Follow-up Study. The patients had all received RAI and none had had cancer when entering the original study. In the new analysis, which included almost 19,000 people from the original cohort, the researchers used a novel, comprehensive method to estimate radiation doses to each organ or tissue.

The researchers found positive dose–response relationships between the dose absorbed by an organ and mortality from cancer at the same site. The relationship was statistically significant for female breast cancer; every 100 milligray (mGy) of dose resulted in a 12% increased relative risk of breast cancer mortality. Further, the relative risk of mortality was increased by 5% per every 100 mGy for all other solid tumors considered together.

Based on their findings, the researchers estimated that for every 1,000 patients aged 40 years with hyperthyroidism, treated with the current, typical radiation doses, a lifetime excess of 19 to 32 solid cancer deaths from radiation would be expected.

In the U.S., about 1.2% of the population has hyperthyroidism, and the condition is much more prevalent in women. Thus, the findings for breast cancer mortality are especially relevant for the large number of women who are treated for hyperthyroidism, particularly as RAI is still widely used.

Source: National Cancer Institute, July 1, 2019

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