The largest and most comprehensive study on the association between cancer and radioiodine (RAI) treatment for hyperthyroidism remains the Cooperative Thyrotoxicosis Therapy Follow-up Study, which had a cohort of more than 35,000 patients with hyperthyroidism (93.7 percent of subjects had Graves' disease) at 25 US medical centers and one center in the UK. Sixty-five percent of these patients were treated with RAI between 1946 and 1964.

Because cancers are known to typically develop 30 years after exposure to RAI, it has been difficult to ascertain the true incidence of cancer mortality although earlier studies have shown a 4-fold increase in thyroid cancer mortality among RAI-treated patients. In this new study, researchers used advanced techniques to analyze data from an extended 28-year follow up of the original Cooperative study to evaluate the absorbed doses of RAI to other solid organ tissues. The researchers found a statistically significant positive dose-response relationship for risk of death for all solid cancers, including breast cancer with a 12 percent increase in risk per 100-mGy dose absorbed to the breast as well as a 6 percent increase in stomach cancer. Researchers also found an inncreased risk for cancer mortality associated with secondary malignant neoplasms, including bone and soft-tissue sarcoma, salivary gland and digestive tract cancers and leukemia.

While the thyroid gland was found to absorb the most radioiodine after RAI ablation, other organs absorb radiodiodine after this procedure. After the thyroid gland, the organs absorbing the most radioiodine in order from highest to lowest amounts include the esophagus, liver, oral mucosa, lung, stomach, red bone marrow, female breast, pancreas, kidney and uterus, brain, bladder, ovary, prostate, and colon or rectum.

What's important to note is that all of the studies using data from the Cooperative Thyrototoxic Therapy Follow-Up Study and related studies such as the Finland Study have only evaluated cancer mortality. The incidence of individuals who developed solid-organ cancers but died from other causes is unknown.

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