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Radioactive iodine treatment absorbed by certain organs was linked with a slightly higher risk for solid cancer mortality, researchers reported.

In an analysis of over 18,800 patients with hyperthyroidism, an estimated 8% of all solid cancer deaths were attributed to radiation exposure targeted at the stomach during an average 26 year follow-up, reported Cari Kitahara, PhD, of the National Institutes of Health in Bethesda, Maryland, and colleagues.

An estimated 7% of all solid cancer deaths in the cohort, excluding female breast cancer, was also attributed to a 100-mGy dose of radiation targeted to the stomach, they wrote in .

This dose of radiation absorbed by the breast tissue accounted for an estimated 14% of female breast cancer deaths, the authors added.

Although statistically significant, the overall relative risk (RR) for all solid cancers, including or excluding female breast cancer, were fairly low:

• All solid cancers combined: RR 1.06 (95% CI 1.02-1.10)
• All solid cancers excluding female breast: RR 1.05 (95% 1.01-1.10)

The dose-response relationship between radioactive iodine targeting breast tissue and female breast cancer was slightly stronger (RR 1.12, 95% CI 1.00-1.32).

Patients with hyperthyroidism treated with radioactive iodine absorbed by other organs or tissues, including the esophagus, colon, rectum, liver, pancreas, lung, bladder, kidney, brain, uterus, ovary, prostate, marrow, or mucosa were not significantly linked to other solid cancer deaths.

"Hyperthyroid treatment decisions should take into consideration the balance of risks with advantages of each available treatment option as well as patient preference, health status, and access to these options," the researchers recommended.

The study was a 24-year extension of the Cooperative Thyrotoxicosis Therapy Follow-up Study, which followed patients treated for hyperthyroidism in the U.S. and U.K. The vast majority of individuals had Graves disease, and nearly 80% of the cohort was female.

A total of 18,805 adults were included in this analysis who received radioactive iodine therapy alone -- a treatment that's been used since the 1940s -- or in conjunction with treatments. Most patients received combination iodine with antithyroid drugs, while only about 38% of the cohort received radioactive iodine alone. The majority of patients who received radioactive iodine only received one treatment. Relative risks for solid cancer were computed 5 years after the last radioactive iodine treatment.

The average total administered activity of sodium iodide I-131 was 375 MBq and 653 MBq for patients with Graves disease and toxic nodular goiters, respectively.

Graves disease patients who received a 100 mGy dose of iodine to the stomach had an average administered activity of 243 MBq, while those who received a 100 mGy dose to the breast tissue had an average administered activity of 266 MBq. However, one limitation to the study included "major uncertainties in the organ (particularly non-thyroid) dose estimations," Kitahara's group explained, adding that these may have "biased the study findings toward the null."

Although the thyroid had the highest average absorbed dose of all the organs or tissues -- 130,000 mGy -- there was no relationship between thyroid cancer and radioactive iodine treatment.

"Previous studies have found that children undergoing high-dose radiotherapy for a first primary cancer were at an increased risk of developing secondary thyroid cancer, but the risk was attenuated at higher doses (>30 Gy), presumably because of cell killing," the researchers explained. "Whether malignant transformation of residual thyroid cells is possible at the higher thyroid doses received by patients with hyperthyroidism, who are generally exposed in adulthood when the thyroid gland is less susceptible to radiation exposure, remains unclear."

Kitahara and colleagues ultimately recommended that future studies look at other hyperthyroidism treatment options, and weigh their risks and benefits against that of radioactive iodine treatment.

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