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Radioiodine treatment for hyperthyroidism increased cancer risk
Incidence was significantly higher in cancer of the stomach, kidney and breast.
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A Finnish study found an increased risk for cancer among hyperthyroid patients who were treated with radioactive iodine when the patients were compared with the general population.
Saara Metso, MD, of the department of internal medicine at Tampere University Hospital in Tampere, Finland, and colleagues conducted a study of 2,793 patients with hyperthyroidism (2,336 women) treated with radioactive iodine between January 1965 and June 2002. The researchers included an age- and sex-matched control group of the same number of individuals using the Population Register Center. Cancer cases among both groups were identified using information from the Finnish Cancer Registry.
The results of their study were published in Cancer.
Patients and controls had a median age of 62 years at the start of treatment or at the beginning of follow-up. The median follow-up time was 9.8 years for the patients and 10 years for the control group, resulting in a total of 30,878 person-years at risk for patients and 32,452 person-years at risk among the control participants.
Increased incidence
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Overall, at least one cancer diagnosis was made in 367 hyperthyroid patients and 308 control participants. The incidence per 10,000 person-years at risk was 118.9 (95% CI, 107.3-131.7) for patients and 94.9 (95% CI, 84.9-106.1) for controls. This higher incidence among hyperthyroid patients treated with radioiodine gave a rate ratio of 1.25 (95% CI, 1.08-1.46). The increased incidence with regard to specific cancers was significant for cancer of the stomach (rate ratio=1.75; 95% CI, 1-3.14), the kidney (rate ratio=2.32; 95% CI, 1.06-5.01), the breast (rate ratio=1.53; 95% CI, 1.07-2.19), and unspecified site (rate ratio=2.22; 95% CI, 1-4.9).
The absolute difference in incidence rates between the groups corresponded to a number needed to harm of 418 (95% CI, 391-446). This means that one excess case of cancer was created by treating 418 patients with radioactive iodine.
“The proportion of [patients] free of cancer at five years of follow-up was 97.4% among the patients vs. 97.5% among the controls,” the investigators wrote. “The difference in cancer incidence between the studied groups emerged thereafter.”
Dose-dependent risk increase
Further analysis based on the cumulative radioactive iodine activity showed that the cancer risk increased with the cumulative dose. The mean total dose administered was 305 MBq; 80.3% of patients received a single dose, 15.6% were given two doses, 2.7% were given three doses, and 1.7% received four or more doses. Overall, the relative risk for cancer increased by 1.06 for an increase of 100 MBq (95% CI, 1.03-1.09).
The researchers noted that in this study “it is not possible to distinguish between the effects of radioactive iodine treatment and those of hyperthyroidism.” The delay in the increased risk over the early years of follow-up suggests that radiation could explain the rise in cancer incidence, however. – by Dave Levitan
For more information:
- Metso S, Auvinen A, Huhtala H, et al. Increased cancer incidence after radioiodine treatment for hyperthyroidism. Cancer. 2007;109:1972-1979.