The frequency of cancer of several organs is slightly increased in patients with hyperthyroidism treated with radioactive iodine
(July 2007)
The background of the study. Patients with hyperthyroidism have long been treated with radioactive iodine (I-131). One anticipated adverse effect of I-131 was an increased risk of thyroid or other cancers, and in some studies there have been small increases in the incidence of some cancers. In this study the long-term risk of cancer was determined in patients with hyperthyroidism treated with I-131.
How the study was done. The study subjects were 2793 patients (2336 women, 457 men) with hyperthyroidism who were treated with I-131 between 1965 and 2002 at a single hospital in Finland. The control group consisted of 2793 age- and sex-matched subjects. Patient and treatment information was obtained from hospital records and the Finnish Cancer Registry. The follow-up period started one year after treatment and ended in 2004. The incidence rates for different cancers were compared in the two groups.
The results of the study. During a follow-up period of 10 years, 367 patients and 308 control subjects had one or more cancers. The incidence rates were 119 per 10,000 subjects in the I-131-therapy group and 95 per 10,000 subjects in the control group (1 excess case per 418 patients treated with I-131). The rates in the two groups did not differ until after five years of follow-up.
Among individual cancers, the incidence of cancers of the breast, stomach, and kidney were slightly higher in the I-131 therapy group. The rates for cancer of the thyroid and other organs were similar in the two groups.
The risk of cancer was slightly increased in patients aged 50 to 59 years and those aged 70 to 98 years at the start of follow-up. The average total dose of I-131 was 8.2 mCi (303 MBq). The overall risk of cancer increased slightly with increasing doses of I-131.
The conclusions of the study. The incidence of cancer of the breast, stomach, and kidney is slightly increased in patients with hyperthyroidism who were treated with I-131.
The original article. Metso S, Auvinen A, Huhtala H, Salmi J, Oksala H, Jaatinen P. Increased cancer incidence after radioiodine treatment for hyperthyroidism. Cancer 2007;109:1972-9.
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