131I Treatment of Hyperthyroidism Does Not Increase the Risk of Cancer Mortality
ABSTRACT & COMMENTARY
Synopsis: 131I treatment of hyperthyroidism appears to be safe and effective.
Source: Ron E, et al. JAMA 1998;280:347-355.
The cooperative thyrotoxicoses follow-up Study began in 1961. It included 35,609 patients with hyperthyroidism treated between 1946 and 1964 at one of 26 study clinics. When the original study ended in 1968, after a mean follow-up of 8.2 years, thyroid cancer incidence and mortality and leukemia incidence were not significantly elevated in the 131I-treated patients compared to other patients.
To assess the long-term carcinogenic effects of treatment for hyperthyroidism, this study was a mortality follow-up through 1990 of the original population minus the one cooperating clinic in England. The total number of cancer deaths from 35,583 patients was close to that expected in the general population (2950 vs 2857). There was, however, a small excess in mortality from lung, breast, kidney, and thyroid cancers and a decrease in death from uterine and prostate cancers. A slight increase in deaths in patients with toxic modular goiter and those treated with antithyroid drugs alone was seen. The latter increase in deaths was associated with other factors, which may have increased the mortality rate.
COMMENT BY RALPH R. HALL, MD, FCAP
This study is particularly helpful in that 88% of the study population had approximately 5-10 years of follow-up, and 76% had 10 years or more. Approximately 65% of the study subjects had been treated with 131I, 25% had received 131I treatment alone, and 39% had received 131I in addition to surgery and/or drugs.
This study is also important because B1I is the treatment of choice for most adults in the Unites States.1 As the authors point out, the public's interest in the late effects of 131I exposure was enhanced by the 1986 Chernoble accident and the acknowledgment of past 131I releases from nuclear reactors and bomb testing. Therefore, physicians often encounter patients who are concerned about the use of 131I.
An interesting subgroup of patients was those under 20 years of age who were treated with 131I. There were no observed or expected thyroid cancer deaths among the 365 patients who were under 20 years of age when treated.
The authors were careful to point out the limitations of the study. Doses of 131I to the thyroid could not be calculated, so exposure response was based on 131I-administered activity. In addition, patients were selected for a specific treatment based on their medical condition. Patients who were ill, etc., may have been given antithyroid drugs rather than 131I.
If there is increased morbidity or mortality from 131I therapy, it is very small. It appears to be a safe therapy.
Reference
1. Klein I, et al. Ann Intern Med 1994;121:281-288.
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