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Radioactive iodine is commonly used to treat hyperthyroidism, such as that which results from Graves disease. However, Cari M. Kitahara, PhD, MHS, and colleagues report that this treatment is associated with cancer mortality and that higher doses are associated with greater risk of death from solid cancers, including female breast cancer. In this interview with i3 Health, Dr. Kitahara, Investigator of the Radiation Epidemiology Branch of the National Cancer Institute's Division of Cancer Epidemiology & Genetics, discusses the implications of her findings, which were recently published in JAMA Internal Medicine.
Can you comment on the significance of your results concerning the link between radioactive iodine for hyperthyroidism and cancer mortality?
Cari M. Kitahara, PhD, MHS: For decades, radioactive iodine has been the preferred treatment option for uncomplicated cases of Graves disease by endocrinologists in the United States, but anti-thyroid medications have gained favor in the United States and have been the preferred option in much of the rest of the world. There has been limited information about whether (and to what extent) radiation exposure from this treatment is associated with risk of cancer later in life.
Our study used data from the largest cohort of patients treated for hyperthyroidism and nearly seven decades of follow-up information on causes of death. For the first time, we evaluated whether higher radioactive iodine treatment doses to the thyroid and other exposed organs were associated with risk of death from cancers at those sites.
The results of our study suggest a modest dose-dependent relationship between radioactive iodine treatment and solid cancer-related death, including breast cancer death, among patients with hyperthyroidism. Our study provides the first direct evidence of a relationship between radioactive iodine treatment and breast cancer risk. This is a particularly important finding, as the majority of patients with hyperthyroidism are women.
Were any of your findings surprising to you?
Dr. Kitahara: We were surprised by the lack of a dose-response relationship for leukemia mortality. Radioactive iodine treatment for thyroid cancer, which involves much higher doses, has been associated with a subsequent increased risk of leukemia. The fact that we did not observe a dose-response relationship may reflect some of the limitations of our study: relatively low doses compared to those used for thyroid cancer treatment, uncertainties in our estimates of red bone marrow doses, and small numbers of leukemia deaths during cohort follow-up.
Your results did not indicate a dose-response relationship between radioactive iodine treatment for hyperthyroidism and thyroid cancer death, despite high doses of radiation to the thyroid. To what do you attribute this finding?
Dr. Kitahara: There are several possible reasons for the lack of a dose-response relationship for thyroid cancer mortality in our study. Other studies have shown that the thyroid gland is much less susceptible to the effects of radiation exposure in adulthood versus childhood. Furthermore, because our study was only designed to capture causes of death, we did not have information on all thyroid cancer diagnoses. Since thyroid cancer is rarely fatal, we could not fully evaluate the relationship between radioactive iodine treatment and the risk of thyroid cancer occurrence in our study.
How do you think that your findings should impact current clinical practice for patients with hyperthyroidism?
Dr. Kitahara: The findings from our study should be communicated by physicians when discussing treatment options for patients with hyperthyroidism. Treatment decisions should be made after carefully weighing the risks and benefits of radioactive iodine and other treatment options, namely surgery and anti-thyroid drugs, and after considering patient preferences.
About Dr. Kitahara:
Cari M. Kitahara, PhD, MHS, is an epidemiologist in the Radiation Epidemiology Branch of the National Cancer Institute's Division of Cancer Epidemiology & Genetics and the study's principal investigator. Her research focuses on cancer risks from low-dose radiation exposure in both medical workers and patients and on the causes of radiosensitive cancers, such as thyroid cancer. She has received multiple awards for her research, including the National Cancer Institute Director's Individual Merit and Intramural Innovation Awards.
For More Information
Kitahara CM, Berrington de Gonzalez A, Bouville A, et al (2019). Association of radioactive iodine treatment with cancer mortality in patients with hyperthyroidism. JAMA Intern Med. [Epub ahead of print] DOI:10.1001/jamainternmed.2019.0981
Transcript edited for clarity. Any views expressed above are the speaker's own and do not necessarily represent those of i3 Health.