Breast cancer survivors are at increased risk of developing thyroid cancer, especially within 5 years of their breast cancer diagnosis, according to a new analysis of a large national database. The study results were presented at ENDO 2015: The 97th Annual Meeting & Expo in San Diego, California.

“Recognition of this association between breast and thyroid cancer should prompt vigilant screening for thyroid cancer among breast cancer survivors,” said lead investigator Jennifer Hong Kuo, MD, assistant professor of surgery at Columbia University, New York City.

Breast cancer survivors, whose numbers are increasing, should receive counseling regarding their higher-than-average risk of thyroid cancer, Kuo recommended.

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Until now, Kuo said, the relationship between breast and thyroid cancer has been controversial, largely based on single-institution studies that have suggested a possible increase in thyroid cancer incidence after breast cancer.

The researchers used the National Cancer Institute’s Surveillance, Epidemiology, and End Results 9 (SEER 9) database to identify the number of people with a diagnosis of breast and/or thyroid cancer between 1973 and 2011. They found 704,402 patients with only breast cancer, 49,663 patients with only thyroid cancer, and 1,526 patients who developed thyroid cancer after breast cancer.

Compared with patients with breast cancer alone, women who had breast cancer followed by thyroid cancer were younger on average at diagnosis of their breast cancer. They also were more likely to have had invasive ductal carcinoma (the most common type of breast cancer), a smaller focus of cancer, and to have received radiation therapy as part of their breast cancer treatment. There was no difference in risk based on whether the breast cancer was hormone receptor positive or had spread to lymph nodes.

Compared with patients who had only thyroid cancer, breast cancer survivors who developed thyroid cancer were more likely to have a more aggressive type of thyroid cancer, but the cancers were smaller in size and fewer patients required additional radioactive iodine treatment. Because thyroid cancer tends to occur at younger ages than breast cancer, survivors who then developed thyroid cancer were older on average than those with only thyroid cancer: 62 years vs 45 years, respectively.

The study findings showed that breast cancer survivors developed thyroid cancer at a median of 5 years. Therefore, Kuo recommended a dedicated thyroid examination every year for the first 5 years after a breast cancer diagnosis, especially for survivors who received radiation therapy.

Kuo said she plans to study whether tamoxifen treatment, typically given for 5 years after a breast cancer diagnosis, may play a role in increasing the risk of thyroid cancer.

Radiation therapy to the head, neck, or chest is a known risk factor for thyroid cancer, according to the Endocrine Society’s Hormone Health Network. Female sex raises the risk of both thyroid and breast cancer. The incidence of thyroid cancer is increasing, and Kuo said researchers must have a better understanding of the etiology for this increase.