FRIDAY, Feb. 5, 2016 (HealthDay News) -- Women who survive either breast or thyroid cancer may be at increased risk for the other tumor type, according to a new analysis.
University of Chicago researchers who reviewed 37 published studies found breast cancer survivors were 1.55 times more likely to develop thyroid cancer than women who hadn't had breast cancer. And, female thyroid cancer survivors were 1.18 times more likely to get breast cancer than women who hadn't had thyroid cancer, researchers said.
"This is a real risk," said study lead author Dr. Raymon Grogan, director of the university's endocrine surgery research program.
"People who have had one of these cancers need to be aware that they are at higher risk for developing the other cancer," he said.
Thyroid cancer cases have nearly tripled in the United States over the past 30 years, and breast cancer is the most common cancer among women, according to background notes with the study. Thanks to medical advances, more women are surviving each cancer, Grogan said.
Doctors need to be more aware of the link between the two cancers, Grogan said.
"It should just become one of the common discussions between a patient and her doctor," he said. "It doesn't change the recommendations for screening, but people need to be aware and be screened at the appropriate time."
The report was published Feb. 5 in the journal Cancer Epidemiology, Biomarkers and Prevention.
Carol DeSantis, director of breast and gynecological cancer surveillance at the American Cancer Society, said the connection between thyroid and breast cancer is known.
She said her concern with this new report is that by lumping together so many studies that differ in their methods and findings, it's impossible to come up with a single number that accurately reflects the risk of having one cancer after having had the other.
"The review of different studies is helpful to see that there is that link, but combining them all together, I am not sure who that would be applicable to," DeSantis said.
Grogan said the research team tried to control for those differences as best they could.
Nineteen of the studies analyzed breast cancer patients and their risk of thyroid cancer. Another 18 looked at thyroid cancer cases and their incidence of breast cancer.
The researchers then combined these data and calculated the odds of a women having thyroid cancer after breast cancer and vice versa.
In addition, the researchers combed through the studies to find reasons why these cancers seemed related. One explanation was that women who survive either cancer were more likely to be screened and examined so that other cancers were found early.
Another possible connection was that breast and thyroid cancers share hormonal risk factors. There is some evidence that exposure to estrogens and to thyroid-stimulating hormones may contribute to both cancers, Grogan said.
A third theory points to radiation therapy, which has been shown to increase the risk for lung, esophageal and blood cancers, and sarcomas. Also, earlier research found that radioactive iodine, used to treat thyroid cancer, may play a small role in the development of other cancers, including breast cancer, but that is not clear, Grogan said.
Finally, it is possible that a genetic mutation might be responsible for the connection, Grogan said.
DeSantis said that cancer survivors should be aware of the increased risk of developing other cancers.
"Generally, cancer survivors are at risk for developing a second cancer," she said. "Breast cancer survivors are at risk for blood cancers, uterine cancer, ovarian cancer and other cancers. Likewise, thyroid cancer survivors are at risk for a number of other cancers, including breast cancer."
For more on cancer, visit the U.S. National Cancer Institute.