Women who survive either breast or thyroid cancer are at greater risk of developing the other type in the future as a secondary malignancy, says research published in Cancer Epidemiology, Biomarkers & Prevention.

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Regular surveillance after cancer treatment can help to catch secondary malignancies in good time.

Breast cancer is the most common type of cancer in women. In 2012, there were 224,147 new cases diagnosed, and 41,150 women died of the disease.

The thyroid gland is located at the base of the throat. The hormones that it produces help to regulate heart rate, body temperature and weight. The shape is like a butterfly, with a right and a left lobe, connected by a piece of tissue.

There are four types of thyroid cancer: papillary, follicular, medullary, and anaplastic. Papillary is the most common type of thyroid cancer. Anaplastic is the most difficult to treat, but the other three can normally be cured, according to the National Cancer Institute (NCI).

The incidence of thyroid cancer has almost tripled in the US in the last 30 years, but progress in detecting and treating both breast and thyroid cancer means that many more women are now surviving their first illness.

It now seems that women who survive one disease have a high risk of developing the other.

Breast cancer survivors have a 1.55 times higher risk of thyroid cancer

Dr. Raymon H. Grogan, assistant professor of surgery and director of the Endocrine Surgery Research Program at the University of Chicago Medicine and Biological Sciences in Illinois, and colleagues carried out a meta-analysis using PubMed and Scopus databases.

They looked at cases of breast and thyroid cancer over several decades and identified who had survived one cancer, and who went on to develop the other type later in life. They also calculated the risk of developing a secondary cancer.

The team found that a breast cancer survivor was 1.55 times more likely to develop thyroid cancer than a woman with no history of breast cancer. A woman who had had thyroid cancer was 1.18 times more likely to develop breast cancer than one with no history of thyroid cancer.

Fast facts about thyroid cancer
  • In 2015, 62,450 Americans were diagnosed with thyroid cancer, 47,230 of them women
  • 1,950 people died of thyroid cancer, of whom 1,080 were women
  • 2 in 3 thyroid cancer diagnoses are for people under 50 years of age.

Learn more about thyroid cancer

The team offers a number of possible explanations.

The first is surveillance bias. A cancer patient is more likely to be followed up and to adhere to a screening regime for some years after the treatment, increasing the chance of having a secondary cancer detected, even at an early stage.

There could be common hormonal risk factors for both diseases. Previous studies suggest that exposure to estrogens and to thyroid-stimulating hormones could, theoretically, contribute to the development of a secondary breast or thyroid cancer.

Treatment for the first cancer could affect the risk of developing the second. Radiation is a common tool in the fight against breast cancer, and studies show this could increase the risk of certain lung, esophageal, blood cancers and sarcomas, among others.

Radiation exposure is a risk factor for thyroid cancer, although protecting the thyroid during treatment should minimize the danger.

The authors say there is evidence that the use of radioactive iodine (RAI) in the treatment of thyroid cancer may have a very small impact on the development of other cancers at a later date, including breast cancer. How great this risk is remains unclear.

Do genetic factors play a role?

Genetic factors, such as a germline mutation, could link breast and thyroid cancer. Cowden syndrome, for example, appears to increase the chance of developing both cancers. However, Dr. Grogan says this alone cannot explain the higher risks observed in the present study.

He would like to see more research to identify other potential genetic factors and adds:

"The connection between breast cancer and thyroid cancer is another risk factor that a doctor and a patient should be aware of."

Limitations of the study include the inability to determine the length of time between the first and secondary malignancies due to differences in study design. This makes it hard to know what role surveillance plays.

Nevertheless, there is a significant increase in risk for both types of malignancy, implying that the phenomenon cannot be entirely due to surveillance bias.

Medical News Today recently reported that a high fiber diet when young might reduce the chance of breast cancer.