Massachusetts General Hospital

Mammograms and thyroid cancer: The facts about breast-cancer screening

By Daniel B. Kopans, MD

20/Apr/2011

Mammography screening is one of the most important advances in women's health in the past 50 years. Since annual screening began, the death rate from breast cancer has decreased by more than 30 percent.

Unfortunately, this undeniable success continues to be clouded by misinformation arising from inappropriate data analyses. Such confusion can do real harm if it dissuades woman from availing themselves of annual screening mammography.

A controversy that erupted last year over whether screening should begin at age 40 or 50 is one example (see the sidebar, "Screening mammography facts"). More recently, a "viral" email claiming a connection between mammography and rising thyroid cancer rates has alarmed many women.

A recent episode of The Dr. Oz Show focusing on this issue pointed out (correctly) that the incidence of thyroid cancers has been increasing among women over the past 30 years.

The host of the show, Mehmet Oz, MD, attributed radiation exposures during dental x-rays and mammograms as possibly contributing to this increase. This was unfortunate since there is absolutely no evidence that mammograms have anything to do with the increase in thyroid cancers.

Here are the facts:

• Dental x-rays and mammograms must be considered separately. X-rays are like a spotlight. If a room is completely darkened, a spotlight only illuminates the area at which it is directed. Similarly, x-rays are confined to a specific area. The thyroid is "illuminated" during dental x-rays, and in fact the American Dental Association agrees that wearing a thyroid shield during dental x-rays is a good idea. However, there is no radiation directed to the thyroid during a mammogram.
• The only radiation that may reach the thyroid during a mammogram is scatter: the tiny amount that bounces off the breast. This has been carefully studied and is so small that it is equivalent to 30 minutes of background radiation (the naturally occurring radiation that we all get every day from the environment). Put another way, a woman could get a mammogram every year for the next 40 years, and her thyroid would receive less radiation than it receives in one day from background radiation. If people are going to encourage women to ask for a thyroid shield during their mammograms, then they should encourage everyone to wear a shield 24/7, because over the course a year, our thyroids receive 17,520 times as much radiation from the background as they would from a mammogram.
• Although the incidence of thyroid cancer has been increasing in females, it has also been increasing with the same rapidity among males. Unless men have been sneaking in at night to have mammograms, mammography has had nothing to do with the increasing incidence of thyroid cancer.
• The risk of x-rays causing cancer of the thyroid is directly related to the age at which the individual is exposed. Children and teenagers have the greatest risk from thyroid irradiation. The risk for women ages 40 and above, even from high doses of radiation, is extremely low and may be nonexistent.
• A thyroid shield can slip down into the field of view of a mammogram, blocking some of the breast tissue and necessitating a repeat mammogram. It has been estimated that this might occur 20 percent of the time.

The bottom line: There is no risk to the thyroid during a mammogram, and a shield can actually compromise optimal imaging. Shields are available for patients who insist on having them. But women must be aware that using one not only confers no benefit in terms of thyroid cancer prevention, but may in fact compromise the mammogram and its lifesaving benefits.

Daniel B. Kopans is a Senior Radiologist in the Breast Imaging Division at Massachusetts General Hospital Imaging and Professor of Radiology at Harvard Medical School.