If you’ve had a breast cancer screening mammogram, your mammogram report told you about any abnormal findings—masses, calcifications, or other atypical structure—in your breast tissue. But it may have also included information about your breast density.

If your report told you that you have dense breasts, you’re far from alone, because about 50% of women in the U.S. who have screening mammograms are diagnosed with dense breasts. You can’t tell if you have dense breasts based on how your breasts look or their firmness—or your body type, weight, or the size and feel of your breasts.

Women who have been told they have dense breasts should be aware that it can make it difficult for screening mammography to detect tumors. What’s more, women who have dense breasts are also at increased risk for breast cancer.

“With conventional mammography, while we can be as accurate as 98% in a fatty breast, our sensitivity can drop to as low as 30% in women with extremely dense breasts, which is why supplementary screening with ultrasound or MRI—depending on the patient’s personal risk factors—can be such an important aid in finding breast cancer,” says Melissa Durand, MD, an Associate Professor of Radiology & Biomedical Imaging at Yale Cancer Center.

The breasts are made up of fatty, glandular, and connective tissues. Breast density is a measure of the amount of dense tissue (glandular and connective tissues) compared to less dense tissue (fatty tissue). Breasts that contain a higher proportion of glandular and connective tissues relative to fatty tissue are said to be dense.

Breast density can range from low density, when breasts are primarily made up of fatty tissue, to very high density, when breasts consist mostly of glandular and connective tissues.

The only way to determine a woman’s breast density is with a mammogram. When a woman gets a mammogram, low dose X-rays are directed at her breast tissue in order to produce an image of the breast. When doctors look at these images, they can distinguish between different tissues based on their appearance in the image. 

Dense tissue, for instance, looks white in the image, while fatty (low density) tissue looks black. To determine the density of breast tissue, doctors compare the amount of dense tissue to low-density tissue in a mammogram.

Doctors classify breast density based on what’s called the Breast Imaging Reporting and Data System, or BI-RADS. This system divides breast density into four categories:

• A. Breasts almost entirely fatty tissue
• B. Scattered areas of glandular and connective tissues (together known as “fibroglandular” tissue)
• C. Breasts are considered heterogeneously dense. They contain more glandular and connective tissue than fatty tissue. Small tumors may be obscured in mammograms
• D. Breasts are considered extremely dense. Breast tissue in this category impairs the ability of mammography to detect tumors

Breast tissue that falls into categories “C” and “D” are categorized as dense breasts.

There are two reasons why this information is important.

• Dense breast tissue can make it harder for doctors to detect small tumors in mammograms. Like dense breast tissue, tumors look white in mammogram images. This makes it more difficult for doctors to distinguish tumors from dense breast tissue. Because of this, for women who have dense breast tissue, mammograms may overlook tumors.
• Women who have dense breasts have a higher-than-average risk of developing breast cancer. This risk rises with increasing breast density.

It’s important to note that if women with dense breasts are diagnosed with breast cancer, they are not at higher risk of dying from breast cancer, compared to women without dense breasts.

The U.S. Food and Drug Administration (FDA) requires mammography facilities to inform patients whether or not they have dense breasts and to recommend that women discuss breast density and breast cancer risk with their health care provider.

Several factors play a role in determining breast density, including:

• Age. Women under 50 tend to have higher breast density than older women. In particular, breast density often declines after menopause, but older women, however, can still have dense breasts.
• Genetic factors. Evidence suggests that breast density is associated with genetic factors that can be inherited.
• Having given birth. Giving birth is associated with lower breast density.
• Tamoxifen. Use of tamoxifen, a medication that may be used to treat or prevent certain breast cancers, can lower breast density.
• Hormone replacement therapy (HRT). Use of HRT after menopause is associated with higher breast density.
• Body mass index (BMI). Women with lower BMIs tend to have higher density breast tissue.

Women with dense breasts should continue to get regular screening mammograms. At Yale, screening and diagnostic mammography is performed with digital breast tomosynthesis (DBT), also known as three-dimensional (3D) mammography, breast tomosynthesis, or “tomo.”

Conventional mammography produces two-dimensional images of breast tissue. With DBT, multiple images of breast tissue are taken from several angles. A computer then combines these images into several 1 mm slices through the breast that the radiologist can scroll through.

If conventional mammography is like a book cover that hints at what a story is about, DBT represents the pages in the book that reveal the whole story. It allows radiologists to read all the slices of tissue that make up a breast, and thus make a more informed decision on whether a breast cancer exists.

Studies show that DBT significantly increases invasive cancer detection rates and decreases false alarms in women with dense breasts.

Women diagnosed with dense breasts should continue to get regular screening mammograms. But there are additional screening tests that can help doctors detect tumors that may not be identified by conventional mammography or DBT:

• Breast ultrasound (whole-breast ultrasound). This test uses sound waves to produce an image of breast tissues.
• Magnetic resonance imaging (MRI). An MRI uses magnetic fields and radio waves to produce images of breast tissue. A contrast dye is injected into a vein prior to the test. This dye enhances the contrast of tissues in the image, which helps doctors better detect tumors.

Though supplementary screening tests can detect cancers that are missed by conventional mammography, not all experts recommend that every woman diagnosed with dense breasts undergo additional screening tests. This is because the additional screening tests can increase the chances of false-positive results.

A false-positive result occurs when a screening test identifies something in the breast that may be cancer, but which upon further investigation (which may involve a breast biopsy) turns out to be non-cancerous. False-positive results can cause substantial stress and anxiety. What’s more, women need to undergo additional testing (such as a breast biopsy) to determine if the finding is cancer or a false-positive. 

Women with dense breasts should talk to their doctor about the benefits and risks of supplementary screening tests, in particular if they have other risk factors for breast cancer.

“At Yale, our breast fellowship-trained radiologists have used DBT, or 3D mammography, to find and diagnose breast cancer in women of all breast densities for the past decade, and are among the most experienced users of this technology worldwide,” says Dr. Durand.