Why Does Breast Density Matter?

By Julie O’Brien Dennis, MD; Jason Wiseman, MD; and Meaghan Mackesy, MD

Lahey Hospital & Medical Center

More than 300,000 women in the U.S. will be diagnosed with invasive breast cancer this year, with about 56,500 new cases of noninvasive or pre-invasive breast cancer anticipated, according to the American Cancer Society. Breast cancer accounts for about 30 percent, or one in three, of all new female cancers each year.

Screening mammography is recommended annually for average risk women age 40 and over. Annual screenings have proven to decrease breast cancer mortality and may allow for less aggressive treatment options by finding cancer earlier.

There has been increased recognition of the importance of breast density in recent years.

Breast tissue is made up of two main components: fibroglandular or dense tissue and fatty tissue. Breast density describes how much of each type is present in the breast, as seen on a mammogram.

There are four classes of breast density:

• Class A: Breasts are almost entirely made up of fatty tissue, with very little dense breast tissue. Mammograms usually appear clearer.
• Class B: Breasts have mostly fatty tissue with scattered areas of dense breast tissue.
• Class C: More of the breast is made up of dense breast tissue, described as heterogeneously dense, making it harder to see small masses.
• Class D: Breast tissue is extremely dense, with very little fatty tissue. Mammograms can be challenging to read.

In general, women are considered to have dense breasts if their mammogram report says they have heterogeneously dense or extremely dense breasts. About half of people having screening mammograms have dense breasts.

Since 2014, breast imaging centers in Massachusetts have been required to inform patients if their breast tissue is dense (Class C or Class D) based on Mammography Quality Standards Act and Program (MQSA) guidelines. As of September 2024, the Food and Drug Administration now requires that all patients be informed of their breast density, including if their breast tissue is not dense (Class A or Class B). This means that all patients will receive clear information about their breast density following their annual mammogram.

Why is this important? Women with dense breast tissue may have an increased risk of developing cancer. At this time, it is unclear why dense breast tissue is linked to breast cancer risk. It may be that dense breast tissue has more cells that can develop into abnormal cells.

Dense breast tissue also makes it more challenging for radiologists to see cancer on mammograms since both dense tissue as well as masses and cancers appear white on the X-ray. In contrast, fatty tissue appears black on mammograms.

Understanding breast density helps raise awareness. For some individuals with dense tissue, doctors may recommend additional imaging tests along with mammograms to better detect cancer. Keep in mind that some insurance plans may not cover further tests solely based on breast density, so it’s crucial for patients to discuss their overall risk with their provider.

Lahey Hospital & Medical Center’s Comprehensive Breast Health Center offers digital breast tomosynthesis, also known as 3D mammography. Tomosynthesis is a more sensitive mammogram, which produces clearer and more accurate images of the breasts. Research on breast tomosynthesis has shown a significantly higher cancer detection rate in women with dense breast tissue over conventional 2D mammography.

The Comprehensive Breast Health Center at Lahey is dedicated to individuals at high risk for breast cancer, and the Breast Imaging Department offers additional screening options for those with dense breast tissue.

It is essential for patients to talk with their health care providers about their breast density, personal risk for breast cancer, family history and any individual concerns.

For more information on breast density, visit densebreast-info.org.

Julie O’Brien Dennis, MD, is director of the Comprehensive Breast Health Center at Lahey Hospital & Medical Center; Jason Wiseman, MD, is a surgical oncologist at Lahey Hospital & Medical Center; and Meaghan Mackesy, MD, is a diagnostic radiologist specializing in breast imaging and oncology at Lahey Hospital & Medical Center. For more on the Comprehensive Breast Health Center at Lahey Hospital & Medical Center, visit www.lahey.org/services/breast-health.