Dense Breasts and What They Mean for Screening

Breasts are made of three kinds of tissue: glandular tissue, fibrous connective tissue, and fatty tissue. Breast density describes the balance of these on a mammogram. Breasts are called dense when they hold relatively high amounts of glandular and fibrous tissue, and relatively low amounts of fatty tissue.

Dense breasts are common and normal. They are not a disease or an abnormal condition. In fact, nearly half of women 40 and older who get mammograms are found to have dense breasts.

Dense breasts are common and normal — but they do affect how mammograms are read and your risk picture.

Here is one surprising fact. You cannot feel breast density. Neither can your doctor during a physical exam. Density is not about how your breasts feel, or how firm they are.

The only way to know is through a mammogram. A radiologist — a doctor who reads medical images — looks at the mammogram and grades the density. That is why density is sometimes called "mammographically dense."

Only a mammogram, read by a radiologist, can reveal breast density — you can't detect it yourself.

Radiologists use a system called BI-RADS. It describes breast density in four categories, from mostly fatty to extremely dense:

• Almost entirely fatty — about 10% of women
• Scattered areas of dense tissue — about 40%
• Many areas of dense tissue (heterogeneously dense) — about 40%
• Extremely dense — about 10%

If your mammogram report says you have dense breasts, you fall into one of the two densest categories.

Here is why density matters for reading a mammogram. On the image:

• Dense tissue appears white
• Some abnormal changes, like tumors, also appear white
• Fatty tissue appears dark

Dense tissue and possible problems both look white. So a problem can be harder to spot, like finding something white against a white background. That makes mammograms less sensitive in dense breasts, meaning they are more likely to miss a cancer. People with dense breasts may also be called back for follow-up testing more often.

Dense tissue and tumors both look white on a mammogram, so cancers are easier to miss.

Density also does something separate from its effect on reading a mammogram. Dense breasts are a risk factor for breast cancer. Women with dense breasts have a higher risk than women with mostly fatty breasts.

It is important to keep this in perspective. Density is one of several risk factors, and it is a modest one. Some factors are linked to higher density, such as using menopausal hormone therapy. Others are linked to lower density, such as increasing age and having had children. Your doctor can help you understand your overall risk.

One reassuring point. Research shows that people with breast cancer who have dense breasts are no more likely to die from it than those with fatty breasts, once other factors are accounted for.

Dense breasts modestly raise risk — but they don't make breast cancer more deadly.

Density affects screening, so updated U.S. FDA regulations now require mammography providers to inform you if your mammogram shows dense breasts. If you get such a notice, treat it as a good prompt to talk with your care team.

You may wonder whether you need extra imaging, such as an ultrasound or MRI. According to the U.S. Preventive Services Task Force, there is not yet enough evidence to recommend for or against additional imaging just because someone has dense breasts. Whether it makes sense depends on your full risk picture. So this is a conversation to have with your doctor.

A density notice is a cue to talk with your doctor about your overall risk and whether extra imaging fits you.