Dense breast tissue: What it means to have dense breasts

Dense breast tissue is detected on a mammogram. Additional imaging tests are sometimes recommended for women with dense breasts.

By Mayo Clinic Staff

If a recent mammogram showed you have dense breast tissue, you may wonder what this means for your breast cancer risk. Doctors know dense breast tissue makes breast cancer screening more difficult and it increases the risk of breast cancer.

Review your breast cancer risk factors with your doctor and consider your options for additional breast cancer screening tests. Together you can decide whether additional screening tests are right for you.

What is dense breast tissue?

Dense breast tissue refers to the appearance of breast tissue on a mammogram. It's a normal and common finding.

Breast tissue is composed of milk glands, milk ducts and supportive tissue (dense breast tissue), and fatty tissue (nondense breast tissue). When viewed on a mammogram, women with dense breasts have more dense tissue than fatty tissue.

On a mammogram, nondense breast tissue appears dark and transparent. Dense breast tissue appears as a solid white area on a mammogram, which makes it difficult to see through.

How do doctors determine if you have dense breast tissue?

The radiologist who analyzes your mammogram determines the ratio of nondense tissue to dense tissue and assigns a level of breast density.

Levels of density are described using a results reporting system called Breast Imaging Reporting and Data System (BI-RADS). The levels of density are often recorded in your mammogram report using letters. The levels of density are:

  • A: Almost entirely fatty indicates that the breasts are almost entirely composed of fat. About 1 in 10 women has this result.
  • B: Scattered areas of fibroglandular density indicates there are some scattered areas of density, but the majority of the breast tissue is nondense. About 4 in 10 women have this result.
  • C: Heterogeneously dense indicates that there are some areas of nondense tissue, but that the majority of the breast tissue is dense. About 4 in 10 women have this result.
  • D: Extremely dense indicates that nearly all of the breast tissue is dense. About 1 in 10 women has this result.

In general, women with breasts that are classified as heterogeneously dense or extremely dense are considered to have dense breasts. About half of women undergoing mammograms have dense breasts.

What causes dense breast tissue?

It's not clear why some women have a lot of dense breast tissue and others do not.

You may be more likely to have dense breasts if you:

  • Are younger. Your breast tissue tends to become less dense as you age, though some women may have dense breast tissue at any age.
  • Have a lower body mass index. Women with less body fat are more likely to have more dense breast tissue compared with women who are obese.
  • Take hormone therapy for menopause. Women who take combination hormone therapy to relieve signs and symptoms of menopause are more likely to have dense breasts.
March 23, 2018 See more In-depth