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 may increase the risk of breast cancer.

In the United States, laws require doctors in some states to inform women when mammograms show they have dense breasts. But just what women should do in response isn't clear.

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 results reporting system called BI-RADS. The levels of density are:

  • Almost entirely fatty indicates that the breasts are almost entirely composed of fat. About 1 in 10 women have this result.
  • 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.
  • 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.
  • Extremely dense indicates that nearly all of the breast tissue is dense. About 1 in 10 women have this result.

In general, a woman whose breasts are classified as heterogeneously dense or extremely dense is considered to have dense breasts. About half of women undergoing mammogram testing 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. Women in their 40s and 50s are most likely to have dense breast tissue. Your breast tissue tends to become less dense as you age, though some women may have dense breast tissue at any age.
  • Are premenopausal. Premenopausal women are more likely to have dense breasts.
  • 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.
Feb. 26, 2015