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What is Breast Density?

Breast density is the description of the mammographic appearance of the tissue within the breast. It describes the relative amounts of fibrous and glandular tissue which show up as white on the mammogram compared to fat, which shows up as black or gray on the mammogram. Some women have denser breast tissue than others for reasons that are not completely understood. For most women, breast density decreases with age. But, in some women, there is little change. Many women have dense breasts and it is not abnormal.

scan showing different breast density classes

Radiologists characterize each mammogram into one of four levels of overall density: almost entirely fatty, scattered areas of fibroglandular density, heterogeneously dense, and extremely dense (see above image).

How many women have breast density in the United States?

  • 10% of women have almost entirely fatty breasts
  • 10% have extremely dense breasts
  • 80% are classified into one of two middle categories

distribution of breast density pie chart

What does it mean if I am told I have dense breasts?

All women who undergo mammography in Michigan will now receive a letter including information about their breast density. Breast density is reported in a standardized way in four categories of increasing density from the least dense- almost entirely fatty, to scattered areas of fibroglandular density, to heterogeneously dense and, extremely dense which is the most dense. “Dense breasts” are considered to be in the higher two categories, either heterogeneously dense or extremely dense.

Why is breast density important when it is seen on a mammogram?

Greater density on a mammogram makes it more difficult to detect a breast cancer due to a “masking” effect. Dense tissue appears white but cancers appear white as well. The expanse of dense white tissue is more likely to obscure cancer then a fatty, less dense background.

In addition, women who have higher breast density are at greater risk of developing breast cancer. This degree of risk is debated.

How high is the cancer risk associated with breast density?

The reporting of risk related to breast density can be misleading. Studies sometimes report the risk of having extremely dense breasts (which represents the highest 10% of breast density) compared to almost entirely fatty breasts (which is the lowest 10% of breast density). A more appropriate representation of breast cancer risk related to breast density is felt to be a comparison with the risk of a woman with average breast density. With this in mind, a patient with extremely dense breasts is considered to have approximately twice (relative risk 2.1) the risk of breast cancer compared to a woman with average breast density. A woman with heterogeneously dense breasts has a 20% increased risk (relative risk 1.2) of breast cancer compared to a woman with average breast density.