How is a woman determined to have dense breast tissue?

Breasts that are heterogeneously dense or extremely dense on mammography are considered “dense breasts.” Breast density matters for 2 reasons: Dense tissue can mask cancer on a mammogram, and having dense breasts increases the risk of developing breast cancer.

Breast density measurement

A woman’s breast density is usually determined during her breast cancer screening with mammography by her radiologist through visual evaluation of the images taken. Breast density also can be measured from individual mammograms by computer software, and it can be estimated on computed tomography (CT) scan and magnetic resonance imaging (MRI). In the United States, information about breast density is usually included in a report sent from the radiologist to the referring clinician after a mammogram is taken, and may also be included in the patient letter following up screening mammography. In Europe, national reporting guidelines for physicians vary.

The density of a woman’s breast tissue is described using one of four BI-RADS® breast composition categories¹ as shown in the FIGURE.

A. ALMOST ENTIRELY FATTY – On a mammogram, most of the tissue appears dark gray or black, while small amounts of dense (or fibroglandular) tissue display as light gray or white. About 13% of women aged 40 to 74 have breasts considered to be “fatty.”²

B. SCATTERED FIBROGLANDULAR DENSITY – There are scattered areas of dense (fibroglandular) tissue mixed with fat. Even in breasts with scattered areas of breast tissue, cancers can sometimes be missed when they look like areas of normal tissue or are within an area of denser tissue. About 43% of women aged 40 to 74 have breasts with scattered fibroglandular tissue.²

C. HETEROGENEOUSLY DENSE – There are large portions of the breast where dense (fibroglandular) tissue could hide small masses. About 36% of all women aged 40 to 74 have heterogeneously dense breasts.²

D. EXTREMELY DENSE – Most of the breast appears to consist of dense (fibroglandular) tissue, creating a “white out” situation and making it extremely difficult to see through and lowering the sensitivity of mammography. About 7% of all women aged 40 to 74 have extremely dense breasts.²

Factors that may impact breast density

Age. Breasts tend to become less dense as women get older, especially after menopause (as the glandular tissue atrophies and the breasts may appear more fatty-replaced).

Postmenopausal hormone therapy. An increase in mammographic density is more common among women taking continuous combined hormonal therapy than for those using oral low-dose estrogen or transdermal estrogen therapy.

Lactation. Breast density increases with lactation.

Weight changes. Weight gain can increase the amount of fat relative to dense tissue, resulting in slightly lower density as a proportion of breast tissue overall. Similarly, weight loss can decrease the amount of fat in the breasts, making breast density appear greater overall. Importantly, there is no change in the amount of glandular tissue; only the relative proportions change.

Tamoxifen or aromatase inhibitors. These medications can slightly reduce breast density.

Because breast density may change with age and other factors, it should be assessed every year.