A woman’s breast density is usually determined during her mammogram by her radiologist by visual evaluation of the images taken. Breast density can also be measured from mammograms by computer software and it can be estimated on computed tomography (CT scan) and MRI imaging. In the U.S., information about breast density is usually included in a report sent from radiologist to the referring doctor after a mammogram. Breast density information may also be included in the patient letter sent after their mammogram. In Europe, national reporting guidelines to physicians vary; no country has a public policy for density reporting to patients.
A woman’s breast tissue is categorized as one of four BI-RADS®  categories:
(A) Fatty; (B) Scattered fibroglandular tissue; (C) Heterogeneously dense; (D) Extremely dense
Breasts which are (C) heterogeneously dense, or (D) extremely dense, are considered “dense breasts.”
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 grey or white.
About 10% of all women have breasts considered to be “fatty.”
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 40% of all women have breasts with scattered fibroglandular tissue.
C. HETEROGENEOUSLY DENSE – There are large portions of the breast where dense (fibroglandular) tissue could hide masses.
About 40% of all women have heterogeneously dense breasts.
D. EXTREMELY DENSE – Most of the breast appears to consist of dense (fibroglandular) tissue creating a “white out” situation, making it extremely difficult to see through.
About 10% of all women have extremely dense breasts.
1. Sickles EA, D’Orsi CJ, Bassett LW, et al. ACR BI-RADS mammography. In: ACR BI-RADS® Atlas, Breast Imaging Reporting and Data System. Reston, VA: American College of Radiology, 2013