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There is a wide variety of how normal breasts appear, both visually and on mammography. However, one of the important ways breasts differ on mammography is breast density.

What is breast density? Breasts are composed of fat, supporting structures like ligaments and vessels, and glandular tissue. The glands that produce milk and the fibrous supportive tissue are collectively called “fibroglandular tissue” which appears dense (white) on x-rays like mammography and tomosynthesis. Some breasts are mostly fatty and some breasts are mostly dense, with the majority somewhere in the middle. The way breasts look and feel does not predict whether or not they are dense on mammography. Radiologists characterize breast density using four categories:  a) almost entirely fatty; b) scattered fibroglandular density; c) heterogeneously dense; and d) extremely dense (see Figure).  The last two categories are considered “dense”.  It is normal to have dense breasts—about 40% of women who participate in mammography have dense breasts.

Figure A
There is a wide range of normal patterns of breast density on mammography, from A) fatty to B) scattered fibroglandular density to C) heterogeneously dense to D) extremely dense. Categories C and D are considered “dense”. As breast density increases, so does the likelihood that cancer, if present, can be hidden by the normal tissue and not detected by mammography. (Courtesy Dr. Wendie Berg)

Why does it matter?  A mammogram is essentially an x-ray of the breast, and x-rays have difficulty penetrating dense tissue. Dense breast tissue shows up as white or opaque on a mammogram. Cancerous masses are also dense and display as white and can therefore be hidden by normal dense tissue. In any woman, cancer may be found because of a lump even after a normal mammogram–but this is much more likely to occur in a woman with dense breasts as a cancer may grow undetected.  Calcifications are even whiter than dense tissue and are still seen on mammograms even in dense breasts.  About half of all invasive breast cancers have calcifications, so mammography should still be performed even in dense breasts.

Figure B
Mammograms from this 46-year-old woman show heterogeneously dense tissue and appear normal. Four months later, the woman noted a lump in her right breast. (Courtesy Dr. Wendie Berg)
Figure C
Ultrasound was performed, showing that the lump the woman was feeling was an irregular mass (arrow). Ultrasound-guided needle biopsy showed high-grade cancer. This 1.5-cm node-negative cancer was not visible on the mammogram even after biopsy as it blended in with the dense breast tissue. (Courtesy Dr. Wendie Berg)

Until recently, the letter sent to a woman after her mammogram did not include information about breast density. Laws have now been enacted in about half of the United States that require that letter to include some level of information about breast density.  If you do not live in a state that requires density notification [see MAP to learn what is required in your state], ask your radiologist or healthcare provider as this information should be in the mammogram report sent from radiologist to healthcare provider.

If you have dense breasts, it is especially important to be aware of how your breasts normally look and feel and to report any changes to your healthcare provider.  There are also options for screening with other tests after mammography that are not limited by the breast density; speak to your health care provider to determine what is right for you.

Despite growing awareness about breast density, there has been little good information on what it means and what to do about it.  Co-developers JoAnn Pushkin, a patient advocate, Dr. Wendie A. Berg, a radiologist and expert in breast imaging and Cindy Henke, a technologist, have developed a website to help women and their health care providers become educated about breast density and screening options. For more information about dense breasts, patient question and answers and screening options, please visit:™ is an information resource focused on providing breast density information to both patients and health care professionals. This medically sourced educational tool is the collaborative effort of breast imaging experts and medical reviewers and has been developed to help guide discussion of the question: “Have dense breasts…now what?”

By JoAnn Pushkin, Executive Director, DenseBreast-info, Inc.,
Dr. Wendie A. Berg, MD, PhD, FACR Professor of Radiology, 

Chief Scientific Advisor,,
Cindy Henke-Sarmento, Technology Director, DenseBreast-info, Inc.

Adapted from Berg, W.  “Breast Density 101”,, April 20, 2015