What is Breast Density?

A woman’s breast is primarily made up of fat and fibroglandular breast tissue, which is the network of milk glands and ducts designed to produce and transport milk to the nipple for breast feeding. Fibrous tissue, also known as connective tissue, supports the breast and holds everything in place. Dense tissue is comprised of fibrous connective tissue and glandular tissue.

When performing a mammogram, breast specialists are taking an X-ray of a 3D organ, the breast, and transforming it into a two dimensional image that can be displayed on film or a monitor. All women have varying volumes of fatty and fibroglandular breast tissues in their breasts, and that will change how the mammogram looks, from person to person.

Breast density is the ratio of volume of dense tissue to the total volume of the breast:

  • High breast density means there is a greater volume of fibroglandular breast tissue compared to fat.
  • Low breast density means there is a greater volume of fat compared to fibroglandular breast tissue.

This is important because on a mammogram, fat in the breast looks dark and the denser fibroglandular breast tissue looks light gray or white. Cancer can also appear white on a mammogram, which makes it is harder to interpret mammograms in women with dense breasts. This results in a decrease in the sensitivity of the mammogram, which may impact early detection in women with dense breasts. According to research, 35 percent of breast cancer goes undetected by mammography in women with dense breasts, as density masks the appearance of tumors.

Example Mammogram with Dense and Fatty Tissue

Fatty tissue appears darker than dense tissue in mammograms as they block less x-rays.











Breast density changes throughout a woman’s life with most studies showing breast density decreasing with age, but it is also known to be influenced by other factors such as genetics, body mass index, monthly hormonal cycles, age at first childbirth, and use of post-menopausal hormone replacement.
In this video, Jean Weigert, MD, FACR, director of Breast Imaging at The Hospital of Central Connecticut, explains breast density and how it affects radiologists’ ability to detect cancers in mammography.

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