The human breast is made up of two main types of tissue: fat and dense tissue. Dense breast tissue refers to that composed of glandular and fibrous tissue (that is why it is also called fibroglandular tissue), and it is formed of epithelial glandular elements (which includes ducts and terminal ductal-lobular units) and stromal elements (the fibrous connective tissue of the breast, which is formed of inter and intralobular stroma).
Breast density is the relative amount of fibrous and glandular tissue that attenuates X-rays on a mammography. It is the ratio of the dense tissue (this appears white on a mammogram X-ray) divided by the total area of the imaged breast (percent mammographic density or PMD). This is a subjective measurement that varies by reader and technique.
It is classified as follows, according to BI-RADS (Breast Imaging-Reporting and Data System):
Type 1: almost entirely fat. Fibroglandular tissue is less than 25%
Type 2: a scattering of glandular and fibrous tissue, from 25-50% of the breast.
Type 3: heterogeneously dense. Glandular and fibrous tissue forms 51-75% of the breast tissue
Type 4: highest category of breast density. The breast contains more than 75% fibroglandular tissue.
The importance of breast density is that it impacts the detection of breast cancer (breast cancer sensitivity being markedly lower in women with very dense breast due to masking by dense tissue, which is a major limitation of mammography). Likewise, breast cancer risk is increased for women with dense breasts (Density 3 and 4) compared to women with less dense breast (Density 1 and 2).
Additional information about fibroglandular density:
Mammographic Breast Density — What It Means
VIDEO: What is New in Breast Imaging Technology
VIDEO: Developments in Breast Density Legislation and Screening Guidelines
Making Sense of Dense Breasts
VIDEO: The Impact of Breast Density Technology and Legislation
Dense Breast Tissue: Supplemental Imaging
Technology to Watch in Breast Imaging
Breast Density Reporting Advances on National and State Levels