Fibroglandular tissue is a term frequently encountered in mammography reports, referring to the mixture of two primary tissue types that compose a woman’s breast. Understanding the composition and proportion of this tissue is relevant to women’s health literacy, particularly regarding breast cancer screening. The amount of fibroglandular tissue present dictates breast density. This measurement has implications for both the effectiveness of standard screening methods and a woman’s inherent level of risk.
Composition and Role in Breast Structure
The breast is composed of three main elements: glandular tissue, fibrous connective tissue, and adipose (fatty) tissue. Fibroglandular tissue is the collective term for the glandular and fibrous components. Fibrous tissue, including structures like Cooper’s ligaments, provides the structural framework and support for the breast. Glandular tissue consists of the lobules (milk-producing glands) and the ducts that transport milk to the nipple. The rest of the breast volume is primarily filled with less dense adipose tissue, which appears different on imaging. The ratio of fibroglandular tissue to fatty tissue varies significantly among individuals and often decreases with age after menopause.
How Fibroglandular Tissue Determines Breast Density
Breast density is measured by the proportion of fibroglandular tissue relative to fatty tissue seen on a mammogram. Fibroglandular tissue contains water and attenuates X-rays, causing it to appear white on the image. Fatty tissue is radiolucent and appears dark. A higher proportion of dense, white-appearing tissue translates to a higher classification of breast density.
Radiologists use the Breast Imaging Reporting and Data System (BI-RADS) to categorize breast density into four groups:
- Category A describes breasts that are almost entirely fatty, containing very little fibroglandular density.
- Category B indicates scattered areas of fibroglandular density, meaning the breast is still mostly fatty tissue with some dense patches.
- Category C is defined as heterogeneously dense, where many areas of dense tissue are present, which may obscure small masses.
- Category D is classified as extremely dense, indicating that the breasts have large amounts of fibroglandular tissue, which significantly lowers the sensitivity of a mammogram.
Categories C and D are clinically considered “dense breasts.” Approximately half of women undergoing screening mammography fall into these dense categories.
Screening Challenges and Supplemental Imaging
High fibroglandular tissue density poses two main challenges for screening and risk assessment. The first is the “masking effect” on mammograms. Cancerous tumors, like dense fibroglandular tissue, appear white on a mammogram. This similarity makes it difficult for a radiologist to distinguish a small tumor from the normal dense tissue surrounding it. This masking effect reduces the sensitivity of a standard mammogram, especially in extremely dense breasts, where up to 50% of cancers may be missed.
The second challenge is the established correlation between high density and an increased lifetime risk of developing breast cancer. Women with extremely dense breast tissue (Category D) have roughly double the risk compared to women with scattered density (Category B). This increased risk, combined with the masking effect, means women with dense breasts may benefit from supplemental screening.
To overcome the limitations of standard mammography, healthcare providers may recommend alternative screening options. Digital Breast Tomosynthesis (DBT), or 3D mammography, provides multiple thin-layer images, which reduces the tissue overlap that causes the masking effect. Other modalities include breast ultrasound, which uses sound waves to create images and is widely available as a supplemental test. For women at higher risk, Magnetic Resonance Imaging (MRI) is also an option, using magnets and radio waves to produce detailed images highly sensitive for cancer detection.