Fibroglandular tissue is central to breast health discussions, especially regarding mammography and breast cancer screening. This dense, structural component of the breast determines a person’s breast density based on its proportion relative to fatty tissue. Understanding this tissue is important because it directly influences how effective a standard mammogram is at detecting potential malignancies. Recognizing dense fibroglandular tissue is the first step toward a more personalized and effective breast screening strategy.
Defining Fibroglandular Tissue and Breast Composition
The breast is primarily composed of fatty tissue and fibroglandular tissue. Fibroglandular tissue is the functional and supportive structure, named for its two main components. The “glandular” part includes the lobules that produce milk and the ducts that transport it, while the “fibrous” part consists of supportive connective tissue.
This combination of glandular and fibrous material appears dense on imaging. In contrast, fatty tissue lacks this structural density. Due to this fundamental difference in physical makeup, the two tissue types appear distinct on a mammogram.
Fatty tissue is more translucent to X-rays, causing it to appear dark or transparent on the image. Fibroglandular tissue is denser, absorbs more X-rays, and consequently shows up as white areas. The proportion of this white, dense tissue compared to the darker fatty tissue determines breast density.
Understanding Breast Density Categories
Radiologists use the standardized Breast Imaging-Reporting and Data System (BI-RADS) from the American College of Radiology (ACR) to categorize breast density. This system classifies breast composition into four categories, labeled A through D, based on the percentage of fibroglandular tissue visible on the mammogram.
Category A represents breasts that are almost entirely fatty, typically making up about 10% of the population. Category B describes breasts with scattered areas of fibroglandular density, meaning there is a mix of dense and fatty tissue, with the majority being fatty. This category is common, representing around 40% of women.
The categories considered “dense” are C and D. Category C (heterogeneously dense) means the breast is composed of an unequal mix of dense and fatty tissue, but is predominantly dense. Category D (extremely dense) means the tissue is overwhelmingly composed of fibroglandular elements, with very little fat present. Categories C and D combined account for approximately half of all women undergoing mammography.
The Impact of Dense Tissue on Screening
The clinical significance of dense breasts stems from two primary concerns. The first is the “masking effect,” a technical limitation of mammography. Since both cancer and dense fibroglandular tissue appear white on a mammogram, a small cancerous tumor can be obscured within the normal dense tissue.
This masking effect significantly reduces mammography sensitivity, especially in the densest breasts. For women in Category A, sensitivity is near 100%, but for those in Category D, it can drop to as low as 50%. This means a substantial number of cancers may be missed during routine screening in extremely dense breasts.
The second concern is that high breast density is an independent risk factor for developing breast cancer. Women with extremely dense breasts (Category D) have a risk of developing cancer four to five times higher than women with almost entirely fatty breasts (Category A). This elevated risk, combined with decreased visibility on a mammogram, makes breast density a major focus in personalized screening.
Management and Supplemental Screening Options
For individuals identified with dense breasts, particularly those in Categories C and D, a conversation about supplemental screening is necessary. Because of the masking effect, a standard mammogram alone may not provide adequate cancer detection. This has led to new federal regulations, effective in 2024, requiring mammography facilities to notify all patients about their breast density.
Supplemental imaging methods are employed to overcome the limitations of mammography in dense tissue. Digital breast tomosynthesis (DBT), often called 3D mammography, is an advanced X-ray technique. It takes multiple images from different angles, which helps minimize the tissue overlap that causes the masking effect.
Other supplemental options include breast ultrasound and magnetic resonance imaging (MRI). Screening breast ultrasound can detect additional cancers not visible on a mammogram, though it is associated with a higher rate of false-positive results. Breast MRI is the most sensitive test available for detecting breast cancer, especially in high-risk women with dense breasts.