Breast composition varies significantly from person to person. Understanding this variability is important because it influences both the appearance of breast tissue on imaging and an individual’s breast cancer risk. The proportion of different tissue types within the breast contributes to its overall structure. This complex interplay of tissues determines what is often referred to as breast density.
The Building Blocks of Breast Tissue
Breasts are comprised of three main types of tissue: fatty tissue, glandular tissue, and connective tissue. Fatty tissue, also known as adipose tissue, largely determines the breast’s overall size and shape. On a mammogram, fatty tissue appears dark because X-rays pass through it easily.
Glandular tissue is responsible for milk production. Connective tissue, or fibrous tissue, provides support and holds both the glandular and fatty tissues in place. Both glandular and connective tissues are denser than fat, appearing white on a mammogram. This combination of fibroglandular (fibrous and glandular) tissue and fatty tissue forms the breast’s internal structure.
Why Breast Composition Varies
Several factors influence the proportion of fatty tissue to glandular and connective tissue in the breasts. Age is a factor, as breasts tend to become less dense with increasing age, particularly after menopause, due to the replacement of glandular tissue with fat. Conversely, younger individuals have denser breasts.
Hormonal changes throughout life also play a substantial role. During puberty, increased estrogen and growth hormone levels stimulate the development of both fatty and glandular tissue, leading to breast growth. Fluctuations during the menstrual cycle, as well as hormonal shifts during pregnancy and lactation, can increase the amount of glandular tissue. Genetics also contributes to breast composition, and higher body mass index (BMI) is associated with a lower mammographic density.
The Significance of Breast Density
Breast density is unrelated to breast size, firmness, or feel. This composition is an important consideration in breast health for two primary reasons. First, dense breast tissue can obscure potential abnormalities on a mammogram. Because both dense tissue and cancerous masses appear white on mammographic images, distinguishing between them becomes more challenging.
Second, dense breasts are associated with an increased risk of developing breast cancer. Women with extremely dense breasts have a risk of breast cancer that is two to six times higher compared to those with almost entirely fatty breasts. This elevated risk is separate from the masking effect on mammograms. To standardize reporting, healthcare providers use the American College of Radiology’s Breast Imaging Reporting and Data System (BI-RADS), which classifies breast density into four categories: A (almost entirely fatty), B (scattered areas of fibroglandular density), C (heterogeneously dense), and D (extremely dense). Recent regulations require that patients be informed if they have dense breasts.
How Breast Composition is Measured
Breast composition is primarily assessed through medical imaging. Mammography is the most common method used to determine breast density, allowing a radiologist to categorize the breast’s density. While radiologists visually assess the images, some advanced systems use computer algorithms to aid objective density measurements.
Beyond mammography, other imaging modalities can provide additional insights into breast composition. Ultrasound is often used as a supplemental tool, especially for individuals with dense breasts, as it can help distinguish between solid masses and fluid-filled cysts obscured on a mammogram. Magnetic Resonance Imaging (MRI) is another advanced technique that offers detailed views of breast tissue and is valuable for supplemental screening in dense breasts. Dual-energy X-ray Absorptiometry (DEXA) can also measure breast fibroglandular density by separating fat and glandular tissue compartments, though it is more common in research settings.