Mammographic density describes the composition of breast tissue as seen on a mammogram. Breasts are composed of fatty tissue and fibroglandular tissue, which includes milk glands and the connective tissue that provides structural support. On a mammogram, fatty tissue appears dark and transparent, while fibroglandular tissue appears white and solid. A higher amount of fibroglandular tissue relative to fatty tissue means the breast is considered dense. This is a common finding, not an illness.
Identifying Breast Density
Breast density is a radiological finding and cannot be determined by how a breast feels during a self-exam or a clinical physical exam. It is identified through mammography, where a radiologist assesses the images using the Breast Imaging Reporting and Data System (BI-RADS). This system classifies density into four categories:
- BI-RADS A: Describes breasts that are almost entirely fatty, with very little fibroglandular tissue. This composition makes it easier for radiologists to detect abnormalities.
- BI-RADS B: Is for breasts with scattered areas of fibroglandular density, though the majority of the breast tissue is still fatty.
- BI-RADS C: Is termed heterogeneously dense, with significant portions of dense glandular and fibrous tissue. It is estimated that about 40% of women fall into this category.
- BI-RADS D: Describes breasts that are extremely dense, where the tissue is almost all fibroglandular. This is found in approximately 10% of women.
Breasts classified as category C or D are considered “dense.”
Factors Influencing Breast Density
A person’s breast density is influenced by a combination of factors. Age is a primary determinant, as breast density decreases with age, particularly after menopause when denser tissue is replaced by fatty tissue. Genetics also plays a role, with studies showing that breast density is a highly heritable trait.
Body Mass Index (BMI) has an inverse relationship with breast density. Women with a lower BMI are more likely to have denser breasts, while women with a higher BMI tend to have a greater proportion of fatty tissue in their breasts, resulting in lower density.
Hormonal factors also have an impact on mammographic density. The use of postmenopausal hormone replacement therapy can increase or maintain breast density. Conversely, medications used to treat breast cancer, such as tamoxifen, can lead to a reduction in density. The number of children a woman has had can also be a factor due to changes in glandular tissue.
Implications for Cancer Risk
Having dense breast tissue affects a woman’s breast cancer risk in two ways. The first is the “masking effect.” On a standard mammogram, both dense fibroglandular tissue and cancerous tumors appear as white areas. When a large amount of dense tissue is present, it can create a camouflage effect, making it harder for a radiologist to distinguish a tumor from the surrounding healthy tissue, which can lead to a delayed diagnosis.
Beyond masking, high mammographic density is an independent risk factor for developing breast cancer. Women with heterogeneously or extremely dense breasts have a higher likelihood of being diagnosed with breast cancer compared to those with less dense breasts. The exact biological reasons for this are still under investigation, but it is thought that the cellular environment in dense tissue may be more conducive to the development of abnormal cells.
This increased risk is a statistical measure, and many women with dense breasts will never develop cancer. However, the information is a component of an individual’s overall risk profile, alongside other factors like family history and genetics.
Supplemental Screening and Management
For individuals with dense breasts (BI-RADS categories C and D), a standard mammogram may not be sufficient for effective screening. Supplemental screening tests are often recommended to improve early cancer detection. These methods are used with mammography, not as a replacement, to provide a more comprehensive view of the breast tissue.
One of the most common supplemental technologies is 3D mammography, also known as digital breast tomosynthesis. This technology takes multiple X-ray images of the breast from different angles, which a computer synthesizes into a three-dimensional image. This allows the radiologist to examine the tissue one thin layer at a time, which helps reduce the issue of overlapping tissue and makes it easier to spot abnormalities.
Other available options include breast ultrasound and breast Magnetic Resonance Imaging (MRI). A breast ultrasound uses sound waves to create images and can be useful for looking at specific areas of concern. A breast MRI uses a magnetic field and a contrast agent to produce detailed images and is often recommended for women at a high lifetime risk for breast cancer.
Many states now have laws requiring facilities to inform patients about their breast density. This encourages proactive conversations with healthcare providers about whether supplemental screening is appropriate.