What Is Breast Density?
Breast density refers to the amount of fibroglandular tissue (glands and fibrous connective tissue) relative to fatty tissue as visualised on mammography. Dense tissue appears white on mammography; fatty tissue appears dark. Density is categorised by radiologists using the ACR BI-RADS system into four categories: A (almost entirely fatty), B (scattered fibroglandular densities), C (heterogeneously dense), and D (extremely dense). Approximately 40–50% of women have dense breasts (categories C or D).
Why Density Matters
Breast density matters for two independent reasons. First, dense tissue is white on mammography — as are tumours. This means dense tissue can mask cancers, reducing mammography sensitivity from ~90% in fatty breasts to ~50–60% in extremely dense breasts. Second, high density is itself an independent risk factor for breast cancer, with extremely dense breasts carrying approximately 4–6 times higher cancer risk than almost entirely fatty breasts, for reasons not fully understood.
Supplemental Screening for Dense Breasts
Because mammography sensitivity is reduced in dense breasts, supplemental screening modalities are recommended. Ultrasound finds additional cancers not visible on mammography in dense breasts but has high false-positive rates (25–30% of ultrasound-detected lesions are benign on biopsy). MRI has the highest sensitivity (90%+) and is recommended for high-risk women, though it has high cost and limited availability. The FDA now mandates that mammography reports include density information.
What Affects Density
Density decreases with age as glandular tissue involutes and is replaced by fat. Hormone replacement therapy (particularly combined oestrogen-progestogen HRT) increases density. Pregnancy and lactation temporarily affect density. Tamoxifen reduces density. Body weight has a complex inverse relationship: higher BMI is associated with lower density (more fat tissue in the breast) but higher overall breast cancer risk due to oestrogen production in adipose tissue.
