Breast density refers to the proportion of fibroglandular (glandular and connective) tissue compared with fatty tissue in the breast, as seen on a mammogram. It cannot be felt on physical examination and is determined only through imaging. Radiologists classify breast density into four standardized categories (BI-RADS): Category A: almost entirely fatty, Category B: scattered areas of fibroglandular density, Category C: heterogeneously dense, and Category D: extremely dense. Women in categories C and D are considered to have “dense breasts.” Dense tissue appears white on a mammogram, similar to most breast cancers, while fatty tissue appears dark.
Breast density is important for two main reasons. First, dense tissue can make cancers harder to detect on mammography because both dense tissue and cancer appear white (the “masking effect”). Second, dense breast tissue is an independent risk factor for developing breast cancer. However, breast density is only one of several risk factors, and most women with dense breasts will not develop breast cancer.
In Canada, mammography remains the foundation of screening and is recommended starting at age 40. For women with dense breasts (categories C or D), the Canadian Society of Breast Imaging supports offering supplemental screening in addition to mammography. MRI is the preferred supplemental test, where available, typically every two years. If MRI is not accessible or suitable, contrast-enhanced mammography (CEM) or ultrasound may be considered. The decision to pursue supplemental screening should be individualized and depends not only on breast density and overall risk factors, but also on local availability of imaging resources across provinces and territories. Supplemental screening does not replace mammography and should be discussed with a healthcare provider as part of shared decision-making.
