What does breast density mean in a mammogram?

Understanding Breast Density in Mammography

Breast density refers to the proportion of fibroglandular tissue compared to fat in the breast as seen on a mammogram, which affects both cancer detection sensitivity and represents an independent risk factor for developing breast cancer. 1

Breast Density Classification

The American College of Radiology (ACR) BI-RADS system classifies breast density into four categories:

1. Almost entirely fatty (BI-RADS a)
2. Scattered areas of fibroglandular density (BI-RADS b)
3. Heterogeneously dense (BI-RADS c) - may obscure small masses
4. Extremely dense (BI-RADS d) - lowers mammography sensitivity

Categories a and b are considered "nondense" while categories c and d are considered "dense" breasts. Approximately 43% of women aged 40-74 years have dense breasts (heterogeneously or extremely dense). 1

Clinical Significance of Breast Density

Impact on Cancer Detection

• Reduced mammogram sensitivity: Sensitivity decreases from 87% in fatty breasts to only 63% in extremely dense breasts 1
• Reduced specificity: Specificity decreases from 96% to 90% as breast density increases 1
• Masking effect: Dense tissue can hide or "mask" cancers on mammograms, making detection more difficult 1
• Increased interval cancer risk: Women with dense breasts have higher rates of cancers detected between screening mammograms, which often have worse prognosis 1

Breast Cancer Risk

• Independent risk factor: Dense breast tissue is an established independent risk factor for developing breast cancer 1
• Relative risk increase:
  • Women aged 40-49 years with dense breasts have a relative risk of 1.23 for invasive breast cancer
  • Women aged 50-64 years have a relative risk of 1.29
  • Women aged 65-74 years have a relative risk of 1.30 1
• Higher risk with extremely dense tissue: Women with extremely dense breast tissue have a 4 to 6-fold greater risk compared to those with fatty tissue 1

Variability in Breast Density Assessment

• Inconsistent classification: A woman's breast density classification can change between mammograms
• Reclassification rate: Major recategorization (from "dense" to "nondense" or vice versa) occurs in approximately 13-19% of women between sequential screenings 1
• Assessment methods: Breast density can be determined by visual assessment or using automated computer software 1

Implications for Screening

FDA Requirements

As of March 2023, the FDA issued a national requirement for dense breast reporting to patients and referring providers (compliance required by September 10,2024). Patients must be notified of their breast density and informed that supplemental imaging studies may be beneficial for cancer detection. 1

Supplemental Screening Options

For women with dense breasts, supplemental screening modalities may be considered:

• Digital Breast Tomosynthesis (DBT): Improves screening sensitivity in all women
• Whole Breast Ultrasound (US): May benefit average-risk women with heterogeneously dense tissue
• Breast MRI or Abbreviated Breast MRI (AB-MRI): Appropriate for women with dense breasts, especially those at intermediate or high risk
• Contrast-Enhanced Mammography (CEM): May be appropriate for intermediate-risk women with dense breasts 1

Key Considerations

• Breast density can change over time and tends to decrease with age 1
• Despite the increased risk, women with dense breasts who develop breast cancer do not have an increased risk of dying from the disease after adjusting for stage, treatment, and other factors 1
• Screening recommendations should consider both breast density and overall breast cancer risk assessment 1