What is dense breast tissue?

What is Dense Breast Tissue?

Dense breast tissue refers to breast tissue with a higher proportion of fibroglandular tissue (glandular and connective tissue) compared to fat, which appears white on mammograms and can both mask cancers and increase breast cancer risk. 1

Definition and Classification

Dense breast tissue is categorized according to the American College of Radiology (ACR) BI-RADS classification system into four categories:

1. Almost entirely fatty
2. Scattered areas of fibroglandular density
3. Heterogeneously dense
4. Extremely dense

Categories 3 and 4 (heterogeneously dense and extremely dense) are considered "dense breasts" 1, 2. Approximately 43% of women aged 40-74 years have dense breasts 2.

Composition and Characteristics

Dense breast tissue consists of:

• Higher proportion of fibroglandular tissue (epithelial and stromal elements)
• Less fatty tissue
• Increased collagen and extracellular matrix components 1
• Greater concentration of breast epithelium 1

The density is determined by the relative proportions of these tissues and their X-ray attenuation characteristics. Fat appears dark (radiolucent) on mammograms, while fibroglandular tissue appears white (radiodense) 3.

Clinical Significance

Cancer Risk

Dense breast tissue is an independent risk factor for breast cancer:

• Women with extremely dense breast tissue have a 4-6 fold greater risk compared to those with fatty tissue 1
• Increases breast cancer risk by 23-30% compared to women with average breast density 2
• Relative risk increase varies by age: 1.23 for women 40-49 years, 1.29 for women 50-64 years, and 1.30 for women 65-74 years 2

Mammographic Sensitivity

Dense breast tissue significantly impacts cancer detection:

• Decreases the sensitivity of mammography due to "masking effect" 1, 2
• Increases the rate of interval cancers (cancers detected between regular screening mammograms) 1
• Cancers detected in dense breasts tend to be larger, more often lymph node positive, and of higher stage 4
• Higher incidence of tumor multifocality and multicentricity 4

Assessment Methods

Breast density is typically assessed through:

• Visual assessment by radiologists using the BI-RADS classification
• Automated computer software that calculates volumetric breast density 1

Regulatory Requirements

The FDA has issued a national requirement for dense breast reporting:

• All mammography facilities must comply by September 10,2024
• Patients must be notified of their breast density
• Patients must be informed that supplemental imaging studies may be beneficial for cancer detection 1, 2

Screening Recommendations for Women with Dense Breasts

For women with dense breasts, supplemental screening options include:

1. Digital Breast Tomosynthesis (DBT): Improves cancer detection rates and reduces recall rates by 18-46% compared to standard mammography 2

2. Breast MRI: Highest sensitivity for detecting additional cancers

   • Preferred supplemental screening for women with dense breasts
   • Reduces interval cancers (as demonstrated in the DENSE trial) 5
   • Detects 15-16 additional cancers per 1,000 examinations 1, 2

3. Whole Breast Ultrasound: Alternative when MRI is contraindicated

   • Detects approximately 3.7 additional cancers per 1,000 screens 2

4. Contrast-Enhanced Mammography (CEM): May be appropriate for intermediate-risk women with dense breasts 2

Important Considerations

• Breast density can change over time and tends to decrease with age 2
• Supplemental screening increases false positive results and unnecessary biopsies 2
• Despite the increased risk of developing breast cancer, 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 2

Understanding breast density is crucial for appropriate breast cancer screening strategies and risk assessment, particularly as new legislation requires informing women about their breast density status.