What is fibroglandular density on a mammogram (magnetic resonance imaging)?

What is Fibroglandular Density on Mammogram

Fibroglandular density refers to the amount of glandular and fibrous connective tissue present in the breast as visualized on mammography, which appears white/bright on mammograms and can obscure underlying cancers while also serving as an independent risk factor for breast cancer development. 1

Tissue Composition and Mammographic Appearance

• The breast consists of two main tissue types: fibroglandular tissue (glandular elements and fibrous connective tissue) and fatty tissue 1
• On mammography, fibroglandular tissue appears white or bright, while fatty tissue appears dark 2, 1
• Fibrous connective tissue, particularly increased stromal collagen, contributes significantly to the white appearance and overall density measurement on mammograms 1
• The upper outer quadrant and area under the areola contain the highest concentration of glandular tissue 1

Standardized Classification System

The American College of Radiology uses the BI-RADS 4-category density classification system 2, 1:

• Category A: Almost entirely fatty breasts
• Category B: Scattered areas of fibroglandular density
• Category C: Heterogeneously dense breasts (may obscure small masses)
• Category D: Extremely dense breasts (lowers mammography sensitivity)

Approximately 43% of women aged 40-74 years (about 25 million women in the United States) have heterogeneously or extremely dense breasts (categories C and D) 2

Clinical Significance: The Dual Problem

Dense breast tissue creates two distinct clinical challenges 3, 1:

1. Reduced Mammographic Sensitivity (Masking Effect)

• Dense fibroglandular tissue can mask cancers on mammography because both cancer and dense tissue appear white 3, 1
• Mammographic sensitivity decreases to as low as 30% in extremely dense breasts compared to 70-85% overall 3
• In heterogeneously dense breasts, sensitivity drops to approximately 63% compared to 87% in fatty breasts 4
• Calcifications associated with DCIS can be identified mammographically independent of breast density, which is an important exception to the masking effect 2

2. Independent Cancer Risk Factor

• Women with extremely dense breast tissue have a 4-6 fold greater risk of developing breast cancer compared to those with fatty tissue 3, 1
• For heterogeneously dense breasts, the relative risk is 1.2 compared to average density 2, 4
• For extremely dense breasts, the relative risk is 2.1 compared to average density 2
• Women aged 40-49 years with heterogeneously or extremely dense breasts have a relative risk of 1.23 for developing invasive breast cancer 2

Relationship to MRI Measurements

• Mammographic breast density correlates highly with volumetric fibroglandular tissue measured on MRI, with correlation coefficients ranging from 0.89 to 0.91 5
• MRI-based fibroglandular tissue percentage shows excellent inter-rater agreement (r = 0.994) and correlates strongly with mammographic density (r = 0.819-0.904) 6
• Automated volumetric measurements from mammography show substantial agreement with MRI measurements (kappa statistics 0.63-0.72) 7

Important Clinical Caveats

• The proportion of women with dense breasts is highest among those aged 40-49 years and decreases with age 2
• Women with dense breasts who develop breast cancer do not have an increased risk of dying from the disease after adjustment for stage, treatment, and other factors 2
• Dense breasts increase the risk of false-positive results, unnecessary biopsies, and false-negative results compared to average density breasts 2
• The FDA now requires that women be notified of their breast density status and informed that supplemental imaging may be beneficial 4