What is Fibroglandular Density?
Fibroglandular density refers to the proportion of fibrous connective tissue and glandular tissue in the breast compared to fatty tissue, which is visible on mammography and classified using the American College of Radiology's BI-RADS 4-category system. 1
Classification System
The American College of Radiology BI-RADS atlas divides breast density into four categories based on the amount of fibroglandular tissue: 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 in the United States have heterogeneously or extremely dense breasts (categories C and D), with the highest proportion occurring in women aged 40-49 years. 1
Tissue Composition
Fibroglandular tissue consists of two main components: 2
- Glandular tissue: Milk-producing lobules and ducts, with highest concentration in the upper outer quadrant and area under the areola
- Fibrous connective tissue: Stromal collagen that provides structural support and contributes significantly to mammographic density
The correlation between mammographic density measurements and actual volumetric fibroglandular tissue on MRI is approximately 0.91, indicating that mammographic density accurately reflects the physical tissue composition. 3
Clinical Significance: Dual Impact on Breast Cancer
Fibroglandular density has two critical clinical implications: 1, 2
1. Masking Effect (Reduced Mammography Sensitivity)
- Mammography sensitivity decreases from 87% in the lowest density category to 63% in the highest density category 1
- Dense fibroglandular tissue appears white on mammography, as do cancers, making tumors difficult to detect 1
- This masking effect can reduce sensitivity to as low as 30% in extremely dense breasts 2
2. Independent Risk Factor for Breast Cancer
- Women with extremely dense breasts have a 4-6 fold greater risk of developing breast cancer compared to those with fatty breasts 1, 2
- Women aged 40-49 years with heterogeneously or extremely dense breasts have a relative risk of 1.23 for developing invasive breast cancer 1
- For women aged 50-64 years, the relative risk is 1.29, and for ages 65-74 years, it is 1.30 1
Important caveat: 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. 1
Variability and Assessment
Breast density classification can be inconsistent over time, with major reclassification (from "dense" to "nondense" or vice versa) occurring in approximately 13-19% of women between sequential screening examinations. 1 This variability complicates risk assessment and screening decisions. 1
Density can be assessed through: 4
- Subjective methods: Visual assessment by radiologists using BI-RADS categories
- Quantitative methods: Automated computer software, interactive thresholding, or volumetric measurements on MRI
Volumetric measures of fibroglandular tissue (percent fibroglandular volume) provide stronger breast cancer risk prediction than percent dense area alone, with odds ratios of 4.1 versus 2.5 in the highest versus lowest quintiles. 5