Breast Density Definitions

Prevalence of Dense Breast Tissue in Women

Breast density is essentially the ratio of fat to fibroglandular tissue in the breast. Radiologists characterize each mammogram into one of four levels of overall density. Based on population studies, and listed in order of least dense to most dense, the approximate frequency distribution of these categories is:

  • Almost entirely fatty: 10 percent
  • Scattered areas of fibroglandular density: 40 percent
  • Heterogeneously dense: 40 percent
  • Extremely dense: 10 percent

All women who fall into the latter two categories (heterogeneously and extremely dense) will be informed that they have dense breasts, under law. Therefore, approximately 50 percent of women will fall into these two categories and be told that they have dense breasts. Currently, the determination of density by radiologists is a qualitative, visual assessment. Evolving technologies may provide more objective and quantitative density assessment in the future.

Significance of Mammographic Breast Density

There are two primary implications of mammographic breast density. One implication is the effect on mammographic sensitivity (i.e., the test’s ability to identify a clinically occult malignancy). This concept is known as masking. The second implication is the increase in breast cancer risk imparted by dense breasts. Overall, the potential masking effect of breast density is likely of greater importance than the minor increase in breast cancer risk

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