Breast Density Definitions

Question: How common is it to have dense breasts?

Answer: 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 frequency distribution of these categories is approximately: almost entirely fatty - 10%; scattered areas of fibroglandular density - 40%; heterogeneously dense - 40%; and extremely dense - 10%. All women who fall into the latter two categories (heterogeneously dense and extremely dense) will be informed that they have dense breasts under the law. Therefore, approximately 50% 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.


Question: What is the significance of mammographic breast density?

Answer: 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. Both these implications are described below. Overall, the potential masking effect of breast density is likely of greater import than the minor increase in breast cancer risk.