Anatomic Pathology: Breast Pathology

• The relative amounts of fat, connective tissue, and epithelial tissue determine the radiographic appearance of the breast on a mammogram. Fat appears as dark or radiologic lucent areas, whereas connective tissue and epithelial tissue appear as areas of high radiologic density; this is usually expressed as a percentage, when percent mammographic density is the percentage of the breast area observed on a mammogram that is radiodense or white.

• Percent mammographic density has been found to be one of the strongest independent predictors of breast cancer risk. Women with the most mammographically dense breasts have a 4 to 6 increased risk of breast cancer compared with women with the least dense breasts.

• Breast density changes throughout a woman’s lifetime; the proportion of the epithelial tissue tends to increase until the third or fourth decade of life and then declines progressively with increasing age. More pronounced reductions occur after each pregnancy and after menopause. Mammographic density also can be altered by endogenous and exogenous hormonal factors.

• Age, parity, and menopausal status account for only 20% to 30% of the percent mammographic density variation observed in the population. Genetic factors might explain a proportion of variation of percent mammographic density. Studies of twins suggest that percent mammographic density is a heritable quantitative trait.

• The American College of Radiology developed a four-category system, the BIRADS (Breast Imaging Reporting and Data System) classification of mammographic density: (1) breast is almost entirely fat (<25% glandular); (2) scattered fibroglandular densities (25% to 50%); (3) heterogeneously dense breast tissue (51% to 75%); and (4) extremely dense (>75% glandular).

Assi V, Warwick J, Cuzick J, et al: Clinical and epidemiological issues in mammographic density. Nat Rev Clin Oncol 2012;9(1):33-40.

Boyd NF, Rommens JM, Vogt K, et al: Mammographic breast density as an intermediate phenotype for breast cancer. Lancet Oncol 2005;6(10):798-808.

Boyd NF, Martin LJ, Yaffe MJ: Mammographic density and breast cancer risk: current understanding and future prospects. Breast Cancer Res 2011;13(6):223-235.

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