Breast Density and its Implications

Screening mammography has made tremendous inroads into reducing the mortality rate from breast cancer over the past 30-years. During that period studies have demonstrated that it is early detection of breast cancer that is responsible for the reduction in mortality, Tabar, et al., Cancer 1999.

Unfortunately, mammography is not a perfect screening tool. Its sensitivity is highly dependent upon the radiographic density of the breast tissue. Digital mammography was intended to increase sensitivity with higher tissue densities, but the large-scale trial of digital mammography still found that mammography was less than 60% sensitive when used with women who have high-density breast tissue, vs. 98% sensitive in women with fatty breast tissue, Pisano et al., Radiology 2008.

The sensitivity problem is caused by the fact that fibroglandular tissue appears white on the mammographic image. Because cancers also appear white, the fibroglandular tissue, which is what causes tissue density, can mimic, obscure or hide breast cancers.

In addition to its confounding effect on the interpretation of mammograms, high tissue density has been determined in many studies to be an independent risk for breast cancer. Women with extremely dense breast tissue have 4-6 times more risk of getting cancer than the women whose breast tissue is mostly fat (low density), Boyd, et al., JNCI 2010. Put another way, women with extremely dense breasts are approximately twice the risk of a woman with average breast density, and a woman with extremely fatty breasts about half the risk of an average women. The significance of this density-related risk is so great that the American Cancer Society ranks its importance just below the risk for women who have tested positive for the BRCA 1 or 2 breast cancer genes, those with 2 or more relatives who have had pre-menopausal diagnoses of breast cancer and women with a prior history of breast cancer, ACS, Breast Cancer Facts and Figures 2009-10.

As might be expected, dense breast tissue is most often found in pre-menopausal women, and tissue density can be expected to decline from menopause on in a woman’s life. It is not, however, unusual to find post menopausal women with relatively high tissue density, and HRT and drugs like tamoxifen have established effects on tissue density. The number of women with high tissue density is estimated at over 50% of women age 50 and under, and over 30% of women who are over the age of 50, Stomper, et al., AJR 1996. The challenge of tissue density is obviously of major importance.