Breast density increases the risk of breast cancer . . . perhaps more than you thought and perhaps in more ways than one

Women with density in 75% or more of a mammogram are at almost five times the risk for breast cancer as women with density in less than 10%. This is largely because the disease is more difficult to detect, according to data from three nested case–control studies involving 1,112 matched case–control pairs. This makes density the third highest risk factor for breast cancer, behind age and mutations in BRCA1 or BRCA2.

The authors of the Canadian study calculated that the women with density in 75% or more of their mammogram had an odds ratio for developing breast cancer of 4.7 (95% CI, 3.0–7.4), whether detected by screening (OR 3.5; 95% CI, 2.0–6.2) or less than 12 months after a negative screening examination (OR 17.8; 95% CI, 4.8–65.9). And the increased risk persisted for at least 8 years, suggesting that masking is not the only force at work and that a biologic connection may exist between increased breast density and an increased risk for the disease.

Younger women were at greater risk than older; for those under 56 years of age, 26% of all breast cancers and half of those detected less than 1 year after a negative screen could be attributed to density in 50% or more of a mammogram.

Boyd NF, Guo H, Martin LJ, et al. Mammographic density and the risk and detection of breast cancer. N Engl J Med. 2007;356:227-236.

Kerlikowske K. The mammogram that cried Wolfe. N Engl J Med. 2007;356:297-300.