Radiologically, the breast varies according to its tissue composition. Fat is radiographically translucent and appears dark on the film. Epithelium and stroma are radiographically opaque or dense and appear light on the film. The amount of radio-opaque tissue is referred to as the breast's density and generally the higher a woman's breast density, the greater her risk of developing breast cancer.
As well as the increased risk, high breast density makes the interpretation of mammography films more difficult, so early lesions may go undetected, leading to poorer outcomes. It is difficult to say whether it is the breast density per se which increases the risk or the delay in diagnosis because of the density which is the problem.
In an attempt to resolve the issue, Boyd et al from Canada (NEJM 2007;356:227-36) noted the woman's breast density at her first mammography and then carefully tracked her risk of subsequent cancer using modern techniques. Thus they hoped to eliminate the detection problem and quantify the risk of high density breast tissue. Their results were conclusive in that women with densities over 75% were five times more likely to develop breast cancer than those with densities below 10%.
Kerlikowske in an editorial (pp 297-9) draws these data together with other risk factors, such as age, family history, BRCA status, and makes a plea for defining each woman's risk status and advise her about mammography accordingly. The density factor is a major risk and seems especially important in women aged 50 to 55 years. Better detection methods, possibly with digital as opposed to film screening may help but the bottom line remains - the greater the breast density, the greater the risk.
