Mammographic breast density and cancer risk: the radiological view.

Breast density refers to the presence of fibroglandular tissue in the breast. Several investigators have demonstrated that extensive mammographic density is strongly associated with increased risk of breast cancer. Density can be assessed either subjectively or quantitatively. Subjective measurements include Wolfe patterns, the Breast Imaging Reporting and Data System and the 6-category scale. However, quantitative density measurements, such as interactive thresholding, planimetry and volumetric density imaging, provide stronger estimates of risk than those which are simply qualitative. Breast density is of particular interest in assessing the etiology of breast cancer, but it may also have potential as a surrogate marker of risk in interventions designed to reduce the risk of breast cancer. However, a surrogate marker is only acceptable if it shows biological feasibility and statistical correlation. To date, we can say that there is undoubtedly an association between breast density and breast cancer, but it will not be until an intervention study using risk as an endpoint has firmly established the connection between change in density and change in risk that density will be absolutely acceptable as a surrogate for breast cancer risk.