Mammographic densities and breast cancer risk

The radiological appearance of the female breast varies among individuals b ecause of differences in the relative amounts and X-ray attenuation charact eristics of fat and epithelial and stromal tissues. Pat is radiolucent and appears dark on a mammogram, and epithelium and stroma are radiodense and a ppear light. We review here the evidence that these variations, known as ma mmographic parenchymal patterns, are related to risk of breast cancer. Studies that used quantitative measurement to classify mammographic pattern s have consistently found that women with dense tissue in more than 60-75% of the breast are at four to six times greater risk of breast cancer than t hose with no densities. These risk estimates are independent of the effects of other risk factors and have been shown to persist over at least 10 year s of follow up. Estimates of attributable risk suggest that this risk facto r may account for as many as 30% of breast cancer cases. Mammographically dense breast tissue is associated both with epithelial pro liferation and with stromal fibrosis. The relationship between these histol ogical features and risk of breast cancer may by explained by the known act ions of growth factors that are thought to play important roles in breast d evelopment and carcinogenesis. Mammographically dense tissue differs from most other breast cancer risk fa ctors in the strength of the associated relative and attributable risks for breast cancer, and because it can be changed by hormonal and dietary inter ventions. This risk factor may be most useful as a means of investigating t he etiology of breast cancer and of testing hypotheses about potential prev entive strategies.