Studying Mammographic Density: Implications for Understanding Breast Cancer

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Abstract
Several epidemiologic studies (1-3) have reported the magnitude of the risk associated with increased mammographic density to be between fourfold and sixfold, exceeding that of most other predictors of breast cancer. In these studies, this increased breast cancer risk was not explained by other traditional breast cancer risk factors. Despite the level of evidence of an independent association with breast cancer and the large number of women who are having routine mammographic screening, mammographic density is perhaps the most undervalued and underutilized risk factor in studies investigating the causes of breast cancer. Many studies of breast cancer risk have not included measures of mammographic density either because of practical reasons, such as difficulty in obtaining mammographic images and in assessing features in a systematic manner, or because of misconceptions stemming from the interpretation of early studies of mammographic features. The fact that this strong predictor of risk has been overlooked in many studies of breast cancer seems particularly unfortunate because of the profound implications to both cancer screening and etiology that attention to these factors may have. Thus, to fully comprehend the causes of breast cancer, we need to understand the role of breast density in the disease process and to identify the determinants of mammographic density. To avoid some of the methodologic problems inherent in the early studies, investigators incorporating mammographic density in their research must pay close attention to the potential for bias inherent in their study design, in their assessment and quantification of mammographic features, and in their interpretation of findings (4-6).