Lynn C. Hartmann, M.D., Amy C. Degnim, M.D., Richard J. Santen, M.D., William D. Dupont, Ph.D., and Karthik Ghosh, M.D

Breast biopsies are commonly performed to evaluate mammographic or palpable findings that are of concern, and the majority reveal benign findings. More than 1 million of the breast biopsies that are performed annually in the United States are found to be benign.1 On the basis of the histologic findings, it is possible to stratify women with benign biopsy findings into groups with significantly different risks of later breast cancer.2,3 Atypical hyperplasia is a high-risk benign lesion that is found in approximately 10%of biopsies with benign findings.4 In this article, we examine these benign lesions because they have special importance as a predictor of future breast cancer. There are two types of atypical hyperplasia, as classified on the basis of microscopic appearance: atypical ductal hyperplasia and atypical lobular hyperplasia; these occur with equal frequency and confer similar risks of later breast cancer ( Table 1 Risk of Breast Cancer among Women with Atypical Hyperplasia.). 2,3,9,10 Thus, throughout this article, the varieties will be referred to together as “atypical hyperplasia.”

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