Atypical breast lesions: a challenging pathological diagnosis and an uncertain malignant potential

Atypical epithelial breast lesions are a heterogeneous group of entities, the diagnosis of which can be challenging. In this review, we discuss the wide spectrum of atypical epithelial breast proliferations including atypical ductal hyperplasia (ADH), atypical lobular hyperplasia (ALH), flat epithelial atypia (FEA), atypia in a papilloma, atypical microglandular adenosis, apocrine atypia and post neo-adjuvant chemotherapy associated atypia. We aim to review the histologic diagnostic criteria for these entities, with an emphasis on the salient morphologic features and differential diagnoses to consider, in order to provide a comprehensive and practical guide to their diagnosis.