Microglandular Adenosis and Associated Invasive Carcinoma.

CONTEXT.—: Microglandular adenosis is a rare borderline neoplastic lesion of the breast composed of haphazardly located small, round tubules with a single cell layer interspersed within breast stroma and/or adipose tissue. Microglandular adenosis is devoid of a myoepithelial cell layer, and has a characteristic immunophenotype, being positive for S100 and negative for estrogen receptor, progesterone receptor, and HER2/neu. When associated with cancer, microglandular adenosis and associated invasive carcinoma share the same molecular alterations, including TP53 mutation; therefore, microglandular adenosis is considered a nonobligate precursor of triple (HER2/neu, estrogen and progesterone receptors)-negative breast carcinoma. Microglandular adenosis is an important diagnostic pitfall as it can be easily mistaken for a low-grade invasive carcinoma. OBJECTIVE.—: To provide a review of the clinicopathologic features of microglandular adenosis and associated invasive carcinoma, with emphasis on key features separating entities in the differential diagnosis. DATA SOURCES.—: Review of current literature on microglandular adenosis and associated invasive carcinoma and personal experience of authors. CONCLUSIONS.—: Microglandular adenosis can mimic breast carcinoma; attention to key features, including morphologic-immunophenotypic correlation, is essential in establishing the diagnosis.