Mucinous breast lesions: diagnostic challenges

Breast lesions with mucin represent a broad spectrum of entities, ranging from benign fibrocystic changes with luminal mucin to mucocele-like lesions (MLL), which can be associated with banal epithelial alterations, atypical ductal hyperplasia or ductal carcinoma in situ. Occasionally invasive mucinous carcinoma can be identified in contiguity with MLL. Diagnostic challenges are enumerated, histological differentials are discussed, and a practical approach towards resolving some of these issues is provided. In addition to these lesions with abundant extracellular mucin, there are also conditions that feature stromal mucinous or myxoid material, as well as rare entities that demonstrate both epithelial extracellular and stromal mucin.