Mucinous carcinoma of the breast: morphological spectrum, diagnostic pitfalls and classification challenges

BackgroundMucinous breast carcinoma (MBC) is a well-recognised special type of invasive breast carcinoma (BC) characterised by abundant extracellular mucin and, in its classical form, a favourable clinical behaviour. However, extracellular mucin production is not unique to classical MBC and extracellular mucin is associated with a spectrum of benign and malignant entities with varying and overlapping morphological patterns but with very different clinical outcomes. Failure to recognise this diversity risks diagnostic imprecision and inappropriate management decisions.This review adopts a clinicopathological and pathogenetic framework centred on BCs associated with extracellular mucin, using classical low-grade MBC as the prototype lesion. The review integrates available molecular data, clinical outcome studies, observations on associated in situ and precursor lesions and accumulated diagnostic experience to generate conceptual models linking morphology, biology and clinical behaviour. Emphasis is placed on reproducible features relevant to routine diagnostic practice to enable pathologists to resolve a differential diagnosis of breast lesions in core biopsies and resections.