Mucin‐producing breast lesions: a practical approach to diagnosis

Mucin-producing breast lesions encompass a diverse range of entities with varied morphologies, distinct molecular genetics and different outcomes. Mucocele-like lesions (MLLs) are being increasingly recognised and sampled due to advancements in imaging techniques. These lesions can present with or without epithelial proliferation and atypia, which hold prognostic significance. Diagnosing MLLs on limited core needle biopsy (CNB) samples can be challenging. Mucinous breast carcinoma (MuBC) generally has an excellent prognosis in its pure form. Recent studies indicate that mucin-producing invasive cancers with micropapillary growth pattern, high nuclear grade or HER2 overexpression/amplification may not fare as well as their pure counterparts, suggesting that they should be distinguished from pure MuBCs. Invasive lobular carcinoma with extracellular mucin (ILCEM) is an emerging subtype of ILC characterised by neoplastic cells in cords, nests and trabeculae, often with signet ring morphology, floating in extracellular mucin. This can lead to misdiagnosis as a ductal phenotype due to varied architectural patterns or a MuBC due to the presence of extracellular mucin. This review highlights the spectrum of mucin-producing breast lesions, focusing on the above-mentioned entities along with recent molecular updates, potential mimics and diagnostic pitfalls on CNB specimens. Awareness of these entities, a practical approach to their diagnosis, combined with judicious use of immunohistochemistry, are crucial for accurate diagnosis by pathologists, which is in turn essential for guiding clinical decision making for optimal patient outcomes.