Invasive Breast Carcinoma With a Multi-cystic Papillary Growth Pattern: A Unique Morphology of Invasion Not Currently Well Classified by the World Health Organization.

Papillary neoplasia of the breast is notoriously difficult to classify, even for seasoned pathologists and those subspecialized in breast pathology. Over the past few decades, the classification of these lesions has undergone multiple updates, and several newly recognized entities have been added. Multiple benign and malignant breast entities are characterized by the World Health Organization (WHO). Despite these categorizations, pathologists are sometimes presented with papillary morphologies and features that do not fit perfectly into any one of these classifications. We present a case of a 62-year-old female with an invasive breast carcinoma of a type that we believe is not currently well characterized by the WHO and that exhibits a papillary growth pattern, specifically invading as multi-cystic papillary structures.