Answer: Intraductal papilloma with ductal carcinoma in situ

Histology: This is a papillary neoplasm, in that the ducts contain fibrovascular cores. The epithelial cells filling these ducts are deceptively bland. They have pale eosinophilic cytoplasm and appear somewhat polymorphous, suggesting the possibility of usual duct hyperplasia. Other areas are somewhat discohesive, and there is focal intracellular mucin which suggests the possibility of lobular neoplasia. However, the cells filling the ducts show significant nuclear irregularity. The cells have plasmacytoid appearance with granular cytoplasm which suggests endocrine differentiation. This is substantiated by immunohistochemistry for synaptophysin which is diffusely positive. Intracellular mucin was identified on mucicarmine stain. The lesion extends over an area larger than 3mm, and E-cadherin was intact. Myoepithelial cells were intact at the periphery of the lesion and within fibrovascular cores, indicating that the lesion arose in an intraductal papilloma. These findings support the diagnosis of ductal carcinoma in situ with neuroendocrine and mucinous features involving a papilloma.

Discussion: The cytologic atypia and neuroendocrine differentiation exclude the possibility of usual duct hyperplasia. The lesion meets size criteria for ductal carcinoma in situ, and thus does not qualify as atypical duct hyperplasia. The lesion demonstrated intact E-cadherin, which argues against atypical lobular hyperplasia.