An invasive ductal breast carcinoma characterized by the presence of a predominantly syncytial architectural pattern. It may have some, but not all the strictly defined additional morphologic criteria which are necessary for the diagnosis of medullary breast carcinoma (presence of a diffuse lymphoplasmacytic infiltrate, neoplastic round cells with abundant cytoplasm and vesicular nuclei, complete histological circumscription, and absence of glandular or tubular structures). It does not have the relatively favorable outcome that characterizes medullary breast carcinoma. [from
NCI]