Expression of the neuroendocrine phenotype in carcinomas of the breast

Neuroendocrine (NE) features are detectable in carcinomas of the breast either as scattered cells immunoreactive for NE markers in carcinoma of the usual type (NOS), or as special type of tumors where the vast majority of the cells display NE characteristics. The former type of lesions, whose biological and diagnostic significance is not clear yet, might reproduce the same phenomenon known to occur in carcinomas of the gastrointestinal tract and pancreas. In the present review we focus on the latter type of lesions, a spectrum of breast tumors largely composed of NE cells. These carcinomas, that we consider the "NE differentiated carcinomas of the breast," are here distinguished from "breast carcinomas NOS with NE differentiation." The diagnostic and histogenetic features of the various types of "NE differentiated carcinomas of the breast," their histological and cytological features and the role and value of ancillary diagnostic techniques, are reviewed. Data of the literature are discussed and related to a relatively large personal series. In addition, divergent differentiation in NE carcinomas of the breast, which is a relatively frequent phenomenon of diagnostic interest but of unknown significance (mainly involving mucinous intra- and extracellular production) is discussed.