[Neuroendocrine differentiation in breast cancer]

To classify as breast cancer with neuroendocrine differentiation a growth pattern as encountered in well differentiated (G1, G2) neuroendocrine tumors (NET) of the gastrointestinal tract and the lung must be present in addition to the immunohistochemical expression of neuroendocrine markers (chromogranin, synaptophysin). The majority of breast cancers fulfilling these criteria show hormone receptor positivity and with regard to prognosis resemble luminal type of breast cancer from which they are not fundamentally different. Despite lacking clinical relevance the up-coming WHO classification of breast cancer will nevertheless introduce the new category of NET luminal type. Immunohistochemical detection of neuroendocrine marker alone cannot be considered as sufficient for classification because it can frequently be encountered in other types of breast cancer (e.g. mucinous, solid papillary). From low grade NET with good differentiation those with poor differentiation and most frequently small cell appearance have to be differentiated. Poorly differentiated NET can primarily originate in the breast, which may be indicated by intraductal growth and co-expression of GATA3 but in these cases metastasis of extra mammary cancers has to be considered and correlation with the clinical findings is required for correct classification.