Mucinous breast carcinomas with abundant intracytoplasmic mucin and neuroendocrine features: light microscopic, immunohistochemical, and ultrastructural study.

Eight mucinous carcinomas of the breast were studied by light microscopy and immunohistochemistry; one was studied by electron microscopy. All 8 cases had abundant, relatively clear cytoplasm that contained mucin. Cells were argyrophil positive and argentaffin negative. Eight cases were positive for neuron specific enolase (NSE), 5 cases for serotonin, 1 case for serotonin and somatostatin and 2 cases for serotonin, somatostatin, and gastrin. None had clinical evidence of abnormal neuroendocrine function. Three patients had axillary lymph node metastases. Only 1 of 5 patients in whom there was clinical followup died of her disease. Electron microscopy of one case showed abundant intracytoplasmic and extracellular mucin, round and pleomorphic dense-core granules, numerous cell processes, and aggregates of intermediate filaments. These cases expand the histologic spectrum of breast carcinomas which may show neuroendocrine differentiation.