Spindle-cell argyrophilic mucin-producing carcinoma of the breast. Histological, ultrastructural, and immunohistochemical studies of two cases.

We report two cases of neuroendocrine carcinomas of the breast displaying unusual histological features: numerous spindle cells and argyrophilic signet-ring cells. Both patients were older than 70 years, and both presented with a bloody nipple discharge. The tumor in both cases was predominantly intraductal. The tumor cells showed little pleomorphism or cytological atypia; because of the presence of spindle cells, benign diagnoses, such as ductal epithelial hyperplasia and intraductal papilloma, were considered for the in situ component. Recognition of the palisading arrangement of the peripheral cells, intracytoplasmic lumina, mitotic figures, and mucin permitted the diagnosis of intraductal carcinoma. Invasive nests composed of identical cells confirmed the diagnosis of malignancy in both cases. Our cases, along with those previously reported, suggest that neuroendocrine carcinoma with mucin production is a distinct breast tumor that usually occurs in older patients who experience bloody nipple discharge. The prognosis may be more favorable than that of the usual type of breast carcinoma. Common histological features include predominantly intraductal growth, an absence of desmoplasia, and low-grade atypia. Awareness of morphological variants of this tumor, such as those reported here, is necessary to avoid erroneous diagnoses.