Invasive mammary carcinoma with neuroendocrine differentiation: histological features and diagnostic challenges.

AIMS: The aim of this study was to review the histomorphological features of primary neuroendocrine carcinomas (NEC) of the breast, in order to identify features useful in recognition of this entity for appropriate classification. METHODS AND RESULTS: 2003 World Health Organization (WHO) classification of tumors of the breast and female genital organs defined NEC of the breast as a subtype of invasive mammary carcinoma in which >50% of the tumor cells express neuroendocrine markers. Seventy-four cases that fulfilled the WHO diagnostic criteria for NEC of the breast, excluding small cell carcinoma and low-grade solid papillary carcinoma with a predominant in-situ component, were identified between 1984 and 2008 from MD Anderson Cancer Center, and were included in the study. NECs of the breast had variable histomorphological features. The most common histologic patterns were papillary (80%) and nested (64%). Mixed growth patterns were common (59%), including admixed ductal component. The tumor cells could be polygonal, round, plasmacytoid, spindled, or with signet ring cell features. The cytoplasm could be granular, eosinophilic, clear, or finely vacuolated. These tumors frequently mimicked invasive or in situ ductal carcinoma, or invasive lobular carcinoma. CONCLUSIONS: NEC of the breast is underrecognized. Careful attention to cytologic and architectural features can help to identify cases that require further immunophenotypic confirmation for correct tumor classification.