RESUMEN
Neuroendocrine carcinoma of the breast is a rare malignant tumor which, with the features of Merkel cells is even rarer. Herein, we report a case of small cell carcinoma with Merkel cell features in a 52-year-old female. Microscopically, the tumor was characterized by diffuse and consistent small round cells that were de-adherent. The tumor cells had round or oval nuclei with delicate chromatin and small nucleoli, the cytoplasm was sparse and eosinophilic. Additionally, the tumor was accompanied by high-grade ductal carcinoma in situ. Immunohistochemical staining showed that infiltrating tumor cells were positive for neuroendocrine markers, and punctately positive for CK20. The patient underwent modified radical mastectomy, axillary lymph node dissection, and postoperative adjuvant chemotherapy. No recurrence or metastasis was observed during follow-up period. Primary breast small cell carcinoma with Merkel cell features is rare and easily misdiagnosed as Merkel cell carcinoma. Early diagnosis and treatment may improve patient prognosis.