Neuroendocrine tumor of the breast showing invasive micropapillary features and multiple lymph node metastases

BACKGROUND: Herein, for the first time, we present a case with mixed invasive micropapillary and neuroendocrine mammary neoplasm. CASE: The patient, a 65‐year‐old postmenopausal woman, had become aware of a tumor in her right breast 11 months prior to presentation at our hospital. The cut surface of the mastectomy specimen contained a well‐circumscribed, multinodular, red‐brown tumor, measuring 15x15x15 cm. Histopathologically, this solid cystic lesion consisted of medullary growth of cancer cells accompanied by a well‐developed vascular network as well as conspicuous hemorrhage. Cancer cell nests of various sizes displayed an “inside‐out” structure surrounded by empty spaces. Most cancer cells were polygonal, though a few were short fusiform‐shaped, and possessed finely granular, eosinophilic cytoplasm and ovoid, fine‐granular nuclei. Eighteen mitotic figures were observed in 10 high‐power fields. Macrometastases, up to 13x8 mm in size, with the same morphological features as the original tumor site, were identified in 3 of 15 dissected right axillary nodes. Immunohistochemically, primary and metastatic cancer cells were diffusely positive for chromogranin A and the estrogen receptor (Allred's total score: 8) and focally reactive for synaptophysin and the progesterone receptor (total score: 5). HER2 and cytokeratin 5/6 were negative, and the MIB‐1 labelling index was 36.2%. MUC1 and EMA lined the stroma‐facing surfaces of the cell membranes, indicating reversed polarity. CONCLUSION: Our current patient, who had an invasive breast carcinoma with concomitant neuroendocrine and micropapillary features, developed multiple nodal metastases in association with a large‐diameter tumor showing a luminal B‐like immuno‐profile. Accordingly, meticulous clinical follow‐up remains essential for this uncommon case.