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.
(© 2023 The Authors. Cancer Reports published by Wiley Periodicals LLC.)