Mucinous Cystadenocarcinoma in Breast: Complicated with Lymph Node, Thoracic Wall Metastasis, and Contralateral Triple Negative Breast Cancer: A Case Report

Background: Mucinous cystadenocarcinoma of the breast is a very rare and special type adenocarcinoma of the mammary tract. We report a rare case of mucinous cystadenocarcinoma of the breast with an unfavourable prognosis which was confirmed after surgical resection pathologically. Case summary: At low power appearance, the tumor formed mucus-filled spaces of varying sizes, and with mucus-rich tumor cells lining the walls. The tumor cells were arranged in papillary structures. At high magnification, the tumor cells shown moderate to severe atypia, which were most of simple columnar cells with nuclei in the base and cytoplasm rich in mucin. In some areas, tumor cells proliferated, stratified, and clustered that protruded into the cyst cavity, or formed papillary structures with thin fibrovascular core. Immunohistochemical staining showed cytoplasm CK7 strong positive of tumor cells and cytoplasm negative of CK20, PAX-8 and CDX2, which could exclude metastatic tumors from ovary and intestine. And the tumor cells also demonstrated the basal-like characters such as negative for ER, PR, HER-2 (triple-negative), CK5/6 focal positive, EGFR positive. Besides, a triple negative breast cancer with basal-like features, lymph node, thoracic wall metastasis of mucinous cystadenocarcinoma was found. Conclusion: Mucinous cystadenocarcinoma of the breast usually have a favorable prognosis, but in this case, it happened lymph node, thoracic wall metastasis, therefore, it needs extra attention of clinical workers.