Peritoneal Metastatic Lobular Breast Carcinoma Cells Masquerading in Atypical Hyperplastic Mesothelial Cell Clusters: Pitfalls Mitigated by Cellblock Immunocytochemistry.

Metastatic lobular carcinoma of the breast in serous effusions is diagnostically challenging because the tumor cells simulate histiocytes and mesothelial cells. The typical metastatic pattern is isolated dispersed tumor cells which are unrelated to surrounding mesothelial cells. Metastatic deposits of lobular carcinoma cells within mesothelial cell clusters have not been previously reported. We report our findings of peritoneal metastatic lobular breast carcinoma cells masquerading in atypical hyperplastic mesothelial cell clusters found in cellblock sections, but were not present in ThinPrep slides. Three patients out of 10 patients diagnosed with breast lobular carcinoma showed infrequent large atypical cell clusters in cellblock sections. They were not identified in ThinPrep slides which showed chronic lymphohistiocytic inflammatory hemorrhagic effusion fluids, that initially were reported as negative for malignant cells and disregarded as mixed inflammatory infiltrates of lymphocytes, plasma cells and histiocytes. However, cellblock sections showed occasional large cell clusters that were confused with atypical mesothelial cell clusters and metastatic adenocarcinoma. Cellblock immunocytochemistry showed peculiar staining patterns. BerEP3 and CEA showed scattered single mononuclear epithelial cells intimately dispersed within WT1 and calretinin-positive mesothelial cell clusters. An extended panel showed intramesothelial epithelial tumor cells expressing GATA3, but were negative for TTF1, Napsin-A, PAX8, CDX2. The tumor cells also expressed ER and mammaglobin. They did not express E-cadherin. The cytologic diagnosis was metastatic lobular carcinoma of the breast. This was confirmed by breast core needle biopsies. Because this phenomenon is not always apparent in ThinPrep slides, cellblock sections supplemented with immunocytochemistry are a valuable diagnostic tool. The remaining seven patients showed the usual dispersed patterns of metastatic lobular carcinoma in ThinPrep slides and cellblock sections in pleural and peritoneal effusions.