Metastatic Lobular Carcinoma of the Breast in Pleural Fluid: Diagnostic Pitfall for Mesothelial Cells

Cytomorphologic patterns of breast carcinoma in effusion include three-dimensional groups of tightly packed clusters for invasive ductal type and single files with cytoplasmic signet ring-like cells for invasive lobular type. A mesothelial-like cell pattern has been described for metastatic lobular carcinoma in pleural fluid. We report a case of a patient with a remote history of lobular carcinoma of the breast who presented with hypocellular pleural effusion with the mesothelial-like cell pattern. Without the previous clinical history and the help of marker studies, the mesothelial-like cell pattern of metastatic lobular carcinoma to pleural fluid may be difficult to distinguish from benign reactive/atypical mesothelial cells.