"Thick" cell membranes revealed by immunocytochemical staining: A clue to the diagnosis of mesothelioma

The distinction of malignant mesothelioma from metastatic adenocarcinoma in pleural effusions and biopsies is frequently a diagnostic problem. Immunocytochemical staining of 13 malignant mesotheliomas, eight primary adenocarcinomas of the lung, five metastatic adenocarcinomas of the lung, and 20 primary adenocarcinomas in extrapulmonary sites with a monoclonal antibody to epithelial membrane antigen (EMA) revealed ‘thick’ cell membranes in all cases of mesothelioma. This distinctive pattern of staining was seen in the periphery of cell clusters and circumferentially around individual cells in cytologic preparations, cell blocks, and tissue sections. Intracel‐Mar and intercellular acini were also outlined by anti‐EMA, and long intraluminal microvillous projections were demonstrated. Weak cytoplasmic staining was only rarely seen in mesothelioma cells. This membranous staining pattern was not observed in adenocarcinomas, which displayed strong and diffuse cytoplasmic staining. The immunocytochemical demonstration of thick and spiky membranes circumferentially disposed around individual cells corresponds to aberrant microvilli, a diagnostic clue in the recognition of malignant mesothelioma.