Diagnostic use of muscle markers in the cytologic evaluation of serous fluids.

Differentiating reactive mesothelial cells from malignant mesotheliomas and from adenocarcinomas can be diagnostically challenging when based solely on the morphologic examination of serous fluids. The diagnosis even after the use of standard immunohistochemical stains may at times be inconclusive because of the variable reactivity of mesothelial cells for these markers. Pathologists and cytologists underutilize reactivity for desmin, a feature of mesothelial cells apparently not shared by adenocarcinomas. The purpose of this study was to evaluate the extent to which mesothelial cells express muscle differentiation and to assess the diagnostic utility of muscle markers in distinguishing reactive mesothelial cells from malignant mesotheliomas and adenocarcinomas. Archival paraffin-embedded cell blocks of serous fluids from 24 cases of reactive mesothelial cells, 14 cases of malignant mesothelioma, and 56 cases (14 cases from each) of metastatic adenocarcinoma from the lung, breast, ovary, and gastrointestinal tract were retrieved. Five cases of omentum with unremarkable mesothelial cells were also included in the study. All cases were stained for desmin, actin, myoglobin, and myogenin and evaluated independently by two observers. Strong cytoplasmic reactivity for desmin was noted in 22 of 24 cases (92%) of reactive mesothelial cells. The reactive mesothelial cells did not express actin, myoglobin, or myogenin. All cases of malignant mesothelioma and metastatic adenocarcinoma were negative for the four muscle markers. The mesothelial lining and scattered subserosal cells in the omental sections were positive for desmin. Because desmin was expressed only in benign mesothelial cells, it may serve as a reliable marker in distinguishing reactive mesothelial cells from mesothelioma or from adenocarcinoma. Awareness of this staining pattern is also important to avoid pitfalls when evaluating body fluid specimens from patients with a history of tumors expressing muscle differentiation.