A malignant neoplasm arising from mesothelial cells in the pleura. It is characterized by the presence of neoplastic cells with an epithelioid appearance. In the majority of cases, the neoplastic epithelioid cells lack significant cytologic atypia; mitotic figures are infrequently seen. In a minority of cases, the neoplastic cells are poorly differentiated and there is evidence of nuclear atypia and increased mitotic activity.