Mesothelial lesions show mesothelial differentiation and arise from the serosal membrane(s), most commonly the pleura, less commonly pericardium, peritoneum, or tunica vaginalis. Mesothelial lesions include benign tumors (adenomatoid tumor, well-differentiated papillary mesothelial tumor), pre-invasive lesion (mesothelioma in situ), and malignant (mesothelioma).¹