Adenomatoid tumor of the pleura

A case of adenomatoid tumor of the pleura is reported, and its differential diagnosis from benign and malignant pleural lesions is discussed. A small pleural nodule was incidentally found during a thoracic operation in a 54-year-old woman with esophageal cancer. The patient had no history of exposure to asbestos, and was well with no sign of recurrence 14 months after the operation. A 7 mm, circumscribed tumor had characteristic features of adenomatoid tumor. The tumor was composed of an aggregation of irregularly shaped tubulocystic spaces with fibrous stoma. The spaces were lined by flattened and occasional cuboidal epithelioid cells with cytoplasmic vacuolization, and several spaces contained pale blue mucinous fluid. On immunohistochemistry the tumor cells were positive for AE1/AE3, CAM5.2, vimentin, cytokeratin 5/6, D2-40, calretinin, thrombomodulin, and WT-1, but negative for CEA, Leu M1 (CD15), thyroid transcription factor-1, epithelial membrane antigen, desmin, glucose transporter-1 (GLUT-1), CD31, and CD34. The MIB-1 (Ki-67) labeling index was 1-2%, indicating low proliferative activity. Adenomatoid tumor of the pleura is rare, and the pathogenesis has not been elucidated. Recognition of these benign mesothelial lesions in the pleura is important to avoid misdiagnosis. The immunohistochemistry in the present case supports its mesothelial origin.