Primary pleural thymoma: a mimic in thoracic pathology.

A case of ectopic thymoma of the pleura with a growth pattern mimicking diffuse pleural mesothelioma is reported. Diagnostic imaging showed that the pleural tumor encased the entire left lung. The specimen biopsied from the tumor was composed of lymphocytes and epithelial cells, consistent with the B1 type of thymoma. The surgical exploration of the anterosuperior mediastinum found no evidence of a thymic tumor. The thymoma was thought to originate from an ectopic thymic tissue in the pleura, as a lesion independent from the primary mediastinal thymoma, and spread along the pleura like diffuse mesothelioma.Because of their peculiar location and variety of histologic patterns manifested, pleural thymomas may be confused with other neoplasms and may cause diagnostic problems clinically, radiologically, and morphologically. A combination of clinical information, histopathologic appearance of the tumor, and immunohistochemical studies will often help to distinguish a primary pleural thymoma from other neoplasms.