Cystic adenomatoid tumor of the mediastinum.

A case of adenomatoid tumor presenting as a mass in the anterior mediastinum is described. The patient was a 56-year-old woman with left side chest wall pain who showed a mediastinal mass on chest x-ray and CT scans. Thorough clinical and radiographic examination did not reveal any evidence of tumor elsewhere. At surgery, the tumor was found adjacent to the anterior pericardial reflection. Grossly, the tumor measured 5.5 x 5.5 x 3 cm and showed a homogeneous cut surface with numerous cystic structures that varied from 0.5 to 1.5 cm in greatest diameter. Histologic examination showed numerous cystic spaces lined by flattened or cuboidal epithelial cells. The walls of the cysts showed a proliferation of small canalicular structures lined by round to polygonal epithelioid cells with vacuolated eosinophilic cytoplasm. Immunohistochemical studies showed strong positivity of the epithelioid cells for AE1/AE3 cytokeratin, CK5/CK6, and calretinin. Stains for CK7, CK20, alpha-fetoprotein, CD31, carcinoembryonic antigen, MOC 31, and chromogranin were negative. Electron microscopic examination showed numerous long microvilli on the cell surface and abundant tonofilaments/desmosomal plaques in the tumor cells, characteristic of mesothelial cells. The patient is alive and well and free of recurrence 1 year following surgery. Adenomatoid tumor is a rare neoplasm that should be added in the differential diagnosis of anterior mediastinal masses. Immunohistochemical and ultrastructural studies may be of aid in identifying the characteristic features of mesothelial cells and to avoid mistaking this lesion for more ominous conditions.