We report two cases of small pleural nodules showing the distinctive histologic appearance of adenomatoid tumor. Both lesions were discovered incidentally during surgery in patients undergoing lung resection for unrelated intrapulmonary masses: lung carcinoma in one case and histoplasmosis in the other. The tumors were composed of a focal proliferation of epithelioid cells forming vacuoles and tubular spaces in a fibrous stroma, as seen in adenomatoid tumors from other sites. The differential diagnosis in both cases included metastatic signet ring cell carcinoma. The mesothelial nature of the lesions was supported by immunohistochemical and ultrastructural evidence. The tumor cells in both cases were positive for cytokeratin but negative for carcinoembryonic antigen and LeuM1. One case was also negative for BER-EP4, B72.3, CD34, and Factor VIII. Electron microscopy in this case demonstrated well-developed basal laminae, desmosomes, and numerous slender microvilli along the luminal surfaces of the tumor cells. Adenomatoid tumors are regarded as a benign variant of mesothelioma. Despite the abundance of mesothelial cells in the pleura, adenomatoid tumors are apparently extremely rare in this location. Separation from malignant lesions such as adenocarcinoma and epithelioid hemangioendothelioma is important.
