Low-grade metaplastic carcinomas of the thymus: Biphasic thymic epithelialtumors with mesenchymal metaplasia - An update

A group of biphasic low-grade thymic epithelial tumors is presented that we suggest calling low-grade metaplastic carcinoma of the thymus [37]. Four o f the patients were men, their age ranging from 44 to 71 years. Three tumor s invaded mediastinal fat or pleura. No metastases were present. Histologic ally, the tumors showed a biphasic pattern with solid carcinomatous areas m erging with a spindle cell component. Only few lymphocytes were present. Cy tological atypia and mitotic activity were variable in the solid areas, but low in the spindle cell component. The tumors showed expression of cytoker atin, vimentin and/or epithelial membrane antigen (EMA), both in the carcin omatous and in spindle cell components. In two cases, actin expression was also present in both components. In one case, chromogranin, S100 protein, g lial fibrillary acidic protein, and neuron specific enolase were expressed in a minority of cells of both components. None of the patients had myasthe nia gravis. All patients are alive without distant metastasis, but meanwhil e one patient suffers from local recurrence. We conclude that metaplastic c arcinoma of the thymus is a clinicopathological entity that is probably dis tinct from the recently described "thymoma with pseudosarcomatous stroma", and should be distinguished from the usually benign medullary thymomas and the highly aggressive carcinosarcomas and sarcomatoid carcinomas.