Low-grade metaplastic carcinomas of the thymus: biphasic thymic epithelial tumors 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 of the patients were men, their age ranging from 44 to 71 years. Three tumors invaded mediastinal fat or pleura. No metastases were present. Histologically, the tumors showed a biphasic pattern with solid carcinomatous areas merging with a spindle cell component. Only few lymphocytes were present. Cytological atypia and mitotic activity were variable in the solid areas, but low in the spindle cell component. The tumors showed expression of cytokeratin, vimentin and/or epithelial membrane antigen (EMA), both in the carcinomatous and in spindle cell components. In two cases, actin expression was also present in both components. In one case, chromogranin, S100 protein, glial fibrillary acidic protein, and neuron specific enolase were expressed in a minority of cells of both components. None of the patients had myasthenia gravis. All patients are alive without distant metastasis, but meanwhile one patient suffers from local recurrence. We conclude that metaplastic carcinoma of the thymus is a clinicopathological entity that is probably distinct 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.