Low-grade metaplastic carcinoma of the thymus

Aims: Five cases of a characteristic low-grade thymic epithelial tumour are described that we suggest calling metaplastic carcinoma of the thymus. Methods and results: The patients' ages ranged from 44 to 71 (mean 56.2) ye ars. Four of the patients were male. Three of five tumours showed invasion into mediastinal fat or pleura but, otherwise, all were well circumscribed. No metastases were present. Histologically, the tumours showed a biphasic pattern with solid carcinomatous areas merging gradually with a spindle cel l component. Lymphocytes were rare. Cytological atypia and mitotic activity were variable in the solid areas, but slight or absent in the spindle cell component. On immunohistochemistry, the tumours showed expression of cytok eratin, vimentin and/or epithelial membrane antigen, both in the carcinomat ous and spindle cell components. In two cases, actin expression was also pr esent in both components. In one case, chromogranin, S100 protein, glial fi brillary acidic protein and neuron-specific enolase were expressed in at le ast some cells of both components. None of the patients had myasthenia grav is. All patients are alive without evidence of recurrence or metastasis. Conclusion: Metaplastic carcinoma of the thymus is a distinct clinicopathol ogical entity that should be distinguished from the usually benign medullar y thymomas and from the clinically aggressive carcinosarcomas and sarcomato id carcinomas.