Low-grade metaplastic carcinoma of the thymus

Five cases of a characteristic low-grade thymic epithelial tumour are described that we suggest calling metaplastic carcinoma of the thymus.

The patients' ages ranged from 44 to 71 (mean 56.2) years. 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 cell 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 cytokeratin, vimentin and/or epithelial membrane antigen, both in the carcinomatous and 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 at least some cells of both components. None of the patients had myasthenia gravis. All patients are alive without evidence of recurrence or metastasis.

Metaplastic carcinoma of the thymus is a distinct clinicopathological entity that should be distinguished from the usually benign medullary thymomas and from the clinically aggressive carcinosarcomas and sarcomatoid carcinomas.