Diagnosis of metastatic thymoma using flow cytometry.

Thymomas are cytologically benign epithelial neoplasms of the thymus gland. They compose 10% of mediastinal tumors, and are most common in the anterosuperior compartment. Seven to 36% of thymomas are malignant, as determined by tissue invasion, yet they metastasize in less than 3% of cases. Distinguishing lymphoma from lymphocyte-predominant thymoma is imprecise due to their histologic similarities. We present a 45-year-old man with intracranial metastatic thymoma. The lesion was interpreted radiographically as meningioma, and as possible lymphoma by frozen section. Flow cytometry proved this neoplasma to be a metastatic thymoma. Sixteen monoclonal antibodies were used to immunophenotype the CD45+ component of this tumor. Coexpression of CD4 and CD8 along with CD1 demonstrated lymphocytes of late cortical thymocyte origin; a second component was cytokeratin positive. This is the first reported case of extrathoracic metastases of thymoma diagnosed using flow cytometry. We propose this method as an invaluable technique to diagnose these histologically difficult neoplasms.