Pulmonary and pleural thymoma. Diagnostic application of lymphocyte markers to the thymoma of unusual site.

Two cases of the thymoma of the unusual sites were examined immunohistochemically. The one was intrapulmonary, and the other was diffuse pleural tumors. The infiltrating lymphocytes were T-cells showing OKT 6 positivity and nuclear immunoreactivity for terminal deoxynucleotidyl transferase (TdT) in the intrapulmonary tumor, like lymphocytes in the mediastinal thymoma of predominantly lymphocytic or mixed types. Although lymphocytes were dispersed in the pleural tumor, there were some TdT(+) OKT 6(-) lymphocytes and the similar finding was observed in the thymoma of predominantly epithelial type. In lung carcinomas and pleural mesotheliomas, there were no TdT(+) OKT 6(+/-) lymphocytes. Immunohistochemical studies using lymphocyte markers may be an useful tool for the diagnosis of the thymoma of ectopic site or unusual presentation.