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Because the histologic features may resemble those of other mediastinal malignancies, thymic carcinoma can be difficult to diagnose, particularly if the primary site is uncertain. In an effort to facilitate this diagnosis, the authors have evaluated the use of immunohistochemistry with an antibody to CD5 (NCL-CD5). Nine thymic carcinomas, 15 thymomas, 8 lymphomas, 10 poorly differentiated lung carcinomas, 10 breast carcinomas, 1 mediastinal seminoma, and 1 thyroid carcinoma showing thymus-like differentiation (CASTLE) were studied. Four of 9 poorly differentiated carcinomas of the thymus were CD5 positive. The one CASTLE was CD5 positive. All other tumors were negative. CD5-positive lymphocytes were internal controls in every case. CD5 labels some thymic carcinomas in paraffin-embedded tissue, whereas other tumors studied were negative. CD5 immunoreactivity of CASTLE appears to support thymic derivation of this tumor.

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