The distribution of epithelial membrane antigen in thymic epithelial neoplasms

Thymic carcinomas arising within a thymoma have been reported, but the relationship between thymoma and thymic carcinoma is poorly understood. Epithelial membrane antigen (EMA) is known to be an effective marker for establishing the epithelial nature of neoplastic cells, and it is reported that staining of tumors is clearly related to the degree of tumor differentiation. Eighty-one thymomas (59 noninvasive, 22 invasive) and 14 thymic carcinomas were studied immunohistologically using antiepithelial membrane antigen (anti-EMA) monoclonal antibody. Thymic carcinomas tended to express much larger quantities of EMA than thymomas, and instances of EMA-positive thymoma were seen significantly more often in invasive thymomas than in noninvasive ones (P < 0.05). However, EMA positivity was also associated with gland-like structures, which were not necessarily associated with malignant disease. Nevertheless, in view of the concept that thymoma and thymic carcinoma show a similar cellular differentiation, EMA-positive epithelial cells in thymoma with no relation to gland-like configurations might represent a pool of cells having a latent potential for malignant disease and might be transformed into thymic carcinoma cells under certain conditions. Immunolabeling for EMA appears to be a useful tool for determining the degree of malignant disease among thymic epithelial neoplasms.