Histopathologic Re-evaluation of Thymoma with Immunonhistochemical Study for bcl-2 and MIC-2 Protein

ABSTRACT

We reviewed 86 thymic epithelial tumors and reclassified them according to the Kirchner and Muller- Hermelink classification. They were subtyped as medullary, mixed, predominantly cortical (organoid), cortical, well differentiated thymic carcinoma, and poorly differentiated thymic carcinoma. The frequency of each subtype was determined and histologic findings were related to stage and myasthenia gravis. Immunohistochemical stains for bcl-2 protein as a marker for medullary thymocytes and MIC-2 protein as a marker for cortical thymocytes were performed in each case. The stages and association of myasthenia gravis was significantly different in each subtypes. The results of this study demonstrate that this histogenetic classification is clinically applicable. The bcl-2 protein was specifically demonstrated in lymphocytes within areas of medullary differentiation and MIC-2 protein in cortical differentiation. The expression of bcl-2 and MIC-2 proteins lend histogenetic support for this new classification of thymoma. Bcl-2 protein is strongly expressed in tumor epithelial cells of every case of poorly differentiated thymic carcinoma whereas the other types of thymic epithelial tumors do not show epithelial expression of this protein. The strong expression of bcl-2 protein in tumor epithelium may be considered as a predictor of aggressive behavior in thymic epithelial tumors.