An immunohistochemical study of thymic epithelial tumors. III. The distribution of interdigitating reticulum cells and S-100 beta-positive small lymphocytes.

We report an immunohistochemical study of the distribution and number of interdigitating reticulum cells (IDC) and S-100 beta-positive small lymphocytes (S-100 beta + lymphocytes) in 53 thymomas and 11 thymic carcinomas. All 53 thymomas showed the presence of IDC in the tumor parenchyma. In most cases of predominantly lymphocytic and mixed-type thymoma, IDC clustered in areas, that corresponded to locations that had medullary differentiation and contained accumulated mature lymphocytes. By contrast, in most of the predominantly epithelial-type thymomas, IDC were scattered rather than forming clusters. The distribution and number of IDC were correlation with the histological type of thymoma but not with invasiveness. In thymic carcinomas, IDC were scattered in tumor nests. In 47 of the 53 thymomas (89%), infiltrating S-100 beta + lymphocytes were readily recognized. The remaining six cases without S-100 beta + lymphocytes were noninvasive thymoma. We conclude that the degree of S-100 beta + lymphocyte infiltration is correlated with the stage of thymoma and may be a marker of thymoma malignancy.