Type B2 Thymoma : Immunostains
Type B2 thymomas show a prominent epithelial network with CK19, p63, and PAX8. There is focal positivity for CK7, CK8, CK14, and CK18. The epithelial component is usually negative for CK20. The background lymphocytic infiltrate is composed predominantly of immature T-cells which are TdT+, CD1a+, CD3+, CD4+, and CD8+. This high power view shows cytologic features of tumor cells in a Type B2 thymoma. The tumor cells are greatly out-numbered by non-neoplastic lymphocytes. The biologic behavior of Type B2 thymoma is intermediate between B1 and B3 thymomas. It is often invasive at the time of diagnosis and has higher rate of recurrences and metastases than B1 thymomas.