Type B1 thymoma vs Hyperplastic thymus: Type B1 thymoma resembles normal non-involuted thymus but has larger lobules, thicker fibrous capsule, shows fibrous septa, and has a predominance of cortical over medullary areas. There are no or very few Hassall corpuscles in type B1 thymoma in comparison to the normal thymus. Type B1 Thymoma vs Type B2 Thymoma: Type B1 thymoma has fewer neoplastic epithelial cells than type B2 thymoma and they are scattered singly. If their numbers appear increased or if they form clusters (3 or more contiguous cells), it supports the diagnosis of type B2 thymoma. Medullary islands with or without Hassall corpuscles are always seen in type B1 thymoma but are rare in type B2 thymoma. Type B1 Thymoma vs Type AB Thymoma: Rare examples of type AB thymoma may show medullary foci in the lymphocyte-rich areas with TdT+ cells. However, type AB thymomas will always show (at least focally) spindled tumor cells, a greater proportion of keratin-positive tumor cells, and epithelial expression of CD20 in about 50% of cases. Type B1 Thymoma vs T-Lymphoblastic Lymphoma (T-LBL): In T-LBL, the cortico-medullary architecture is effaced. The blasts of T-LBL are monomorphic, atypical, and frequently infiltrate into mediastinal fat. Necrosis is frequently present. Epithelial stains such as CK19 and p63 fail to reveal epithelial cell network in T-LBL. In contrast, type B1 thymomas will show a population of CK19+ and p63+ epithelial cells and the T-cells are not monomorphic and lack atypia. The image shows scattered neoplastic thymic epithelial cells surrounded by non-neoplastic lymphocytes. The epithelial cells have pale oval or round nuclei with inconspicuous nucleoli.