Type B1 Thymoma (Lymphocyte-rich) : Microscopic
The neoplastic epithelial cells in type B1 thymoma are scattered singly in a background of densely-packed non-neoplastic immature T-cells. They are difficult to appreciate at low magnification. If the epithelial cells are increased in number or form clusters (3 or more cells), the diagnosis of type B2 thymoma should be considered. The epithelial cells have pale eosinophilic cytoplasm with ill-defined cytoplasmic borders, uniform round to oval vesicular nuclei, and variably-prominent nucleoli.