The neoplastic epithelial cells of type B3 thymoma are arranged in clusters or solid sheets. They are polygonal and contain moderate amount of eosinophilic or clear cytoplasm. The nuclei are round, oval, or irregular and may show prominent nucleoli. Moderate nuclear atypia is commonly seen. Mitotic activity may be increased. The background contains a small number of non-neoplastic lymphocytes which are predominantly immature T-cells. Tumors with type B3 areas (epithelial-predominant) intermingled with type B2 component (more numerous lymphocytes than B3) are common.