High power view of Type B1 thymoma showing scattered neoplastic thymic epithelial cells surrounded by non-neoplastic lymphocytes. The epithelial cells have pale oval or round nuclei with inconspicuous nucleoli. Type B1 thymoma is considered to a tumor of low-grade malignant potential since it is completely encapsulated in more than 50% of cases. In another 25% of cases, it may invade mediastinal adipose tissue. Distant metastases are rare.
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