Type B1 thymoma is considered to a tumor of low-grade malignant potential since it is completely encapsulated in more than 50% of cases. Almost 90% of cases are considered completely resectable. The 5-yr and 10-yr survival rates are 95% and 90% respectively. Recurrences may occur in 10% to 15% of cases, but distant metastases are rare. This high power view of a type B1 thymoma shows scattered neoplastic epithelial cells surrounded by a dense infiltrate of non-neoplastic lymphocytes most of which are TdT+ immature T-cells. The epithelial cells have pale eosinophilic cytoplasm with ill-defined cytoplasmic borders, uniform round to oval vesicular nuclei, and punctate nucleoli.