Type B2 thymoma is composed of clusters of neoplastic epithelial cells in a background of abundant immature T-cells. The epithelial cell density is higher than that seen in type B1 thymoma or the normal thymus. Foci of medullary differentiation may or may not be seen. It makes up 25% to 30% of all thymomas. This image shows thick fibrous septa dividing the tumor into lobules.