fig4

Figure 4. Predominantly lymphocyte-rich AB thymoma: (A) the tumors are composed of a predominantly lymphocyte-rich population of cells harboring scattered small spindle cells in the background; these tumors can resemble on cursory examination a type B1 thymoma (2×); (B) an immunoperoxidase stain for p63 shows on higher magnification the scattered proliferating thymic epithelial cells (dark brown nuclei) surrounded by a large number of small lymphocytes. Notice the spindle shape of the nuclei, unlike the round shape of the nuclei characteristic seen in type B thymoma (60×).