Microscopic examination reveals that the tumor has a ribbon-like or gyriform architecture and is richly vascularized (Images 03, 04, and 05). The individual cells range from cuboidal to columnar, with eosinophilic to amphophilic cytoplasm. There is mild nuclear pleomorphism, and the cells have an overall slightly increased nuclear/cytoplasmic ratio. The nuclear chromatin is coarse, and small, inconspicuous nucleoli are present in many tumor cells. Mitotic figures are extremely rare to absent. Approximately 30-40% of the tumor cells are positive for insulin (Image 06), and about 10% are positive for somatostatin (Image 07). A rare glucagon positive cell is seen. A Congo red stain for amyloid is negative. The tumor is surrounded by a fibrous capsule, which is thickened in many areas. No invasion through the capsule into the surrounding parenchyma is seen. Discrete richly vascular islands of tumor are present within the capsule (Images 08 and 09). An immunostain for endothelial cells (CD31) highlights the prominent capillary network around the tumor nests (Image 10), but no tumor cells are found within the vascular spaces (Image 11). Nine peripancreatic and perisplenic lymph nodes are free of tumor. No islet cell hyperplasia is seen in the uninvolved pancreatic parenchyma (Image 12).