Histology of endoscopic resection specimen. ( A) Cross-section of the resected specimen (hematoxylin-eosin stain). The tumor surface showed a central depression and bilateral elevations. ( B) Dense tumorous glands with papillary growth were seen in the depressed portion. The bilateral elevations were covered by normal duodenal villi. Tumorous glands with an inverted growth downward (arrow heads) led to the bilateral elevations. ( C) The tumor invasion was confined to the muscularis mucosae with no submucosal involvement (Desmin stain). ( D) High magnification of the tumor illustrated in Figure 3A showing the complex papillary architecture and considerable cytological atypia.