Fig. 2

Histopathological findings and immunohistochemical staining test. The resected specimen showed a 40 × 30 mm tumor on the descending part of the duodenum slightly more distal to the papilla of Vater, and a white thickening of the duodenal wall was observed on the sectioned surface of the same area (A, B) (red arrow). There was an infiltrative growth of carcinoma cells with a predominant component of well to poorly differentiated tubular adenocarcinoma under the normal duodenal mucosa (C, D). The non-tumor duodenal epithelium is observed in the upper right corner of this image (red arrow), showing the continuous infiltration of the intramucosal hypodense lesion into the submucosal layer of the duodenal epithelium (C). Cancer cells were developing under the duodenal submucosa and infiltrated into the duodenal muscularis propria (D). The immunostaining of the same area showed that MUC5AC, which stained the gastric crypt epithelium, was diffusely positive in the intramucosal and tumor-infiltrated area, whereas MUC6 was negative to partially positive in the mucosa and positive in approximately 50% of the infiltrated area (E, F)