Difficult-to-diagnose duodenal adenocarcinoma with rare invasive form.

A 73-year-old man presented with liver dysfunction secondary to a distal bile duct stenosis. Initially, an ampullary neoplasm was suspected. Cytological and histological examinations were repeated, but no definite malignant cells were detected. The bile duct stricture improved with temporary metal stent placement. However, 21 months after the initial visit, stenosis of the second part of the duodenum was observed. Despite the absence of malignant cells, pancreaticoduodenectomy was performed due to strong suspicion of cancerous stenosis. Post-operative pathological findings revealed primary advanced duodenal carcinoma (pT4, pN1, cM0, and pStage IIIA), with the tumor extensively involving the submucosal tissue of the duodenum and extending to the muscular to the serosal layers. Invasion into the bile duct, pancreas, and ampulla or peri-ampullary duodenum was observed, but tumor cells were scattered within normal mucosa, complicating preoperative diagnosis. He received capecitabine/oxaliplatin as adjuvant chemotherapy for 6 months, and 14 months postoperatively, no recurrence was observed.This is an extremely rare case of duodenal carcinoma extending into the submucosal tissue, with no previous reports of such an extension. Bile duct strictures can be challenging to differentiate between benign and malignant causes, and duodenal carcinoma should be considered as a contributing factor.