An extramural gastrointestinal stromal tumor of the duodenum mimicking a pancreatic head tumor

We report the case of a 53‐year‐old woman with a gastrointestinal stromal tumor (GIST) of the duodenum that showed only extramural growth, mimicking a pancreatic tumor. Preoperatively, computed tomography (CT) and angiography revealed a hypervascular mass, 3.0 cm in diameter, in the pancreatic head. Hypotonic duodenography showed compression of the second and third portions of the duodenum by the pancreatic lesion. Endoscopic examination showed no specific mucosal abnormalities in the duodenal lumen. The pancreatic head tumor was diagnosed preoperatively as a nonfunctioning islet cell tumor of the pancreas, and the patient underwent pylorus‐preserving pancreaticoduodenectomy. A hard mass was palpated intraoperatively in the pancreatic head region, and neither peritoneal dissemination nor metastasis was detected. Histologically, the tumor was composed of spindle‐shaped cells with a fascicular growth pattern, and only a few mitotic features were seen. Immunohistochemically, most of the tumor cells were positive for c‐kit oncoprotein and CD34, but negative for alpha‐smooth muscle actin and S‐100 protein. Therefore, this neoplasm was finally diagnosed as a duodenal GIST of the uncommitted type. This is a rare case of a duodenal GIST with exclusively extramural growth mimicking a pancreatic head tumor.