Collision Tumor of Adenocarcinoma and Gastrointestinal Stromal Tumor in the Small Bowel

We report a very rare case of collision tumor composed of primary adenocarcinoma of the jejunum and gastrointestinal stromal tumor (GIST). The patient was a 63-year-old man who visited our hospital for epigastralgia and vomiting. Abdominal computed tomography revealed a mass in the upper jejunum, with gastric and duodenal dilatation. Endoscopy of the small bowel showed a circumferential tumor in the upper jejunum, which was diagnosed as primary adenocarcinoma by tissue biopsy. Thereafter, partial resection of the small bowel from the third part of the duodenum over the upper jejunum was performed. A tumor colliding with the primary adenocarcinoma was identified on the serosal side of the jejunum in the excised specimen and was histologically diagnosed as GIST. The annual incidence of primary adenocarcinoma of the small bowel (i.e., jejunum and ileum excluding the duodenum) has been reported to be 7 in 1 million people, and only 6 cases of collision tumor of the small bowel (i.e., duodenum: 5, ileum: 1) have been reported thus far. Although esophageal, gastric, and large intestinal collision tumors composed of primary cancer and GIST have been reported, to our knowledge, the present patient is the first case of the small bowel. The cause of or correlation between 2 tumors forming a collision tumor remains unclear. In the present patient, there was no pathological finding of infiltration between the 2 tumors. Although the collision of the 2 tumors was unclear, the findings indicate their independent development in closely located regions consistent with collision tumors.