[A case of small intestinal gastrointestinal stromal tumor (GIST) with peritoneal dissemination, treated effectively with molecular target drug after operation].

A 54-year-old man, presenting with sudden onset of abdominal pain, was admitted to our hospital. Blood examination revealed high white blood cell counts and elevated C-reactive protein (CRP) levels. Ultrasonography and computed tomography detected a 12 cm mass in the lower abdomen, some ascites, and multiple small nodules spread through the abdomen. The preoperative diagnosis of the tumor was either a gastrointestinal stromal tumor (GIST) or a possible lymphoma. The 12 cm tumor and greater omentum with multiple nodules were resected. Upon pathological examination, the tumor was diagnosed as a GIST, and appeared KIT positive by immunostaining. After the operation, imatinib was administered; however, psoriasis vulgaris developed within 5 months. As the next line of therapy, sunitinib treatment was initiated; however, since peripheral nerve disorder developed, sunitinib dose was halved and maintained. Two years after the operation, the patient is still alive. Small intestinal GISTs, which make up only 20-30% of all GISTs, are considered to be more malignant than others. We report a rare case of GIST with peritoneal metastases originating from the small intestine, which was treated effectively with molecular target drugs.