We report a case of gastrointestinal stromal tumor (GIST) with peritoneal dissemination. A 73-year-old man was diagnosed to have a lesion of the stomach by an upper gastrointestinal barium study at an annual physical examination. Subsequently, melena was recognized and the patient was referred to our hospital. Gastrointestinal endoscopy revealed a huge submucosal tumor extending from the gastric body to the fundus which was ulcerated and hemorrhagic. Under the suspected diagnosis of the huge submucosal tumor of malignancy, total gastrectomy, splenectomy, pancreatectomy, and D2 dissection were performed. During the operation, another mass was found on the serosa of the sigmoid colon and resected. Histological examination revealed that the tumor was a GIST of the uncommitted type and the second resected mass from the colon was a metastasis from the primary tumor. No lymph node metastases or invasion into the resected adjacent organs were recognized. Immunohistological study indicated that this tumor originated from the interstitial cells of Cajal. The tumor in this case was unequivocaly malignant, hence strict follow up for a long time would be necessary.
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