[A Case of Peritoneal Metastasis Resection from GIST of the Small Intestine with Focal Resistance Against Imatinib Mesylate].

A 69-year-old man was referred to our institute for the surgical resection of focal resistant peritoneal GIST during imatinib administration. He had been diagnosed with GIST of the small intestine with liver and peritoneal metastases, and imatinib treatment was initiated. Shortly after imatinib administration, the primary lesion perforated, and thus, partial resection of the small intestine was performed. Imatinib treatment was resumed after the first surgery, and he achieved partial response. However, computed tomography scans obtained 7 months after the first surgery showed focal progression, a peritoneal metastasis near the right kidney. Under the diagnosis of focally imatinib-resistant GIST, local resection of the metastatic tumor was performed. In this case, an exon 11 mutation of c-kit was noted initially. After the imatinib treatment, an additional point mutation was observed in exon 18 that caused resistance to imatinib. Therefore, imatinib treatment was reinitiated after the second surgery, and other metastases were well controlled. In case of GIST with multiple metastases, appropriate treatment should be selected based on the resistance of each lesion.