A case of gastrointestinal stromal tumor (GIST) with peritoneal dissemination － Imatinib re-challenged case －
Unresectable, metastatic, recurrent gastrointestinal stromal tumor (GIST) is primarily treated with a molecular-targeted therapeutic agent, imatinib. However, after an initial response, a secondary resistance to the drug often occurs after a few years. We report here a case of a resected giant GIST of the jejunum that disseminated following treatment with imatinib. A 59-year-old male presented with a giant tumor in the abdominal cavity, which was diagnosed as GIST by needle biopsy; he was administered 400 mg/day imatinib. Eight months later, the tumor had considerably decreased, but multiple tumors in small intestine and mesenterium, indicating dissemination, appeared. Administration of imatinib was continued for 36 months from the initial treatment and the dissemination gradually reduced and almost disappeared except for a tumor in the right upper abdomen. Three years later, follow-up computed tomography revealed that the disseminated lesions had enlarged; a part of the intrapelvic tumor was suspected to be viable. We deduced that the tumor developed partial resistance to imatinib: therefore, we surgically removed as many disseminated tumors as possible. Pathologically, resected tumors appeared to have no viable tumor cells except for a small part of the primary tumor in which mitosis was 0-1/50 high-power fields. Genetic analysis of surgically resected specimen for c-kit mutation revealed an exon 11 c554-559 deletion. 17 months after operation, another disseminated tumor was detected. Imaninib therapy was re-introduced. The dissemination was diminished after three months re-challenged imatinib and continues to be recurrence-free for two years. When partial resistance to imatinib is observed, combined modality therapy that involves chemotherapy with surgical intervention at early stages is expected to improve the outcome. doi：10.11482/KMJ-E43（1）5 (Accepted on January 11, 2017)
