Imatinib Dose Escalation to 800 mg/Day in Korean Patients With Metastatic or Unresectable GIST Harboring KIT Exon 9 Mutation: KENEDI

The patients will receive 400 mg per day of imatinib for 4 weeks, and then 600mg per day (300 mg po bid) for 4 weeks if tolerable to 400 mg per day, and then 800 mg per day (400 mg po bid)