Inclusion Criteria:

1. Age 20 years or older, at the time of acquisition of informed consent
2. Histologically confirmed GIST with CD117(+), DOG-1(+), or mutation in KIT or PDGFRα gene
3. (1) Completely resected localized GIST (R0 resection) within 12 weeks prior to the start of the adjuvant imatinib.

(2) After complete resection(R0 resection), High risk GIST according to Modified NIH criteria and ongoing adjuvant imatinib treatment.

4) High risk GIST according to Modified NIH criteria,

1. Tumor rupture according to Nishida classification or

2. tumor size >10cm and mitosis >10/50 HPF 5) Eastern Cooperative Oncology Group (ECOG) performance status 0 ~ 2 6) Adequate bone marrow, hepatic, renal, and other organ functions, before adjuvant imatinib treatment

   • Neutrophil >1,500/mm3
   • Platelet > 100,000/mm3
   • Hemoglobin >8.0 g/dL
   • Total bilirubin < 1.5 x upper limit of normal (ULN)
   • AST/ALT < 2.5 x ULN
   • Creatinine <1.5 x ULN 7) Provision of a signed written informed consent

Exclusion Criteria:

1. Women of child-bearing potential who are pregnant or breast feeding
2. Women or men who are not willing to use effective contraception entering the study period or until at least 3 months after the last study drug administration.
3. If any of the following applies within ≤ 6 months prior to starting study enrollment : Myocardial Infarction, severe instable angina, coronary/peripheral bypass, NYHA class III or IV congestive heart failure, stroke or transient ischemic attack, treatment required severe arrhythmia.
4. Uncontrolled infection
5. Acute and chronic liver disease and all chronic liver impairment.(But Patients with stable chronic hepatitis B are eligible)
6. Patients who had reduced the dose of imatinib to less than 300 mg/day due to toxicity.
7. Acute, or chronic medical or psychiatric condition or laboratory abnormality such as active uncontrolled infection that difficult to study participation in the judgment of the investigator
8. Known diagnosis of HIV infection (HIV testing is not mandatory).
9. History of another primary malignancy that is currently clinically significant or currently requires active intervention.
10. Alcohol or substance abuse disorder.
11. The patients with PDGFRα D842V mutation