Rechallenge of Imatinib in GIST Having no Effective Treatment: RIGHT

Inclusion Criteria: * Patients aged 18 years and older * Patients with metastatic or unresectable malignant gastrointestinal stromal tumour which has been histologically confirmed by the detection of CD117 on immunohistochemical staining or genetically confirmed by the detection of mutation in KIT or PDGFRα genes on direct sequencing of tumor DNA. * Prior benefit from 1st line imatinib defined as complete response, partial response, or stable disease at 6 months after the start of 1st line imatinib * Patients whose disease has progressed despite at least both prior imatinib therapy (400mg/day) and then subsequently also failure of prior sunitinib therapy. * ECOG(Eastern Cooperative Oncology Group) performance status 0 \~ 3 * Adequate bone marrow function as defined by platelets ≥ 75 x 109/L and neutrophils ≥ 1.5 x 109/L * Adequate renal function, with serum creatinine \< 1.5 x upper limit of normal * Adequate hepatic function with serum total bilirubin \< 1.5 x upper limit of normal, alanine aminotransferase or aspartate aminotransferase \< 2.5 x upper limit of normal in the absence of liver metastases, or \< 5 x upper limit of normal in the presence of liver metastases. * Expected life expectancy of greater than 12 weeks in the absence of any intervention * No other malignant disease apart from non-melanotic skin cancer or carcinoma in situ of the uterine cervix or any other cancer except where treated with curative intent \> 5 years previously without evidence of relapse * Written, informed consent to the study Exclusion Criteria: * Medical or psychiatric conditions that compromise the patient's ability to give informed consent or to complete the protocol or a history of non- compliance * Last dose of radiotherapy received within 4 weeks before the start of study treatment, excluding palliative radiotherapy * Obstruction of gastrointestinal tract * Active gastrointestinal bleeding * Myocardial infarction within 6 months prior to the study medication, and other clinically significant heart disease (e.g., unstable angina, congestive heart failure or uncontrolled hypertension) * Evidence of severe or uncontrolled systemic disease or any concurrent condition which in the investigator's opinion makes it undesirable for the patient to participate in the study or which would jeopardise compliance with the protocol * Female patients who are pregnant or breast-feeding. Female patients must have had a negative pregnancy test within one week before starting imatinib.