Three Versus Five Years of Adjuvant Imatinib as Treatment of Patients With Operable GIST

Inclusion Criteria: * Age ≥ 18 years. * Morphological and immunohistological documentation of GIST (immunostaining for KIT and/or DOG-1 positive, or mutation of KIT or PDGFRA present in tumor tissue). * Macroscopically complete surgical resection of GIST (either R0 or R1 resection). * Mutation analysis of KIT and PDGFR genes has been carried out. * A high risk of GIST recurrence; either gastric GIST with mitotic count \>10/50 HPFs, or non-gastric GIST with mitotic count \>5/50 HPFs, or tumor rupture. * Eastern Cooperative Oncology Group performance status ≤ 2. * Adequate organ function. * Female patients of childbearing potential must have a negative pregnancy test within 14 days before initiation of study drug dosing. Postmenopausal women must have amenorrhea for at least 12 months to be considered of non-childbearing potential. Male and female patients of reproductive potential must agree to employ an effective barrier method of birth control throughout the study and for up to 3 months following discontinuation of study drug. * Patient willing to be followed up at the study site regardless of the result of randomization. * Patient has provided a written, voluntary informed consent prior to study-specific screening procedures. Exclusion Criteria: * Presence of distant metastases or local recurrence of GIST. * Not willing to donate tumor tissue and/or blood samples for the study molecular studies. * Presence of a substitution mutation at PDGFRA codon D842 (usually D842V). * Administration of adjuvant imatinib longer than for 3 years is planned regardless of the result of randomization, or "life long" imatinib administration is planned. * Prior adjuvant (+ neoadjuvant) therapy with imatinib mesylate for at least 35 months has not been completed, or the total duration of prior adjuvant (+ neoadjuvant) imatinib administration exceeds the total duration of 37 months. * Neoadjuvant imatinib for a duration that exceeds 9 months. * Longer than 4-week break during adjuvant imatinib administration. * The dose of imatinib at completion of 3 years of adjuvant imatinib was 200 mg per day or less or greater than 800 mg per day. * Patient has received any investigational anti-cancer agents during adjuvant imatinib or between completion of adjuvant imatinib and the date of randomization. * Patient has been free of another malignancy for less than 5 years except if the other malignancy is not currently clinically significant nor requiring active intervention, or if the other malignancy is a basal cell skin cancer or a cervical carcinoma in situ. Recent existence of any other malignant disease is not allowed. * Patient with Grade III/IV cardiac disease as defined by the New York Heart Association Criteria (i.e., congestive heart failure, myocardial infarction within 6 months of study entry). * Female patients who are pregnant or breast-feeding. * Severe and/or uncontrolled medical disease (i.e., uncontrolled diabetes, severe chronic renal disease, or active uncontrolled infection). * Known diagnosis of human immunodeficiency virus (HIV) infection. * Patient with a significant history of non-compliance to medical regimens or with inability to grant reliable informed consent.