Imatinib Mesylate or Observation Only in Treating Patients Who Have Undergone Surgery for Localized Gastrointestinal Stromal Tumor

DISEASE CHARACTERISTICS: Histologically confirmed gastrointestinal stromal tumor Localized disease Meets 1 of the following criteria: At high-risk of relapse, defined by 1 of the following criteria: Tumor size > 10 cm Mitotic rate > 10/50 high-power field (HPF) Tumor size > 5 cm AND mitotic rate > 5/50 HPF At intermediate-risk of relapse, defined by 1 of the following criteria: Tumor size < 5 cm AND mitotic rate 6-10/50 HPF Tumor size 5-10 cm AND mitotic rate < 5/50 HPF Tumor must stain positive for Kit (CD117) by polyclonal DAKO antibody staining Must have undergone complete resection of the primary tumor at least 2 weeks, but no more than 3 months, before study entry Meets criteria for 1 of the following resection levels: R0 (clear margins) R1, defined by 1 of the following criteria: Margins of resection are contaminated by tumor, but no macroscopic tumor is left behind Intraoperative tumor rupture Shelling-out procedure Endoscopic maneuver No residual macroscopic disease after surgery Regional positive lymph nodes allowed provided they have been macroscopically excised No distant metastases*, including any of the following: Peritoneal lesion not contiguous to the primary tumor Liver metastases Hemoperitoneal metastases NOTE: *Even if a complete resection (R0) was performed PATIENT CHARACTERISTICS: Age 18 and over Performance status WHO 0-2 Life expectancy Not specified Hematopoietic Absolute neutrophil count ≥ 1,500/mm^3 Platelet count ≥ 100,000/mm^3 Hemoglobin ≥ 9 g/dL (transfusions allowed) Hepatic Bilirubin ≤ 1.5 times upper limit of normal (ULN) AST or ALT ≤ 2.5 times ULN No uncontrolled liver disease No chronic viral hepatitis at risk of reactivation Renal Creatinine < 1.5 times ULN No uncontrolled chronic renal disease Cardiovascular No New York Heart Association class III-IV cardiac disease No congestive heart failure No myocardial infarction within the past 2 months Other Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception during and for up to 3 months after study participation No uncontrolled diabetes No uncontrolled active infection No HIV infection No psychological, familial, sociological, or geographical condition that would preclude study compliance or participation No other severe and/or uncontrolled medical disease No other malignancy within the past 5 years except adequately treated basal cell or squamous cell skin cancer or carcinoma in situ of the cervix PRIOR CONCURRENT THERAPY: Biologic therapy No other prior molecular targeted or biologic therapy No concurrent filgrastim (G-CSF) or sargramostim (GM-CSF) to support blood counts No concurrent anticancer biologic agents Chemotherapy No prior chemotherapy for gastrointestinal stromal tumors No concurrent anticancer chemotherapy Endocrine therapy Not specified Radiotherapy No prior radiotherapy No concurrent anticancer radiotherapy Surgery See Disease Characteristics Prior non-curative surgery allowed (e.g., surgery with main diagnostic intent or emergency surgery with symptomatic intent) Other No prior imatinib mesylate No prior randomization to this study No concurrent therapeutic anticoagulation with coumarin derivatives Concurrent therapeutic low-molecular weight heparin or mini-dose coumarin derivatives (equivalent to oral warfarin 1 mg/day) allowed for prophylaxis of central venous catheter thrombosis No other concurrent antitumoral therapy No other concurrent anticancer agents No other concurrent investigational drugs