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