Imatinib Mesylate in Treating Patients With Unresectable or Metastatic Gastrointestinal Stromal Tumor

DISEASE CHARACTERISTICS: * Histologically confirmed gastrointestinal stromal tumor (GIST) * Metastatic or unresectable disease * Immunohistochemical confirmation of KIT (CD117) expression by tumor as documented by DAKO antibody staining * Measurable or non-measurable disease by conventional imaging (CT scan or MRI) or physical examination * If a target lesion has been previously embolized or irradiated, there must be objective evidence of progression to be considered for response assessment * No known brain metastasis PATIENT CHARACTERISTICS: * WHO performance status 0-3 * Bilirubin ≤ 1.5 times upper limit of normal (ULN) * AST and ALT ≤ 2.5 times ULN (≤ 5 times ULN if hepatic metastases are present) * Creatinine ≤ 1.5 times ULN * ANC ≥ 1,000/mm³ * Platelet count ≥ 100,000/mm³ * Hemoglobin ≥ 9 g/dL (transfusions allowed) * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective barrier contraception during and for up to 3 months after completion of study therapy * No NYHA class III-IV cardiac disease * No congestive heart failure or myocardial infarction within the past 2 months * No severe and/or uncontrolled concurrent medical disease (e.g., uncontrolled diabetes, uncontrolled chronic renal or liver disease, or active uncontrolled infection \[e.g., HIV\]) * No other prior malignancy except adequately treated basal cell or squamous cell skin cancer, carcinoma in situ of the cervix, adequately treated stage I or II cancer from which the patient is currently in complete remission, or any other cancer from which the patient has been disease-free for 5 years * No medical, psychological, familial, sociological, or geographical condition that, in the opinion of the investigator, may preclude the patient's ability to tolerate or complete study treatment, comply with study protocol and follow-up schedule, or give reliable informed consent PRIOR CONCURRENT THERAPY: * Recovered from all prior therapy * More than 28 days since prior chemotherapy, biologic therapy, or any other investigational drug * More than 14 days since prior major surgery * No concurrent therapeutic anticoagulation with coumarin derivatives * Concurrent therapeutic anticoagulation with low-molecular weight heparin allowed * Concurrent mini-dose coumarin derivatives (i.e., equivalent to 1 mg of oral warfarin daily) as prophylaxis allowed * No concurrent cytokines (i.e., filgrastim \[G-CSF\] or sargramostim \[GM-CSF\]) to support blood counts * No other concurrent investigational drugs * No other concurrent anticancer agents, including chemotherapy, radiotherapy, or anticancer biologic therapy