Inclusion Criteria

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