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