DISEASE CHARACTERISTICS:

• Histologically confirmed primary gastrointestinal tumor (GIST)
• Tumor size at least 3 cm in maximum dimension
• No peritoneal or distant metastasis

• Prior complete gross resection of a primary GIST within the past 14-70 days

  • R0 resection (negative microscopic margins) OR
  • R1 resection (positive microscopic margins)
• Tumor must stain positive for Kit receptor tyrosine kinase by immunohistochemistry using the Dako anti-CD117 antibody
• No objective evidence of residual disease on the postoperative CT scan or MRI of the abdomen or pelvis

PATIENT CHARACTERISTICS:

Age:

• 18 and over

Performance status:

• ECOG 0-2 OR
• Zubrod 0-2

Life expectancy:

• Not specified

Hematopoietic:

• WBC at least 2,000/mm^3
• Platelet count at least 100,000/mm^3

Hepatic:

• Bilirubin no greater than 1.5 times upper limit of normal (ULN) (unless elevation is secondary to Gilbert's disease)
• AST and ALT no greater than 2.5 times ULN

Renal:

• Creatinine no greater than 1.5 times ULN

Cardiovascular:

• No New York Heart Association class III or IV cardiac disease

Other:

• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective barrier contraception during and for 3 months after study participation

• No other malignancy within the past 5 years except:

  • Effectively treated basal cell or squamous cell skin cancer
  • Carcinoma in situ of the cervix effectively treated by surgery alone
  • Lobular carcinoma in situ of the ipsilateral or contralateral breast treated by surgery alone
• Prior malignancies must be deemed at low risk for recurrence
• No active infection requiring antibiotics within the past 14 days

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• No concurrent anticancer biologic agents

Chemotherapy:

• No prior postoperative chemotherapy
• No concurrent anticancer chemotherapy

Endocrine therapy:

• Not specified

Radiotherapy:

• No prior postoperative radiotherapy

Surgery:

• See Disease Characteristics

Other:

• No prior postoperative investigational treatment
• No prior imatinib mesylate (Gleevec; STI571)
• No other concurrent anticancer agents
• No other concurrent investigational drugs
• No concurrent full-dose warfarin for therapeutic anticoagulation (concurrent mini-dose warfarin [1 mg orally per day] for prophylaxis of central venous catheter thrombosis allowed)