DISEASE CHARACTERISTICS:

• Histologically confirmed high-risk primary gastrointestinal stromal tumor

  • High-risk is defined as any of the following:

    • Tumor at least 10 cm in greatest dimension
    • Presence of tumor rupture before or during surgery
    • Intraperitoneal hemorrhage
    • Multifocal intraperitoneal tumors

• Complete gross tumor resection performed within the past 70 days

  • Includes R0 (negative microscopic margins) and R1 (positive microscopic margins) resection
• Kit protein positive
• No residual disease on CT scan or MRI of the abdomen or pelvis

PATIENT CHARACTERISTICS:

Age:

• 16 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)
• AST/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 heart disease

Other:

• No active infection requiring antibiotics within the past 14 days

• No other prior 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
• At low risk for recurrence of curatively treated prior malignancies
• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective barrier-method contraception during and for 3 months after study

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• Not specified

Chemotherapy:

• No post-operative chemotherapy

Endocrine therapy:

• Not specified

Radiotherapy:

• No post-operative radiotherapy

Surgery:

• See Disease Characteristics

Other:

• No prior imatinib mesylate
• No post-operative investigational treatment
• No concurrent full-dose warfarin