Imatinib Mesylate in Treating Patients With Gastrointestinal Stromal Tumor That Has Been Completely Removed During Surgery

Inclusion Criteria: Patient must have an ECOG/Zubrod performance status of ≤ 2 Patient must have a diagnosis of high-risk primary GIST; NOTE: High risk is defined as tumor size ≥ 10 cm in maximum dimension, or the presence of tumor rupture before or during surgery, intraperitoneal hemorrhage or multifocal (< 5) intraperitoneal tumors Patient must have undergone complete gross resection (includes R0 [negative microscopic margins] and R1 [positive microscopic margins] resections) of a primary GIST within 70 days prior to registration Patient must have a histologic diagnosis of GIST that is confirmed by central pathology review Patient's tumor must stain positive for the Kit receptor tyrosine kinase on immunohistochemistry as determined by the central pathologist using the Dako (Dako Corp., Carpinteria, CA) anti-CD 117 antibody Patient must have a chest x-ray completed within 28 days prior to registration Patient must have a post-operative CT scan with IV and PO contrast or MRI with contrast (if allergic to CT contrast) of abdomen and pelvis within 28 days prior to registration Creatinine ≤ 1.5 times the institution ULN WBC ≥ 2,000/mm^3 Platelet ≥ 100,000/mm^3 Total bilirubin ≤ 1.5 times the institution ULN AST and ALT ≤ 2.5 times the institution ULN Female of childbearing potential must have negative serum pregnancy test Patient or the patient's legally acceptable representative must provide a signed and dated written informed consent prior to registration and any study related procedures If patient is a cancer survivor, each of the following criteria must apply: Patient has undergone potentially curative therapy for all prior malignancies, No evidence of any prior malignancies for at least 5 years with no evidence of recurrence (except for effectively treated basal cell or squamous carcinoma of the skin, carcinoma in-situ of the cervix that has been effectively treated by surgery alone, or lobular carcinoma in-situ of the ipsilateral or contralateral breast treated by surgery alone) Patient is deemed by their treating physician to be at low risk for recurrence from prior malignancies Exclusion Criteria: Patient has received post-operative chemotherapy Patient has received post-operative radiation therapy Patient has received post-operative investigational treatment Patient has received prior therapy with STI571 Patient has had an active infection requiring antibiotics within 14 days prior to registration Patient has objective evidence of residual disease on the post-operative CT scan or MRI of the abdomen or pelvis Patient, if female and breastfeeding; NOTE: It is not known whether STI571 or its metabolites are excreted in human milk; however, in lactating female rats administered 100 mg/kg, a dose approximately equal to the maximum clinical dose of 800 mg/day based on body surface area, STI571 and/or its metabolites were extensively excreted in milk; it is estimated that approximately 1.5% of a maternal dose is excreted into milk, which is equivalent to a dose to the infant of 30% the maternal dose per unit body weight; because many drugs are excreted in human milk and because of the potential for serious adverse reactions in nursing infants, women should be advised against breastfeeding while taking STI571 Patient has New York Heart Association class 3 or 4 cardiac disease Patient is taking full dose warfarin; NOTE: The use of mini-dose warfarin (1 mg orally per day) for prevention of central line-associated deep venous thrombosis is permitted