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

Inclusion Criteria: Patient must have an ECOG/Zubrod performance status of =< 2 Patient must have a histologic diagnosis of primary GIST (without peritoneal or distant metastasis) that expresses Kit protein by immunohistochemistry and have tumor size >= 3cm in maximum dimension Patient must have undergone complete gross resection (includes R0 [negative microscopic margins] and R1 [positive microscopic margins]) of a primary GIST within 70 days prior to registration Patient must have a chest x-ray completed within 28 days prior to registration; NOTE: Chest CT within 28 days prior to registration can be used in lieu of chest x-ray 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 Platelets >= 100,000/mm^3 Total Bilirubin =< 1.5 times the institution ULN; NOTE: Patients with elevated bilirubin secondary to Gilbert's disease are eligible to participate in the study AST =< 2.5 times the institution ULN ALT =< 2.5 times the institution ULN Female of childbearing potential must have negative serum pregnancy test; NOTE: Post-menopausal women must be amenorrheic for at least 12 months to be deemed not of reproductive potential 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 Patient must provide written authorization to allow the use and disclosure of their protected health information; NOTE: This may be obtained in either the study-specific informed consent or in a separate authorization form and must be obtained from the patient prior to study registration (non-US sites are exempt from HIPAA regulations) If patient is a cancer survivor, ALL of the following criteria 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 postoperative CT scan or MRI of the abdomen or pelvis Patient, if female and breastfeeding; NOTE: It is not known whether STI571or its metabolites are excreted in human milk; however, in lactating female rats administered 100mg/kg, a dose approximately equal to the maximum clinical dose of 800mg/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; 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 (1mg orally per day) for prevention of central line-associated deep venous thrombosis is permitted