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