Imatinib Mesylate in Treating Patients With Locally Advanced Gastrointestinal Stromal Tumor

DISEASE CHARACTERISTICS: * Histologically confirmed gastrointestinal stromal tumor * Locally advanced disease * Potentially resectable disease\* * No tumor that can be completely resected (R0) with sufficient margins NOTE: \*Multivisceral resection may be necessary * Tumor must stain positive for c-Kit (CD117) or platelet-derived growth factor receptor-alpha (PDGFRA) by immunohistochemistry * At least 1 site of measurable disease * No known brain metastases PATIENT CHARACTERISTICS: Age * 18 and over Performance status * ECOG 0-3 Life expectancy * Not specified Hematopoietic * Platelet count \> 100,000/mm\^3 * Absolute neutrophil count \> 1,500/mm\^3 Hepatic * AST and ALT \< 2.5 times upper limits of normal (ULN) (5 times ULN if hepatic metastases are present) * Bilirubin \< 1.5 times ULN * No chronic active hepatitis * No cirrhosis * No other chronic liver disease Renal * Creatinine \< 1.5 times ULN * No chronic renal disease Cardiovascular * No New York Heart Association class III-IV cardiac disease * No congestive heart failure * No myocardial infarction within the past 6 months Immunology * No active uncontrolled infection * No known HIV positivity Other * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective barrier contraception during and for 3 months after completion of study treatment * Must be medically fit to undergo surgery * No other primary malignancy within the past 5 years except basal cell skin cancer, carcinoma in situ of the cervix, or a primary malignancy that is not currently clinically significant and does not require active intervention * No gastrointestinal obstruction or major bleeding episode requiring immediate surgical intervention * No uncontrolled diabetes * No other severe or uncontrolled medical disease * No significant history of noncompliance to medical regimens PRIOR CONCURRENT THERAPY: Biologic therapy * No concurrent anticancer biologic agents Chemotherapy * More than 4 weeks since prior chemotherapy (6 weeks for nitrosourea or mitomycin) unless disease is rapidly progressing * No concurrent anticancer chemotherapy Endocrine therapy * No concurrent systemic corticosteroid therapy unless approved by the study sponsor Radiotherapy * More than 4 weeks since prior radiotherapy * No prior radiotherapy to ≥ 25% of bone marrow Surgery * More than 2 weeks since prior major surgery except tumor biopsy Other * More than 4 weeks since prior investigational drugs unless disease is rapidly progressing * No other concurrent anticancer therapy * No other concurrent investigational agents * No concurrent warfarin for therapeutic anticoagulation * Concurrent low molecular weight heparin (e.g., enoxaparin sodium) or heparin for therapeutic anticoagulation allowed * Concurrent mini-dose warfarin (e.g.,1 mg/day) for prophylaxis of central venous catheter thrombosis allowed