Study of Dose Escalation Versus no Dose Escalation of Imatinib in Metastatic Gastrointestinal Stromal Tumors (GIST) Patients

Inclusion Criteria: * Age ≥ 18 years * Unresectable and/or metastatic GIST * Currently receiving imatinib 400 mg per day for a minimum of 4 weeks prior to registration, and for no more than 6 months prior to registration. This must be the first time that the patient has been treated for metastatic and/or unresectable GIST * For patients who received imatinib following surgery at the time of an initial diagnosis of GIST, there must be a 6 month interval between completion of imatinib and the diagnosis of metastatic GIST * Good physical functioning (ECOG Performance Status of 0 or 1) * Generally, good function of organ such as liver and kidneys Exclusion Criteria: * Disease progression during adjuvant therapy with imatinib (adjuvant treatment is treatment that is given after surgery for GIST) * Known intolerance of imatinib at a dose of 400 mg/day or higher * Prior systemic therapy for advanced GIST with imatinib or those who have been on imatinib for longer than 6 months for unresectable and/or metastatic disease * Major surgery within 2 weeks prior to Day 1 of study or who have not yet recovered from prior surgery * Use of coumadin derivatives (i.e. warfarin, acenocoumarol, phenprocoumon) * Patients who have received wide field radiotherapy ≤ 4 weeks or limited field radiation for palliation \< 2 weeks or who have not recovered from side effects of this therapy