Imatinib can lead to long recurrence free survival in patients diagnosed with gastrointestinal stromal tumors (GIST); however side effects can significantly hinder quality of life for our patients. This study will use therapeutic drug monitoring to improve quality of life and symptoms and assess how many patients maintain therapeutic levels. Free drug levels and pharmokinetics of imatinib will also be monitored.

Inclusion Criteria:

• Histologically or cytologically confirmed GIST with KIT mutation or PDGFRA mutation (non-D842V)
• Currently receiving imatinib initiated within the last 3 months or to be started in the next 1 month
• Prior systemic chemotherapy for other malignancy is allowed as long as it was completed within the past 6 months and there is no evidence of disease
• Age ≥18 years
• ECOG performance status of 0 or 1
• Normal organ function

Exclusion Criteria:

• Presence of PDGFRA D842V mutation
• Known allergy to imatinib or allergic reactions to compounds of similar chemical or biologic composition to the study drug
• Concomitant anticoagulation with oral warfarin.
• Use of strong inhibitors or inducers of CYP3A or CYP3A4, drugs metabolized by CYP3A4 or CYP2D6
• Uncontrolled intercurrent illness
• Concurrent malignancy