Inclusion Criteria:  Patients ≥18 years of age; Histologically documented diagnosis of malignant advanced/metastatic GIST with immunohistochemical documentation of c-kit (CD117) expression either by the primary tumor or metastases; Eastern Cooperative Oncology Group (ECOG) - Performance status (PS) 0 to 2 evaluated within 7 days prior to the date of inclusion. Patient must be under imatinib treatment (at 300 or 400mg/day) maintained for 10 years or over with no more than 12 months in total or 3 consecutive months of interruption during the treatment period; Patient with controlled disease (without any progression under imatinib); Willingness and ability to comply with scheduled visits, treatment plans , laboratory tests, and other study procedures; Covered by a medical/health insurance; Signed and dated informed consent document indicating that the patient has been informed of all aspects of the trial prior to enrolment.  Exclusion Criteria:  Patient concurrently using other approved or investigational antineoplastic agents; Patient with GIST harboring the mutation D842V in PDGFRA; Major concurrent disease affecting cardiovascular system, liver, kidneys, haematopoietic system or else considered as clinically important by the investigator and that could be incompatible with patient's participation in this trial or would likely interfere with study procedures or results; Prior history of other malignancies other than study disease (except for basal cell or squamous cell carcinoma of the skin or carcinoma in situ of the cervix) unless the patient has been free of the disease for at least 3 years; Patient receiving concurrent treatment with warfarin (acceptable alternative: low-molecular weight heparin) or any prohibited concomitant and/or concurrent medications Patient has a known diagnosis of human immunodeficiency virus (HIV) infection; Major surgery within 2 weeks prior to study entry. Known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the study. Pregnant or breastfeeding woman Patient requiring tutorship or curatorship or patient deprivied of liberty.