Efficiency of Imatinib Treatment After 10 Years of Treatment in Patients With Gastrointestinal Stromal Tumours (GIST) (Gist-Ten)

This is a 2 arms study concerning patients under imatinib treatment for at least 10 years of treatment with locally advanced/metastatic GIST. In the first arm, patients will discontinue Imatinib treatment. This arm will allow to determine if the re-introduction of Imatinib at relapse is still an efficient treatment for the control of disease. In the second arm, patients will continue Imatinib treatment, allowing to determine if the continuation of this treatment is efficient for disease control, by the rate of non-progression disease.