Unresectable Gastrointestinal Stromal Tumor (GIST)

This trial is a prospective, randomized (1:1 ratio), multicenter, comparative and phase II study, conducted in patients with unresectable advanced gastrointestinal stromal tumors (GIST) after failure of imatinib (disease progression),sunitinib and regorafenib (either disease progression or intolerance) In the first arm, patients will be treated with imatinib + atezolizumab (experimental arm), whereas in the second arm, patients will be treated with imatinib alone (control arm). The comparison between this two arms will allow to compare whether or not atezolizumab and imatinib is efficient for disease control, in terms of Progression-Free Survival improvement.