Type of progression in patients treated with imatinib for advanced gastrointestinal stromal tumor (GIST): A study based on the EORTC-ISG-AGITG trial 62005

10536 Background: Development of resistance in GIST patients responding to targeted agents is heterogeneous, and different types of mutations may be observed in different lesions of the same patient. Progressive disease (PD) is defined as increase in size of initial lesions, occurrence of new lesions, or both (RECIST): mixed progression (MXPD), were new lesions develop when old lesions are still responding could represent a different type of biological event. We evaluated the impact of progression type and time to first PD (TTP) on survival after progression (SAP) in a randomized phase III trial comparing two doses of imatinib as first line treatment of GIST in 946 patients. Methods: PD was classified into 5 categories: PD in old lesion without new lesions (PD-), and new lesions associated with CR (CR+), PR (PR+), NC (NC+) and finally PD (PD+); MXPD was defined as CR+ or PR+. We investigated the correlations between type of PD, baseline patients’ characteristics, imatinib dose, TTP and SAP with univariate...