Efficiency of Imatinib Treatment Maintenance or Interruption After 3 Years of Adjuvant Treatment in Patients With Gastrointestinal Stromal Tumours (GIST)

Inclusion Criteria: * Age ≥ 18 years at the day of consenting to the study * Patients must have histologically confirmed diagnosis of localized GIST with documented KIT (CD117) positivity (by polyclonal DAKO antibody staining) * Documented macroscopically complete surgical R0 or R1 resection of primary GIST lesion with no evidence of residual lesions or metastases on the baseline CT-scan or MRI performed no more than 4 weeks before randomization. * Risk of tumor recurrence ≥ 35% according to National Comprehensive Cancer Network Task Force on GIST (NCCN) risk classification (Demetri et al., 2010) (See Appendix 1) * Eastern Cooperative Oncology Group (ECOG) performance status 0, 1 or 2 * Patients must be under imatinib treatment (at 300 or 400 mg/day) initiated immediately after resection and maintained for 3 years (i.e. 36 months ± 3 months at the time of randomization) with no more than 3 consecutive months or 6 months in total of interruption during these past 3 years. * Patients must have normal organ and bone marrow function at baseline as defined below: * absolute neutrophil count (ANC) ≥ 1.5 G/L, platelet count ≥ 100 G/L, and haemoglobin of ≥ 9 g/dL). * Serum total bilirubin ≤ 1.5 (upper limit of normal (ULN), aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 3 x ULN (or 5 x ULN in case of hepatic metastases at time of reintroduction) * Adequate renal function assessed by at least one of the following: * 1\) Serum creatinine ≤ 1.5 x ULN …