The purpose of this study is to evaluate WB DWI as early predictor of response to treatment with regorafenib or placebo in patients with advanced metastatic GIST.

Aim of the study

- To assess whole body diffusion-weighted magnetic resonance imaging (WB-DWI) for the assessment and early prediction of response of treatment with regorafenib or placebo in patients with advanced, metastatic gastro-intestinal stromal tumors (GIST)

1. Evaluation of pretreatment apparent diffusion coefficient (ADC) and b1000 signal intensity (SI) of GIST visualized on the WB-DWI as predictor of time to progression, determined by progression-free survival (PFS)
2. Evaluation of WB-DWI using changes of high b-value SI and ADC early during treatment (2weeks after start of therapy; allowed optimal window 10-14 days) as early predictor of time to progression or patient benefit according to RECIST (stable disease + partial response + complete response)
3. Evaluation of WB-DWI for treatment follow-up 3 months after initiation of treatment. Confirmation of prior published pilot study (Dunet V et al, J Nucl Med 2010)
4. Comparison of WB-DWI with conventional CT imaging for response assessment

Inclusion Criteria:

• patients with advanced, metastatic gastro-intestinal stromal tumors treated with regorafenib or placebo

Exclusion Criteria:

• in case of a known contraindication for MRI (eg. pacemaker), the patient will not be admitted to the study

patients with advanced, metastatic gastro-intestinal stromal tumors treated with regorafenib or placebo