Inclusion Criteria:  Patients with GIST with a biopsy confirmed primary KIT exon 11 mutation covered by our KIT exon 11 ddPCR assay (mutation/deletion within target sequence of c.1665_1736); Patients with an indication for at least 4 CT-scans concomitant with regular laboratory examination in a neoadjuvant, adjuvant and/or palliative care trajectory within the time frame of the study; Age ≥18 years; Written informed consent provided.  Exclusion Criteria:  1. Patients who are unable to comply with study procedures and follow up.