In this study, the investigators seek to investigate biological aspects in patients receiving targeted treatment for rare subtypes of cancer of the Gastrointestinal.~The targeted treatment will be given as per standard of care. Translational blood samples will be drawn pre-treatment, before the third cycle of chemotherapy, and hereafter corresponding to the planned imaging during treatment and follow up.~The total cell free DNA will be quantified in all samples. The samples will be analyzed for tumor specific mutations such as the KRAS, BRAF, and NRAS oncogenes, by ddPCR. Circulating tumor DNA will also be identified by hypermethylation markers, and a focused panel of next generation sequencing can be applied. The samples will also be analyzed for immune-related biomarkers.~The investigators expect to include up to 130 patients.~This is a purely observational translational study. Results will be analysed in relation to outcome data.
A single-arm prospective observational translational study of biomarkers in patients receiving targeted treatment for rare subtypes of cancer of the Gastrointestinal Tract.