The theranostic principle is based on the use of radiolabeled compounds which can be applied for diagnostic molecular imaging and targeted delivery of radiation to the tumor. Gastroenterologic tumors (GET), including hepatocellular Carcinoma, Colorectal Carcinoma, Pancreatic Adenocarcinoma, Cholangiocellular Carcinoma, gastroenteropancreatic neuroendocrine neoplasms also express a phenotypic biomarker called prostate-specific membrane antigen (PSMA), thereby rendering it a potential diagnostic (through positron emission tomography (PET) scan imaging) and therapeutic target for radioligand therapy. Aim is to evaluate whether PSMA-directed in-vivo imaging can be also applied to GET patients to determine if i) biopsy-derived tissue of newly diagnosed patients exhibit a PSMA expression profile, ii) PSMA-PET shows upregulated PSMA expression in-vivo, iii) such a molecular imaging approach identifies more disease sites relative to conventional imaging, and iv) if the PSMA PET signal predicts further clinical course and outcome under guideline-compatible treatment.