Systemic Targeted Adaptive RadioTherapy of NeuroEndocrine Tumors.

Inclusion Criteria: * Age ≥18 years * Written informed consent * Eastern Cooperative Oncology Group (ECOG) 0-1 * Presence of histologically confirmed, advanced, well-differentiated, inoperable neuroendocrine tumors (NET) of any primary tumor origin and any grade, except for pheochromocytoma and paraganglioma. * Somatostatine receptor (SSTR)-expression in tumor lesions \> basal liver uptake on 68Ga-DOTA-PET * Radiologically progressive disease within the last 1-24 months according to common clinical criteria and confirmed by the institutional multidisciplinary conference for the treatment of NETs. The CT/MRI that shows tumor progression compared to screening/baseline must have been performed 1-24 months earlier. * All previous anti-tumor treatment except SSA must be terminated at least 4 weeks before start of treatment within the trial. * Measurable disease according to RECIST v 1.1 * Given the available, approved anti-tumor treatments and the specific characteristics of the patient and the tumor, the investigator judges peptide receptor radionuclide therapy (PRRT) to be the treatment of choice * GFR \> 50 ml/min/1.73 m2 as determined by iohexol- or 51Cr-EDTA clearance, calculated according to a combination of LMR18 and CAPA formulas, or equally accurate method * Hemoglobin \> 90 g/L, platelets \>100 x109/L, leukocytes \> 3.0x109/L, neutrophils \> 1.5 x109/L, aspartate transaminase (ASAT)/alanine aminotransferase (ALAT) \< 3 x ULN, bilirubin \< 2 x upper limit of normal (ULN), albumin \> 25 g/L * For women of child-bearing potential, highly effective contraception should be used from the time of inclusion up to at least six months after the end of treatment (EOT) visit. Exclusion Criteria: * Pregnancy or lactation * Previous treatment with PRRT * Concomitant systemic anti-tumor therapy other than somatostatin analogue (SSA) * Contraindications for treatment with capecitabine according to the approved label. * Discordance between CT/MRI/18F-FDG-PET and 68Ga-DOTA-PET, with evidence of tumor lesions without uptake on 68Ga-DOTATOC. * Any other serious, uncontrolled medical or psychiatric condition that, in the opinion of the investigator, precludes the patient from participation in the trial * Unwillingness, or inability, to participate in any part of the trial procedures or treatments.