Collection of Liquid Biopsy Samples of Neuroendocrine Neoplasms (NEN) Patients - Collection of NET (CollectNET) 2.0, a Study by the BE-FORCE Consortium

Inclusion Criteria: * Male or female ≥ 18 years of age on the day of signing informed consent. * Written informed consent must be obtained from the patient or patient's legal representative. * Patient is willing and able (in the investigator's opinion) to comply with all trial requirements. * For inclusion in the Regular Sampling Group: patients must have (had) a histologically confirmed NEN diagnosis, patients must have measurable tumor burden on imaging, patients must be in follow-up in one of the participating hospitals and patients who have progressed or completed follow-up for 3y in the Intensive Sampling Group. * For inclusion in the Intensive Sampling Group: patients who are included in the Regular Sampling Group and where either a baseline sample (RSG-B) or a recent RSG follow-up sample (RSG-V…) has been collected before the start of 1st systemic treatment (as defined below), patients must be diagnosed with a histologically confirmed NEN diagnosis of a WHO 2019 grade 1-3 NET or NEC of pancreatic, colorectal, or small intestinal origin and patients must start any kind of 1st line systemic treatment (e.g. somatostatin analogues, targeted therapy, chemotherapy, etc.). Exclusion Criteria: * Patients who are unable to give informed consent. * Patients for which blood sampling would compromise their overall health. * Patients pregnant at time of study entry or are willing to become pregnant during the study. * Patients with a history or current evidence of any condition or abnormality that might confound the results of the study, interfere with the patient's participation for the full duration of the study, or is not in the best interest of the patient to participate, in the opinion of the Investigator.