The study was approved by the Danish Data Protection Agency at Rigshospitalet (ref. no. 2007-58-0015) and by the Danish Patient Safety Authority (31-1521-453). Due to the retrospective design, informed consent approval from the Ethical Committee was not required. Data collectionData were collected from the prospective NET database at Rigshospitalet and the Danish National Pathology Database. Data included gender, age at diagnosis, date of diagnosis, performance status (PS) at diagnosis, location of primary tumor, metastases, and TNM-classification at diagnosis. Surgical and oncological treatment data were retrieved from the date of diagnosis until date of death or end of follow-up, dated on the 21 January 2020. Progression-free survival (PFS), recurrence-free survival (RFS), and overall survival (OS) were recorded. The surgical data included the type and anatomical location of the surgical intervention and its radicality and oncological data. The pathological data included Ki67 proliferation index, pTNM (pathological TNM-staging), cTNM (clinical TNM-staging) in patients not eligible for resection, large vs. small cell morphology, tumor differentiation (poorly vs. well-differentiated morphology), determination of largest