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Purpose: Neuroendocrine neoplasms of the appendix (aNET) are rare tumors that are often diagnosed by pathology as an incidental finding after appendectomy for acute appendicitis. Several guidelines proposed risk criteria to indicate oncological completion surgery after appendectomy. The aim of this study was to evaluate the reliability of proposed criteria for completion surgery of aNET. Methods: Patients with aNET treated at ENETS center of excellence Marburg between 2002 and 2022 were retrieved from a prospective data base. Demographic data, histopathological findings, including formerly proposed criteria to indicate oncological completion surgery, histological results of the completion resection and disease-free survival were evaluated. Results: 82 patients with a median age of 35 (range 8–82) years were analysed. 72 (88%) patients underwent an emergency appendectomy because of acute appendicitis. 11 (13%) patients received an ileocecal resection or right hemicolectomy. Seven (8.5%) patients had lymph node metastases and three (3.6%) patients had distant metastases at the initial operation. 27 (33%) patients underwent completion surgery by right hemicolectomy according to guideline criteria, but postoperative histology detected lymph node and distant metastases in only six (22%) and zero patients resulting in an overtreatment of 21 (75%) patients. A tumor size of > 2 cm was the only significant criterion which was associated with lymph node metastases (p < 0.05). After a median follow-up of 62 months (range 2-264) 76 (96%) of the patients in stages I to III were alive with no evidence of disease. Conclusion: aNET have an excellent prognosis in stages I-III and distant metastases are rare. Formerly proposed criteria for oncological completion surgery have to be adopted and discussed for every patient, as they might result in an overtreatment in at least 75% of patients. [ABSTRACT FROM AUTHOR]