Background: Rectal neuroendocrine tumor (NET) smaller than 10 mm is typically treated with endoscopic resection. Rectal NET larger than 20 mm should be treated with radical surgical resection. However, proper treatment for 10 - 20 mm sized rectal NET is controversial. The purpose of this study was to investigate the appropriate treatment strategy for 10-20 mm sized rectal NET by verifying factors that can predict metastasis.Methods: A total of 23 patients with 10–20 mm sized NET who were

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... at Pusan National University Yangsan Hospital from January 2009 to February 2020 were included. The patients were divided into metastasis group and non-metastasis group and their respective data were analyzed.Results: Six patients had metastasis (26.1%) while 17 patients had no metastasis (73.9%). Tumor size in endorectal ultrasound (EUS) was significantly larger in the metastatic group than in the non-metastatic group (p = 0.036), however pathological tumor size did not show significant difference (p = 0.087). The rate of involvement of muscularis propria was 50% in the metastatic group, which was significantly higher than that in the non-metastatic group (p = 0.040). Lymph node enlargement was observed in a single patient (16.7%) in the metastatic group for each of EUS and computed tomography (CT) imaging. Ki-67 index in the metastatic group was higher than in the non-metastatic group. The two groups also showed a significant difference in the proportion of patients with tumor grade 2 (66.7% vs. 5.9%, p = 0.008). In multivariate analysis, tumor grade was the only independent metastasis-predicting factor (p = 0.010). Conclusions: EUS should be performed to evaluate tumor size, and muscularis propria involvement along with CT imaging to check lymph node enlargement before the treatment of 10–20 mm sized rectal NET. If endoscopic resection was performed, tumor grade should be the most important factor to determine whether additional radical resection is necessary.