Factors predicting metastasis in patients with 10–20-mm sized rectal neuroendocrine tumor

Background: Rectal neuroendocrine tumors (NET) <10 mm are typically treated with endoscopic resection, while those >20 mm should be treated with radical surgical resection. However, the proper treatment methodology for 10-20-mm sized rectal NETs remains controversial. The purpose of this study was to investigate the appropriate treatment strategy for 10-20-mm sized rectal NETs by verifying factors that can predict metastasis.Methods: A total of 24 patients with 10–20-mm sized NETs who were treated at Pusan National University Yangsan Hospital from January 2009 to September 2020 were included. The patients were divided into the metastasis and non-metastasis groups, and their respective data were analyzed.Results: Seven patients had metastasis (29.2%) while 17 patients had no metastasis (70.8%). The tumor size, based on the endorectal ultrasound (EUS) findings, was significantly larger in the metastatic than in the non-metastatic group (14.4 vs. 10.2 mm, p = 0.029); however, the pathological tumor size did not show significant differences (13.3 vs. 11.8 mm, p = 0.065). Lymph node enlargement was observed in a single patient (14.3%) in the metastatic group, based on EUS and computed tomography (CT) imaging findings. The mitotic count (p = 0.017) and Ki-67 index (p = 0.014) were significantly higher in the metastatic group than in the non-metastatic group. The two groups also showed a significant difference in the proportion of patients with tumor grade 2 (71.4% vs. 5.9%, p = 0.003). In multivariate analysis, the tumor grade was the only independent metastasis-predicting factor (odds ratio = 49.25; 95% confidence interval = 2.55–950.83; p = 0.010). Conclusions: Regarding the treatment of 10–20-mm sized rectal NETs, tumor grade 2 should be the most important factor to determine whether additional radical resection is necessary.