Background: As the World Health Organization grading system for gastroenteropancreatic-neuroendocrine tumors\n(GEP-NETs) may not always correlate with tumor progression, it is imperative that other independent predictors of\ntumor progression be established. To identify such predictors, we conducted a retrospective histopathological study\nof hindgut NETs, obtained from endoscopic procedures, and used statistical analyses to evaluate predictive factors.\nMethods: We first obtained clinicopathological data of cases of hindgut NETs. Tissue sections from tumor samples\nwere prepared and subjected to pathological examination. In particular, we calculated the microvessel density\n(MVD) and lymphatic microvessel density (LMVD) values, and performed appropriate statistical analyses.\nResults: A total of 42 cases of hindgut NETs were selected for the study, 41 from the rectum and 1 from the\nsigmoid colon. Based on the Ki-67 labeling index, 34 cases were classified as NET G1 tumors and 8 as NET G2\ntumors. MVD values ranged from 1.4/mm2 to 73.9/mm2 and LMVD values from 0/mm2 to 22.9/mm2. MVD and\nLMVD were identified as risk factors for venous and lymphatic invasion of hindgut NETs. Moreover, MVD positively\ncorrelated with the maximum diameter of the tumor.\nConclusions: Tumor progression of NETs may cause angiogenesis and lymphangiogenesis, via an unknown\nmechanism, as well as lymphovascular invasion. Angiogenesis likely plays an important role in occurrence and\nprogression in the initial phase of hindgut NETs.
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