GAST and neoplasm: Som-NETs and GSSN-NETs were more commonly located in the ampullary region and showed significantly larger size, more frequent lymphatic and/or vascular invasion, and higher pT, pN, and American Joint Committee on Cancer (AJCC-9th edition) stages compared to Gas-NETs, which were often (77%) diagnosed at AJCC stage I. Ampullary Som-NETs showed a more invasive and metastatic potential compared to non-ampullary Som-NETs, while, among plurihormonal NETs, the predominantly expressed hormone influenced tumor biological features, with gastrin-predominant NETs showing less invasive potential.