The incidence and prevalence of neuroendocrine neoplasms (NENs) in many organs are increasing. Although such NENs have similar grades, they may exhibit quite different behaviors. In this multicenter study, we aimed to investigate the prevalence and distribution of different morphological NEN variants in the non-pancreatic gastrointestinal (GI) tract and determine whether they can guide prognosis prediction.

Two hundred and fifty-six patients diagnosed with NENs originating from the GI tract from 7 different centers were included in the study. In 89 (36.6%) cases, different morphological variants were detected.

When the variants were grouped according to their aggressiveness as described in the literature, a statistically significant relationship between aggressiveness and the variables organ and age was found (p < 0.05). The oncocytic variant was found to metastasize more than the other aggressive types (42.9%). The paraganglioma-like variant was found to have a smaller size, lower proliferation index, and a more benign clinical course.

This study demonstrated that well-differentiated GI neuroendocrine tumors (GI-NETs) have considerable morphological diversity. Generally, case reports of rare morphological variants of GI-NETs are available in the literature. We believe that our study contributes to a better understanding of the prevalence, localization, and significance of morphological variations in GI-NETs.