Gastroenteropancreatic neuroendocrine neoplasms: selected pathology review and molecular updates.

Gastroenteropancreatic (GEP) neuroendocrine neoplasms can be broadly separated into well- and poorly differentiated categories. Tumours within each category have similarities in morphology and immunophenotype, but vary in grade, behaviour, molecular signature and responses to therapy. The aetiology of these differences is multifactorial. Site of origin, mucosal milieu and hereditary influences are some of the currently known factors. Given these differences, staging and grading systems continue to evolve, and the most recent World Health Organization classification of pancreatic neuroendocrine neoplasms reflects this by introducing a grade 3 neuroendocrine tumour category for morphologically well-differentiated tumours with an elevated Ki-67 proliferation index and/or mitotic count. This review aims to highlight current classification guidelines with discussion of unique site-specific features of selected GEP neuroendocrine neoplasms and an emphasis on practical issues related to daily reporting.