Tumours of the duodenum and the ampulla of Vater: An overview of the key criteria for their classification, pathogenesis, and molecular assessment.

Duodenal and ampullary tumours represent rare neoplasms that arise sporadically or in the context of tumour predisposing syndromes, especially familial adenomatous polyposis. This heterogeneous group of tumours encompasses both non-invasive and invasive epithelial neoplasms, which are associated with variable clinical presentations and prognoses. Defining the exact origin-whether duodenal or ampullary-is determined by the localization of the centre of the tumour, the identification of which requires meticulous specimen grossing. While histomorphological evaluation remains the cornerstone of establishing a correct diagnosis, immunohistochemistry may be required for characterizing less common neoplasms. Moreover, antibody panels targeting lineage-specific markers (intestinal, pancreatobiliary, and gastric) yield ancillary information for the subtyping of ampullary carcinomas, thereby guiding the selection of systemic chemotherapy. Furthermore, in view of the rapid expansion of targeted therapies, routine molecular profiling for relevant predictive biomarkers is warranted. This review outlines the morphological and immunohistochemical diagnostic criteria for the classification of this group of neoplasms and addresses their pathogenesis and the clinical relevance of molecular testing within the current standard of care.