Changes in the 6th edition of the World Health Organization classification of tumours of the digestive system

The 6th Edition of the WHO Classification of Digestive System Tumours represents a significant update to the 5th edition. It integrates pathological, new molecular, and clinical insights to refine the taxonomy of digestive system neoplasms. The revised classification continues to emphasise standardisation in terminology, coding, and diagnostic criteria to facilitate global consistency in diagnosis, treatment, epidemiological reporting and research. Structural reorganisation of book chapters describes epithelial tumours by anatomical site, while separating neuroendocrine, mesenchymal and haematolymphoid tumours into dedicated chapters that are aligned with other WHO tumour volumes. Genetic tumour syndromes are classified by mechanisms, pathways and genes, whereas metastatic disease is comprehensively covered under other tumours and metastases. Key structural and diagnostic refinements include consolidation of gastric dysplasia entities; separation of duodenal/ampullary from jejuno-ileal tumours; clearer categorisation of colorectal serrated polyps and novel carcinoma grading; introduction of small- and large-duct intrahepatic cholangiocarcinoma as separate entities, and redefinition of undifferentiated carcinoma to include 'carcinoma with mesenchymal differentiation'. Several new entities are introduced, including oesophageal epidermoid metaplasia, colorectal intramucosal adenocarcinoma, low-grade tubuloglandular adenocarcinoma and lymphoglandular complex-like adenocarcinoma, intraductal tubulopapillary and intraductal oncocytic papillary neoplasms of the bile ducts and sonic hedgehog hepatocellular adenoma. The concept of amphicrine-like carcinoma (ALC) is distinguished from MiNEN and broadens the understanding of tumours with dual neuroendocrine-non-neuroendocrine differentiation. Grading systems are simplified to two-tier classifications (low/high grade) across precursor lesions, with enhanced criteria for neuroendocrine tumour grading. Anal canal neoplasia terminology is harmonised with human papillomavirus (HPV) related Lower Anogenital Squamous Terminology (LAST) and mass-forming biliary and gallbladder cancer precursors share similar terminology. Finally, carcinoma of unknown primary (CUP) is included in a separate section for the first time, classified by molecular and immunophenotypic profiles to guide therapy. Overall, the 6th edition strengthens tumour diagnostic precision and molecular alignment across the digestive system.