Knowledge gaps in tubular gut tumours: a critical appraisal of the 6th edition of the World Health Organization classification of tumours

Gastrointestinal cancer is a global health problem. In the new 6th edition of the World Health Organization Classification of Tumours (WCT) of the Digestive System, updated evidence and guidance is provided for the aetiology, pathogenesis, diagnosis, classification, grading, staging and prognosis of these tumours. However, significant knowledge gaps remain, some of which are addressed in this review. To provide for a research framework to fill these knowledge gaps, we focused on precursor lesions and rare cancers, where the need for new evidence is most urgent. Areas with a need for further research are discussed for neoplastic precursors of the oesophagus (squamous epithelium), stomach (gastric dysplasia and adenomas), ampulla-duodenum, jejunoileum, anal canal, as well as for colorectal serrated polyposis and inflammatory bowel disease associated dysplasia (conventional and non-conventional). Knowledge gaps are also presented as they exist in rare cancers of the oesophagus (mixed type), appendix (mucinous neoplasms and mucinous adenomas), colorectum (three new subtypes) and anus, as well as for neuroendocrine tumours (grading, necrosis and two new gastric subtypes) and undifferentiated carcinomas (deficiencies of the SWI/SNF chromatin remodelling complex and mesenchymal differentiation). We identify and present research questions and persistent challenges for these entities and thus propose research frameworks that focus on specific priority areas. Progress in these core research topics for both neoplastic precursors and rare cancers will hopefully be reflected in future editions of the WCT, but many of these knowledge gaps will only be resolved with collective registries and collaborative research efforts.