Reappraisal of the so-called 'villous tumours' of the rectosigmoid, based on histological, immunohistochemical and genotypic features.

Background: Villous tumours of the rectosigmoid are historically defined as broad-based lesions associated with secretory diarrhoea.

Objective: This study aimed to perform a reappraisal of these tumours, on the basis of newly introduced histological, immunohistochemical and molecular parameters.

Methods: For this study, 22 villous tumours, diagnosed by endoscopic criteria (19 Paris 0-IIa, three Paris 0-Is), were evaluated according to WHO classification. Microsatellite instability status, KRAS and BRAF mutations, MGMT status of villous tumours and associated invasive carcinoma were determined.

Results: The 22 villous tumours fell into four groups: 1) nine villous adenomas, 2) six tubulovillous adenomas, 3) three filiform traditional serrated adenomas, and 4) four traditional serrated adenomas with conventional dysplasia. Filiform serrated adenomas displayed a distinctive endoscopic protruding pattern (Paris 0-Is). Villous adenomas were strongly associated with secretory diarrhoea. All the villous tumours were microsatellite stable. Five tumours exhibited MGMT abnormalities. KRAS mutations were frequent in villous adenomas, whereas BRAF mutations were essentially detected in serrated lesions. Invasive carcinomas (n = 7) maintained the histopathological and molecular imprint of the prior villous tumour.

Conclusion: The rectosigmoid villous tumours are histologically and molecularly heterogeneous, including serrated neoplasias. Endoscopic and clinical findings are predictive of the histopathological diagnosis of some of these distinct entities.