Supplementary Material for: Gastric polyps in Familial Adenomatous Polyposis Portuguese patients: the first Western cohort with Asian features

Introduction: Chronic atrophic gastritis may contribute to gastric polyps (GP) phenotype in familial adenomatous polyposis (FAP). Considering the high prevalence of Helicobacter-pylori (HP) infection in Portugal, we aim to characterise GP in a series of Portuguese patients.
Methods: In a retrospectively-selected series of 53 FAP patients, clinical data and histopathological features of GP and background gastric mucosa were studied. SPSS (27.0) was used for statistical analysis.
Results: Thirteen patients (24.5%) developed fundic gland polyps (FGP), seven (13.2%) gastric adenomas (GA) and ten (18.9%) both FGP and GA. Out of 100 GP, four were hyperplastic polyps, 58 FGP (24 with dysplasia), 35 intestinal-type GA (intGA) and three foveolar-type GA (fovGA). IntGA were larger (60% >7mm, p=0.03), occurred predominantly in the distal stomach (66.7%, p=0.024), in patients harbouring gastric intestinal metaplasia (IM) (86.7%, p<0.001) and duodenal adenomas (86.7%, p<0.001)
Conclusion: This is the first Western series showing high prevalence of intGA in FAP patients, comparable to Asian cohorts. HP infection and chronic atrophic gastritis/intestinal metaplasia are likely responsible for this difference, with risk of neoplastic transformation and management implications. Biopsy/excision of GP >7mm, in the distal stomach, and in patients harbouring gastric intestinal metaplasia/duodenal adenomas should be considered.