see related editorial on page x nature publishing group COLON/SMALL BOWEL 1 © 2014 by the American College of Gastroenterology The American Journal of GASTROENTEROLOGY ORIGINAL CONTRIBUTIONS INTRODUCTION Colonoscopy is the most efective method for detecting and removing precursors of colorectal cancer (1). If a colon polyp is restricted to the supericial layers of the colonic wall (i.e. mucosa and upper part of the submucosa) it can be safely removed by endoscopy preventing potential malignant transformation. Japa- nese and Western studies have shown that the endoscopic appear- ance of a polyp can predict invasion into the submucosa, which is associated with an increased risk of nodal metastases (2–7). Because of this predictive value, polyp morphology can be an important determinant of when to apply endoscopic therapy and when to refer for surgery. Pedunculated lesions are less likely to demonstrate invasive growth compared with sessile lesions (5% vs. 34%) (8), and depressed lesions have been reported to be invasive in up to 61% (3,8,9). Polyp morphology has also been described to be associated with incomplete endoscopic resec- tion during colonoscopy (10). Previous studies have suggested that insuicient awareness and recognition of non-polypoid (lat) lesions contributes to the development of right-sided interval car- cinomas (11–14). Flat lesions were initially thought to be conined to the Japanese population, but more recently they have been described and recognized worldwide (8,9,15–17). A validated and reproducible classiication is necessary to compare the prevalence of diferent morphological types of polyps in diferent parts of the Polyp Morphology: An Interobserver Evaluation for the Paris Classification Among International Experts Sascha C. van Doorn, MD 1 , Y. Hazewinkel, MD 1 , James E. East, MD 2 , Monique E. van Leerdam, MD, PhD 3 , Amit Rastogi, MD 4 , Maria Pellisé, MD, PhD 5 , Silvia Sanduleanu-Dascalescu, MD, PhD 6 , Barbara A.J. Bastiaansen, MD 1 , Paul Fockens, MD, PhD 1 and Evelien Dekker, MD, PhD 1 OBJECTIVES: The Paris classification is an international classification system for describing polyp morphology. Thus far, the validity and reproducibility of this classification have not been assessed. We aimed to determine the interobserver agreement for the Paris classification among seven Western expert endoscopists. METHODS: A total of 85 short endoscopic video clips depicting polyps were created and assessed by seven expert endoscopists according to the Paris classification. After a digital training module, the same 85 polyps were assessed again. We calculated the interobserver agreement with a Fleiss kappa and as the proportion of pairwise agreement. RESULTS: The interobserver agreement of the Paris classification among seven experts was moderate with a Fleiss kappa of 0.42 and a mean pairwise agreement of 67%. The proportion of lesions assessed as “flat” by the experts ranged between 13 and 40% ( P<0.001). After the digital training, the interobserver agreement did not change (kappa 0.38, pairwise agreement 60%). CONCLUSIONS: Our study is the first to validate the Paris classification for polyp morphology. We demonstrated only a moderate interobserver agreement among international Western experts for this classification system. Our data suggest that, in its current version, the use of this classification system in daily practice is questionable and it is unsuitable for comparative endoscopic research. We therefore suggest introduction of a simplification of the classification system. SUPPLEMENTARY MATERIAL is linked to the online version of the paper at http://www.nature.com/ajg Am J Gastroenterol advance online publication, 21 October 2014; doi:10.1038/ajg.2014.326 1 Department of Gastroenterology and Hepatology, Academic Medical Centre, University of Amsterdam, Amsterdam, The Netherlands; 2 Translational Gastroenterology Unit, University of Oxford, Oxford, UK; 3 Department of Gastroenterology and Hepatology, Netherlands Cancer Institute, Amsterdam, The Netherlands; 4 Endoscopy Department, University of Kansas School of Medicine, Kansas City , Kansas, USA; 5 Department of Gastroenterology, Hospital Clínic, Centro de Investigación Biomédica en Red en Enfermedades Hepáticas y Digestivas (CIBERehd), IDIBAPS, Barcelona, Spain; 6 Division of Gastroenterology and Hepatology, Maastricht University Medical Center , Maastricht, The Netherlands. Correspondence: Evelien Dekker, MD, PhD, Department of Gastroenterology and Hepatology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, Amsterdam, AZ 1105, The Netherlands. E-mail: e.dekker@amc.uva.nl Received 14 May 2014; accepted 23 August 2014