Abstract

The new WHO classification of tumors of the digestive system not only redefines common diagnostic terms, such as intraepithelial neoplasia and dysplasia but also introduces changes in the nomenclature and diagnostics of colorectal tumors which will be important in daily practice. Changes in nomenclature and classification include the introduction of serrated adenocarcinoma, cribriform comedo type adenocarcinoma and micropapillary adenocarcinoma as new distinct histological subtypes of colorectal cancer. The grading of mucinous and signet ring carcinomas, which were previously invariably graded as G3/high grade, is now dependent on the microsatellite instability (MSI) status as a high MSI (MSI-H) indicates a better and low or no MSI (MSI-L/MSS) a worse prognosis. Thus, analysis of microsatellite instability via immunohistochemistry or fragment length analysis must be included in the pathological report of these tumors. Serrated polyps/adenomas and their potential of progression into colorectal cancer via the alternative pathway of colorectal carcinogenesis will be discussed as well as new insights into prognostic and predictive markers of colorectal cancer. This manuscript will give an overview of the most important changes within the new WHO classification of colorectal tumors.