Histological grading in colorectal cancer: new insights and perspectives

Poor histological differentiation is currently considered among the adverse histopathological factors associated with unfavourable clinical course of colorectal carcinoma (CRC). At present, the histological grade of CRC is assessed based on the percentage of glandular differentiation in the tumor according to the World Health Organization (WHO) criteria. However the prognostic value of the WHO grading system is limited by significant inter-observer variability in its assessment. In addition, the prognostic significance of WHO grading seems to depend on the microsatellite instability (MSI) status of the tumor. Finally, this grading does not apply to rarer histotypes of colorectal adenocarcinomas, such as the micropapillary, medullary, mucinous and signet ring cell variants. Recently a novel grading system based on the counting of clusters of five or more cells lacking a glandular structure (poorly differentiated clusters) and set in the tumor stroma or at invasive edge has been proposed in CRC. There is evidence that grading based on poorly differentiated clusters (PDC) is more reproducible and has more robust prognostic significance compared to WHO grading in CRC. In the present review we discuss the morphological features, criteria for the assessment, prognostic significance and correlation with biomolecular profiles of grading based on PDC counting in CRC.