[Colorectal carcinoma in consideration of the new German S3 guideline 2013].

In the current German S3 guidelines for colorectal carcinoma (CRC), morphologically based tumor grading is extended by molecular grading for poorly differentiated and undifferentiated carcinomas, as well as for special morphological subtypes. These CRC are classified as low-grade when microsatellite instability (MSI) is found. In routine diagnostics, immunohistochemistry for hMLH1 and hMSH2, capturing MSI-CRC with high sensitivity and specificity, can be used as an inexpensive substitute for molecular MSI-testing. In patients with positive Bethesda criteria, a stepwise immunohistochemical and molecular diagnostic scheme is proposed. The detection of a BRAF mutation in tumors with hMLH1 loss allows distinguishing between sporadic and HNPCC-associated MSI-CRC. For rectal cancer the residual tumor classification (R-status) is completed by the circumferential resection margin classification (CRM).