[Sessile serrated lesion with dysplasia and invasion].

Sessile serrated lesions (SSLs) are not uncommon entities per se, usually localised to the proximal half of the colon and are often flat on endoscopic examination. The presence of dysplasia associated with SSL and, even more so, invasion are considered rare, while dysplastic SSLs (SSL-D) very quickly transform into invasive carcinoma. MLH1 immunohistochemical reaction may help to establish the diagnosis of SSL-D. We present the case of an 88-year-old female patient who presented for colonoscopic resection of suspicious tumours in the coecum and transverse colon. Histological examination revealed medullary carcinoma in the coecum with MLH1 and PMS2 mismatch repair protein deficiency. The polyp removed from the transverse colon was histopathologically confirmed to be SSL with central low-grade dysplasia (SSL-D), confirmed by loss of MLH1, and submucosal invasion was seen at a single focus. A BRAF V600E sequencing was performed to rule out Lynch syndrome, which confirmed the presence of the mutation. Risk factors for SSL-D include older age, the presence of multiple synchronous SSLs and a size greater than 10 mm. In SSL-D, due to the high malignant transformation potential, a complete resection should always be sought. In the SSL – SSL-D – carcinoma sequence, the main driver mutations are in the BRAF and less frequently KRAS genes. In cases with numerous SSLs, the possibility of sessile polyposis syndrome, and less frequently MUTYH-associated polyposis and hereditary mixed polyposis syndrome should also be raised. In both SSL and colorectal carcinoma, microsatellite instability, CpG island methylation phenotype and the presence of BRAF V600E mutation are common. The mismatch repair status can be determined by immunohistochemistry using MLH1, PMS2, MSH2 and MSH6 reactions, the 2 tests complementing each other. Determination of mismatch repair status is crucial for the diagnosis of colorectal carcinoma and the potential indication of immune checkpoint inhibitor treatment. Orv Hetil. 2025; 166(11): 427–433.