Histologic Reappraisal and Evaluation of MLH1 Protein Expression in Sessile Serrated Lesions of the Proximal Colon

Changes in nomenclature and in the criteria for histological diagnosis of the serrated lesions have occurred over the years. Some of these lesions, the sessile serrated lesions (SSLs), progress to adenocarcinoma via suppression of the MLH1 gene, an important carcinogenic pathway.

Evaluate the frequency of reclassification in histological diagnosis from hyperplastic polyps (HPs) to SSL after reappraisal using the 2019 World Health Organization (WHO) classification, to determine the occurrence of previously undiagnosed dysplasia and to study the expression of the MLH1 protein in SSLs and SSLDs.

Lesions with histological diagnosis of HP, SSL, and SSLD resected by colonoscopies performed between 2005 and 2015 located in the proximal colon were studied. All HPs were submitted for histological review by two pathologists (Examiners 1 and 2), and a third experienced pathologist (Examiner 3) made the final decision when the other examiners did not agree. Interobserver agreement was analyzed. MLH1 protein expression was assessed by immunohistochemistry in lesions diagnosed as SSL and SSLD before and after reappraisal. These lesions were reviewed again for missed dysplasia.

A total of 308 lesions were assessed being 287 with the initial diagnosis of HP and 21 SSL. Thirty-eight (13.3%) lesions with an initial diagnosis of HP had their diagnosis reclassified to SSL. No dysplasia was found. There was a moderate agreement (Kappa 0.52) between Examiners 1 and 2 regarding the diagnosis of SSL. Between Examiners 1 and 3, there was no agreement (Kappa -0.19), and between Examiners 3 and 2, the agreement was poor (Kappa 0.13). All 38 lesions analyzed by immunohistochemistry had MLH1 expression.

Changes in diagnosis from HP to SSL occurred in 13.3%. No dysplasia or lack of MLH1 expression was observed.