BRAF V600E immunohistochemistry demonstrates that some sessile serrated lesions with adenomatous dysplasia may represent collision lesions

Aims Sessile serrated lesions ( SSL ) with dysplasia are uncommon polyps with a high risk of rapid malignant transformation. Most of these lesions have a BRAF mutation and 75% show loss of MLH 1 expression in their dysplastic component. Different morphological patterns of dysplasia occurring in these polyps have recently been described. We hypothesised that a subset of SSL s with dysplasia mimicking the dysplasia seen in conventional adenoma (adenomatous dysplasia) may represent a collision lesion between an ordinary SSL and a conventional adenoma. Methods and results We selected 80 SSL s with dysplasia, including 19 with adenomatous dysplasia, 18 with serrated dysplasia and 43 with dysplasia not otherwise specified ( NOS ). BRAF mutation analysis was performed using molecular testing (allelic discrimination) and the mutation‐specific BRAF ‐V600E immunohistochemistry (clone VE 1). The overall BRAF ‐ V600E mutation rate was 84% in all lesions, 68% in SSL s with adenomatous dysplasia, 89% in SSL s with serrated dysplasia and 88% in SSL s with dysplasia NOS . From the 63 SSL s with dysplasia that were positive for the BRAF ‐ V600E mutation, a negative BRAF ‐V600E immunostaining was observed in the dysplastic component of 83% of SSL s with adenomatous dysplasia, 0% of SSL s with serrated dysplasia and 3% of SSL s with dysplasia NOS ( P  < 0.001). Conclusions These findings suggest that SSL s with adenomatous dysplasia may not represent advanced SSL s, but instead may be a collision between a non‐dysplastic SSL and a conventional adenoma.