BRAF V600E immunohistochemistry is reliable in primary and metastatic colorectal carcinoma regardless of treatment status and shows high intratumoral homogeneity.

In colorectal carcinoma the evaluation of BRAF mutation status is increasingly being performed given its utility as a prognostic and predictive biomarker. However, there are conflicting reports of the sensitivity and specificity of BRAF V600E immunohistochemistry (IHC), and little is known about its reliability in tissues collected from metastatic sites or after chemotherapy, radiation therapy and/or targeted therapy. The degree of intratumoral staining heterogeneity is also not well established. We performed IHC for BRAF V600E (VE1) on 204 cases of colorectal carcinoma including 59 with the BRAF V600E mutation. These included primary (n=147) and metastatic/recurrent (n=57) tumors, collected before (n=133) or after (n=71) chemotherapy, radiation therapy and/or targeted therapy. Evaluation of a test cohort (39 cases) with knowledge of mutation status established a specific staining pattern for the mutation: diffuse cytoplasmic staining of near-uniform intensity, regardless of strength of staining. Using this pattern, pathologists at 3 levels of training independently performed blinded evaluation of the remaining cases. BRAF V600E staining was 96.3% sensitive and 98.5% specific for the mutation, including both pretreatment and posttreatment specimens. Fleiss κ for interobserver agreement was 0.96. Staining of whole sections of the BRAF mutants showed diffuse staining in all cases and uniform or near-uniform intensity in 91%. In 20 cases with both pretreatment and posttreatment specimens, there was 100% accuracy and agreement in staining between samples. We conclude that BRAF V600E IHC is reliable for the evaluation of mutational status in colorectal carcinoma regardless of site or prior treatment history, and staining shows a high degree of intratumoral homogeneity.