BRAF V600E immunohistochemistry is reliable in primary and metastaticcolorectal carcinoma regardless of treatment status and shows high intratumoralhomogeneity

摘要

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, and little is known about its reliability in tissues collected from metastatic sites or following chemo/radiation or targeted therapy. The degree of intratumoral staining heterogeneity is also not well established. We performed immunohistochemistry 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) chemo/radiation 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 three 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 pre- and post-treatment specimens. Fleiss’kappa for interobserver agreement was 0.96. Staining of whole sections of theBRAF mutants showed diffuse staining in all cases anduniform or near-uniform intensity in 91%. In 20 cases with both pre- andpost-treatment specimens there was 100% accuracy and agreement instaining between samples. We conclude that BRAF V600E immunohistochemistry isreliable for the evaluation of mutational status in colorectal carcinomaregardless of site or prior treatment history, and staining shows a high degreeof intratumoral homogeneity.