Detection of BRAF p.V600E Mutations in Melanoma by Immunohistochemistry Has a Good Interobserver Reproducibility.

Context: Assessment of BRAF p.V600E mutational status has become necessary for treatment of patients with metastatic melanoma. Detection of p.V600E mutation by immunohistochemistry was recently reported in several tumor types.

Objective: To evaluate the interobserver reproducibility of BRAF p.V600E detection by immunohistochemistry in melanoma.

Design: Immunohistochemistry with VE1 antibody was performed on metastatic melanomas of 67 patients. Staining interpretation was performed on digital image virtual slides of tissue microarrays. The p.V600E status was determined by 7 pathologists from 3 European laboratories, blinded for other interpretations and for molecular biology results.

Results: Melanomas had p.V600E (n = 30), p.V600K (n = 4), p.K601E (n = 1), p.600-601delinsE (n = 1), or no p.V600 mutations (n = 31). Staining of p.V600E within mutated cells was cytoplasmic and diffuse, and for each case the staining on the 3 tissue microarray cores was similar. In 53 cases (79.1%) the 7 pathologists had perfect concordance. Agreement of interobserver reproducibility was almost perfect (κ = 0.81 [0.77-0.85]). Only 2 false-positive responses (0.9%) were obtained. The specificities reported were 100% for 5 pathologists (two of whom previously trained for p.V600E interpretation), and 97% for 2 untrained pathologists.

Conclusions: Detection of BRAF p.V600E mutation by immunohistochemistry in melanomas has an excellent interobserver reproducibility. Our results suggest that immunohistochemistry could be used as a first step for detection of BRAF p.V600E mutation, to identify patients with melanoma as candidates for BRAF inhibitors.