BRAFV600E immunohistochemistry can reliably substitute BRAF molecular testing in the Lynch syndrome screening algorithm in colorectal cancer

Aims: The Lynch syndrome (LS) screening algorithm requires BRAF testing as a fundamental step to distinguish sporadic from LS-associated colorectal carcinomas (CRC). BRAF testing by immunohistochemistry (IHC) has shown variable results in the literature. Our aim was to analyse concordance between BRAFV600EIHC and BRAF molecular analysis in a large, mono-institutional CRC whole-slide, case series with laboratory validation. Methods and results: MisMatch repair (MMR) protein (hMLH1, hPMS2, hMSH2, and hMSH6) and BRAFV600EIHC were performed on all unselected cases of surgically resected CRCs (2018–2023). An in-house validation study for BRAFV600EIHC was performed in order to obtain optimal IHC stains. BRAFVV600EIHC was considered negative (score 0), positive (scores 2–3), and equivocal (score 1). Interobserver differences in BRAFV600EIHC scoring were noted in the first 150 cases prospectively collected. Nine-hundred and ninety CRCs cases (830 proficient (p)MMR/160 deficient (d)MMR) were included and all cases performed BRAFV600EIHC (BRAFV600EIHC-positive 13.5% of all series; 66.3% dMMR cases; 3.4% pMMR cases), while 333 also went to BRAF mutation analysis. Optimal agreement in IHC scoring between pathologists (P < 0.0001) was seen; concordance between BRAFV600EIHC and BRAF molecular analysis was extremely high (sensitivity 99.1%, specificity 99.5%; PPV 99.1%, and NPV 99.5%). Discordant cases were reevaluated; 1 score 3 + IHC/wildtype case was an interpretation error and one score 0 IHC/mutated case was related to heterogenous BRAFV600EIHC expression. Among the 12 IHC-equivocal score 1+ cases (which require BRAF molecular analysis), three were BRAF-mutated and nine BRAF-wildtype. Conclusion: BRAFV600EIHC can be used as a reliable surrogate of molecular testing after stringent in-house validation.