Purpose: Perineural invasion (PN) diagnosed by hematoxylin-eosin (HE) staining is an important prognostic factor after curative-intent surgery in patients with colorectal cancer. However, the clinical significance of PN diagnosed by immunohistochemistry (IHC) has not been investigated. The present study assessed the clinical significance of PN diagnosed by IHC with an anti-S100 antibody in patients with colorectal cancer. Methods: We retrospectively enrolled 184 consecutive patients with stage I-III colorectal cancer who had undergone curative-intent surgery. We analyzed the absence/presence of PN diagnosed by HE staining (HE-PN) compared to that diagnosed by IHC with the anti-S100 antibody (S100-PN). Potential prognostic factors were identified by univariate and multivariate analyses of the overall and relapse-free survival. The κ statistics were used to assess the inter-observer reproducibility. Results: The incidence of HE-PN and S100-PN among the 184 patients was 60 patients (32.6 %) and 113 patients (61.4 %), respectively (P < 0.001). A multivariate Cox proportional hazards regression model analysis indicated that S100-PN was an independent prognostic factor for both the overall and relapse-free survival. The κ value was 0.77 for S100-PN and 0.47 for HE-PN. Conclusion: PN diagnosed by IHC is an important prognostic factor in patients with colorectal cancer. An inter-observer assessment showed superior judgment reproducibility for S100-PN compared with HE-PN.
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