Performance of the Pattern Based Interpretation of p53 Immunohistochemistry as a Surrogate for TP53 Mutations in Vulvar Squamous Cell Carcinoma.

INTRODUCTION: The most commonly mutated gene in vulvar squamous cell carcinoma (VSCC) is TP53 and its prognostic value, particularly in HPV-independent VSCC, is uncertain. In other tumors, p53 immunohistochemistry (IHC) is an excellent surrogate marker for TP53 mutations. In order to study this in VSCC, we assigned six p53-IHC patterns into two final classes: 'wildtype' or 'mutant'. We determined the performance and interobserver variability of this pattern-based p53-IHC approach. METHODS: Two experienced gynecologic pathologists scored the predefined p53-IHC patterns of 59 VSCC, independently and blinded for molecular data. Agreement was calculated by Cohen's kappa. All disagreements regarding p53-IHC patterns were resolved by a consensus meeting. After DNA isolation, the presence of pathogenic TP53 variants were determined by next-generation sequencing (NGS). Sensitivity, specificity, and accuracy of p53-IHC as a surrogate marker for TP53 mutation status were calculated. RESULTS: Initial p53-IHC pattern interpretation showed substantial agreement between both observers (k=0.71, p<0.001). After consensus, 18 cases (30.5%) were assigned a final p53-IHC class as TP53 wildtype and 41 cases (69.5%) as 'mutant'. The accuracy between the p53-IHC class and TP53 mutation status, after the consensus meeting, was 96.6%. Moreover, the sensitivity and specificity were high 95.3% (95%CI 82.9-99.1%) and 100% (95%CI 75.9-100%)). CONCLUSIONS: Pattern-based p53-IHC classification is highly reproducible amongst experienced gynecologic pathologists and accurately reflects TP53 mutations in VSCC. This approach to p53-IHC interpretation offers guidance and provides necessary clarity for resolving the proposed prognostic relevance of final p53-IHC class within HPV-independent VSCC.