Proficiency Testing of p53 Immunohistochemistry Pattern Read-out in Vulvar Biopsies Demonstrates Frequent Basal Overexpression Interpretation in TP53 Wild-type Cases

Recently, criteria for p53 immunohistochemistry (IHC) interpretation were described in squamous neoplasia of the vulva. This pattern-based approach detailed 2 wild-type patterns (scattered and basal-sparing) and 4 mutant patterns (parabasal/diffuse overexpression, basal overexpression, null, and cytoplasmic). However, the proficiency of pathologist read-out has not been studied. We created an online tool to evaluate p53 IHC interpretation proficiency. p53 IHC on 90 vulvar biopsies (n=31 squamous in situ/premalignant and n=59 benign lesions) were scanned (without corresponding H&E). Fifteen pathologists assessed 45 cases in Module A and assigned each case as wild-type or mutant via the 6 p53 IHC patterns. Following Module A, participants were given the suggested p53 IHC pattern and TP53 sequencing data for each case. After self-review, pathologists completed a second 45 case set (Module B). The average pathologist score per case increased from Module A to Module B (69.8%–87.7%, P=0.0005). Pathologist proficiency was excellent in the parabasal/diffuse (100%–100%), null (93.3%–90.0%), and basal-sparing (88.9%–100%) patterns. The greatest discrepancy was due to the interpretation of the basal overexpression pattern in cases that were TP53 wild-type by sequencing, but this improved with educational intervention. Scores for the scattered pattern improved from 64.9% to 82.8% and basal overexpression from 73.3% to 91.1% after completion of the training module. Pathologists should exhibit caution when interpreting p53 IHC as basal overexpression, as this pattern can be seen in the absence of TP53 alterations. There were 2 cases with convincing p53 IHC abnormal patterns (1 parabasal/diffuse and 1 null) without TP53 mutations by sequencing.