HPV Genotyping and p16 Expression in Biopsy Specimens from Patients with Anal Squamous Cell Carcinoma: A Review

Abstract

Almost every anal squamous cell carcinoma is caused by human papillomavirus, and p16 immunohistochemistry is used as a cheap surrogate for detecting it. The two agree most of the time. The clinically interesting patients are the ones where they do not.
Objectives: To review the published evidence on HPV genotyping and p16 expression in anal squamous cell carcinoma: how often each is positive, how well p16 substitutes for direct viral detection, what each predicts for outcome, and what happens to patients whose two results disagree. Material and Methods: Narrative review of genotyping series, immunohistochemical cohorts, molecular characterisation studies and meta-analyses indexed in PubMed and the major oncology and pathology journals. Studies were eligible if they reported HPV or p16 prevalence in a defined anal carcinoma population, the concordance between the two, or outcomes stratified by either. No new patient data were generated; every estimate quoted is one published by the original investigators. Results: Across the studies pooled in a meta-analysis of 398 cases, HPV prevalence ranged from 67.9 to 95.8 per cent and p16 positivity from 65.3 to 90.7 per cent. HPV16 dominates the genotype distribution, accounting for 78.9 per cent of cases as a single infection in one series of 95 tumours, with a further 9.5 per cent carrying HPV16 alongside a second type. Against direct viral detection in 116 tumours, p16 immunohistochemistry showed 100 per cent sensitivity and 85 per cent specificity. Tumours positive for both markers had markedly better outcomes, with a pooled hazard ratio of 0.30 (95% CI 0.17-0.51) for overall survival and 0.23 (0.14-0.36) for disease-free survival, while patients discordant in either direction did not share that advantage. Conclusion: p16 is a highly sensitive surrogate and an imperfect specific one, so a positive result does not establish viral causation. The prognostic benefit belongs to the doubly positive group rather than to either marker alone, which is an argument for testing both rather than substituting one for the other.
Abbreviations: ASCC – Anal Squamous Cell Carcinoma, HPV – Human Papillomavirus, p16 – p16INK4a, IHC – Immunohistochemistry, PCR – Polymerase Chain Reaction, CISH – Chromogenic In Situ Hybridisation, FFPE – Formalin-Fixed Paraffin-Embedded, CRT – Chemoradiotherapy, OS – Overall Survival, DFS – Disease-Free Survival, LRR – Locoregional Recurrence, HR – Hazard Ratio, pRb – Retinoblastoma Protein.