Interpretation of p16 and p53 in the Classification of Squamous Cell Carcinoma of the Vulva—An Interobserver Agreement Study

Squamous cell carcinoma of the vulva (vSCC) is currently categorized either as human papillomavirus (HPV) associated or independent. Immunohistochemical stains, p16 INK4a (p16) and p53 are helpful biomarkers to support the designation of vSCC into 1 of the 3 tumor pathways: (1) HPV-associated, (2) HPV-independent, TP53 mutant, or (3) HPV-independent, TP53 wild type. Recently, a framework of p53 expression patterns in vSCC was proposed. In this international and multi-institutional study, we evaluated the interrater agreement for p53 and p16 and tumor pathway classification in a cohort of 50 invasive vSCC across a variety of practice settings (private practice, academic medicine) and levels of expertise (trainees, gynecologic pathologists, dermatopathologists, private practice pathologists). Our study shows that the overall interrater agreement for the interpretation of p16 in vSCC is strong to near perfect, while the agreement for p53 and tumor pathway assignment is overall moderate. Interrater agreement for p53 and tumor pathway is higher (strong) in the academic practice setting. Pathologists without gynecologic subspecialty expertise benefited the most from a brief educational module, which fostered a better understanding and improved comfort level with the p16/p53 stain interpretation and tumor pathway designation in the diagnosis of vSCC. Some interpretative challenges remain, particularly in regard to select p53 patterns and high-risk HPV-in situ hybridization utilization, warranting additional research.