Pathological extranodal extension in head and neck cancer: A prognostic biomarker with therapeutic ramifications and diagnostic pitfalls.

Extranodal extension (ENE), defined as the pathological spread of metastatic tumor cells beyond the lymph node capsule into adjacent soft tissues, represents a critical prognostic biomarker in head and neck squamous cell carcinoma (HNSCC). Its presence correlates with aggressive tumor biology, increased risk of locoregional recurrence, distant metastasis, and reduced survival, thereby influencing staging systems and therapeutic strategies. The 8th edition of the AJCC Cancer Staging Manual has formally incorporated ENE into nodal classification for select HNSCC subsites, with the 9th edition of UICC (2025) extending this to HPV-positive oropharyngeal carcinomas. Despite its prognostic significance, diagnostic evaluation of ENE-particularly microscopic ENE-remains challenging due to interobserver variability, ambiguous histological thresholds, and limitations of imaging modalities. The emergence of artificial intelligence and radiomics offers promising tools for improving ENE detection. Moreover, recent studies underscore the importance of quantifying ENE extent, with macroscopic ENE (>2 mm) portending a worse prognosis and guiding the need for adjuvant chemoradiotherapy. However, controversies persist, especially regarding its role in HPV-positive tumors and the utility of traditional cutoffs. This review synthesizes current evidence on the biological underpinnings, diagnostic complexities, therapeutic implications, and future research directions related to ENE in HNSCC, advocating for standardized assessment protocols to optimize patient management and outcomes.