Invasive-Front P21 Expression Is Associated With Tumor Aggressiveness in Head and Neck Squamous Cell Carcinoma

Background/Aim: P21 is a key cyclin-dependent kinase inhibitor linked to cellular senescence. However, its spatial expression patterns within tumor compartments and their relationship with tumor aggressiveness remain poorly characterized in head and neck squamous cell carcinoma (HNSCC). This study investigated the spatial distribution of P21 across distinct tumor regions and evaluated its association with tumor aggressiveness, including stage and adverse pathological features, in HNSCC. Materials and Methods: Formalin-fixed, paraffin-embedded tissues from patients with HNSCC were assessed using immunohistochemistry for P21. The intensity, percent positivity, and composite scores were quantified in the surface, center, and invasive-front regions. All variables were transformed using log (x+1), and spatial ratios comparing surface-to-center and invasive-front-to-center expressions were calculated. Non-parametric analyses were performed to compare early- and advanced-stage HNSCC, with additional subgroup analyses according to extracapsular extension (ECE), lymphovascular invasion (LVI), perineural invasion (PNI), and tumor subsites. Results: Advanced-stage HNSCC demonstrated significantly higher invasive-front-to-center log ratios for both percent positivity and composite P21 scores (p=0.0275), indicating relative enrichment of P21 expression at the invasive-front. Among advanced-stage HNSCC, PNI-positive cases showed the most pronounced increases in both surface-to-center and invasive-front-to-center spatial ratios (p<0.05), whereas no significant spatial differences were observed for ECE or LVI. Conclusion: P21 expression demonstrated region-specific differences associated with tumor aggressiveness, with PNI-positive tumors showing the clearest enrichment of P21 expression at the invasive-front. These findings suggest that spatial, rather than absolute, P21 expression patterns may reflect progression-related phenotypes in HNSCC. Spatial profiling of P21 may provide exploratory insights into tumor biology and could help identify tumors with a higher invasive potential.