Importance of the Number and Location of Lymph Node Metastasis in Oropharyngeal Cancer

The 8^th edition of the American Joint Committee on Cancer staging system classifies oropharyngeal cancer (OPC) by the expression of p16. The discrepancy observed in this system between pathological and clinical N-stages in p16-positive OPC has provoked controversy. Therefore, this study investigated prognostic factors not included in the new staging system for p16-positive OPC patients.

Patients with non-metastatic OPC receiving radiotherapy were reviewed. Clinical lymph node statuses were reassessed based on contrast-enhanced computed tomography and fluorodeoxyglucose positron emission tomography. Overall survival (OS) and cause-specific survival (CSS) were analyzed using multivariate analyses to adjust baseline imbalances.

In total, 166 OPC patients were reviewed. Among them, 81 patients with p16-positive were analyzed. Three or more lymph node metastases (LNM) were observed in 21 p16-positive OPCs. Retropharyngeal lymph node metastasis (Rp) was found in 12. Three-year OS, CSS, and progression-free survival rates in p16-positive patients were 76, 88, and 81%, respectively. In multivariate analyses of p16-positive OPC, LNM ≥3 was a prognostic factor of OS (hazard ratio=9.30, p<0.001) and CSS (hazard ratio=17.80, p=0.005). Rp was associated with poor CSS (hazard ratio=8.73, p=0.03). In N0-1 p16-positive patients, LNM ≥3 trended to be associated with poor OS (hazard ratio=3.93, p=0.06). CSS in patients with Rp was unfavorable (hazard ratio=70.16, p=0.05).

LNM ≥3 and Rp may be predictive of OS and CCS in p16-positive OPC. These are also possibly used to subcategorize p16-positive cN0-1 OPC. Further validation of lymph node staging is needed to refine the clinical staging system.