Background Prognostic significance of posterior cervical lymph node metastasis in nasopharyngeal cancer is largely unknown. This study aims to determine the added prognostic significance of cervical lymph node group V to the standard American Joint Committee on Cancer (AJCC) staging system (eighth edition AJCC) of nasopharyngeal patients with cancer treated with intensity‐modulated radiation therapy (IMRT) in terms of overall survival (OS), distant metastatic‐free survival (DMFS), and disease‐free survival (DFS). Methods and Materials A retrospective cohort of 199 consecutively diagnosed nasopharyngeal patients with cancer treated with definitive radiotherapy (RT) or concurrent chemoradiotherapy (CCRT) in the era of IMRT in a large university hospital in endemic area of Southeast Asia. Pre‐treatment imaging studies were thoroughly re‐evaluated and re‐staged by a board‐certified radiologist using radiographic criteria for cervical lymph node metastasis. T and N classifications were reclassified according to the eighth AJCC staging system. Group V (Va and Vb) cervical node was evaluated for its added prognostic significance. Cox's proportional hazard model was used to retrieve hazard ratio (HR), 95% confidence interval and P value for N classification. Harrell's C‐statistic (concordance index) was used for test of discrimination and internal validation was calculated by bootstrap method. Results This study demonstrated greater separation of OS with HR of 6.75 (95%CI 1.94‐23.51, P = .003) by using group Vb only as N3 compared to HR of 4.70 (95%CI 1.37‐16.13, P = .014) by using current standard N3 disease (groups IV and Vb). Similarly, N2 with presence of Va shows worsened DFS with HR of 8.70 (95%CI 1.08‐69.67, P = .042) compared to N2 without Va with HR of 5.93 (95%CI 0.76‐46.00, P = .089). After incorporating cervical group V into nodal staging, the HR and 95%CI among each group was better separated than the eighth AJCC staging system but without significant improvement in C‐index. Conclusion Cervical lymph node group V is a potentially added prognostic factor to standard TNM staging.

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