BACKGROUND. Retropharyngeal lymph node (RLN) metastases have profoundprognostic implications in patients with nasopharyngeal carcinoma (NPC). However,the AJCC staging system does not specify a size threshold for determining RLN involve-ment, resulting in inconsistent thresholds in practice.OBJECTIVE. The purpose of this article was to determine the optimal size thresholdfor determining the presence of metastatic RLNs on MRI in patients with NPC, in termsof outcome predictions.METHODS. This retrospective study included 1752 patients (median age, 46 years;1297 men, 455 women) with NPC treated by intensity-modulated radiotherapy (RT)from January 2010 to March 2014 from two hospitals; 438 patients underwent MRI 3–4months after treatment. Two radiologists measured the minimal axial diameter (MAD)of the largest RLN for each patient using a consensus process. A third radiologist mea-sured MAD in 260 randomly selected patients to assess interobserver agreement. Ini-tial ROC and restricted cubic spline (RCS) analyses were used to derive an optimal MADthreshold for predicting progression-free survival (PFS). The threshold’s predictive util-ity was assessed in multivariable Cox regression analyses, controlling for standard clin-ical predictors. The threshold’s utility for predicting PFS and overall survival (OS) wascompared with a 5-mm threshold using Kaplan-Meier curves and log-rank tests.RESULTS. The intraclass correlation coefficient for MAD was 0.943. ROC and RCSanalyses yielded an optimal threshold of 6 mm. In multivariable analyses, MAD of 6 mmand greater independently predicted PFS in all patients (HR = 1.35, p = .02), patients withN0 or N1 disease (HR = 1.80, p = .008), and patients who underwent posttreatment MRI(HR = 1.68, p = .04). In patients with N1 disease without cervical lymph node involve-ment, 5-year PFS was worse for MAD greater than or equal to 6 mm than for MAD thatwas greater than or equal to 5 mm but less than 6 mm (77.2% vs 89.7%, p = .03). OS wassignificantly different in patients with stage I and stage II disease defined using a 6-mmthreshold (p = .04), but not using a 5-mm threshold (p = .09). The 5-year PFS rate was as-sociated with a post-RT MAD of 6 mm and greater (HR = 1.68, p = .04) but not a post-RTMAD greater than or equal to 5 mm (HR = 1.09, p = .71).CONCLUSION. The findings support a threshold MAD of 6 mm for determining RLNinvolvement in patients with NPC.CLINICAL IMPACT. Future AJCC staging updates should consider incorporation ofthe 6-mm threshold for N-category and tumor-stage determinations.