목적: Extra-nodal extension (ENE) indicates a known poor prognostic factor in head and neck squamous cell carcinoma (HNSCC), its pre- treatment detection leads to proper treatment planning. We therefore investigated whether the ENE of HNSCC is accurately detected by CT versus ¹⁸F-FDG PET/CT and significantly impacts patient prognosis. 방법:This prospective study was approved by the Institutional Review Board and written informed consent was obtained from all enrolled patients. In total, 186 patients with HNSCC were preoperatively evaluated with contrast-enhanced CT and ¹⁸F-FDG PET/CT. Histopathological results of neck dissection samples were served as the gold standard. McNemar's test and logistic regression using generalized estimating equations was utilized to compare the diagnostic value of CT and ¹⁸F-FDG PET/CT and Kaplan-Meier methods were used for assessing the prognostic value of ENE. 결과:Of the 186 patients, 113 (60.8%) had neck metastasis in 131 neck sides and 211 levels of the neck, and 42 (22.6%) had ENE in 46 (17.5%) neck sides. Radiologic ENE on CT was documented in 48 (25.8%) patients and 52 (19.8%) neck sides. Using the cut-off value of maximum standardized uptake value at 5.7, ¹⁸F-FDG PET/CT documented radiological ENE in 44 (23.7%) patients and 48 (18.3%) neck sides. There were no significant difference in sensitivity, specificity and accuracy between CT and ¹⁸F-FDG PET/CT for detection of pathological ENE and macroscopic ENE (P>0.05). CT- or ¹⁸F-FDG PET/CT-defined ENE was not the independent predictors of poor disease-free and cancer-specific survivals (P> 0.05). 결론:The ENE of HNSCC can be detected by pretreatment contrast-enhanced CT or ¹⁸F-FDG PET/CT with acceptable sensitivity and accuracy but they were not significant prognostic indicators.