[Evaluation of CT, MRI and PET-CT in detecting retropharyngeal lymph node metastasis in nasopharyngeal carcinoma].

Background & Objective: At present, the detection of retropharyngeal lymph node (RLN) metastasis in nasopharyngeal carcinoma (NPC) mainly depends on imaging, but the diagnosis criteria are controversial. Recently, the 18F-fluoro-2-deoxy-D-glucose positron emission tomography ((18)FDG-PET-CT) has been widely applied in diagnosing cancer. This study was to evaluate CT, MRI, and (18)FDG-PET-CT in detecting RLN metastasis of NPC, and explore the imaging diagnosis criteria.

Methods: From Dec. 2003 to Aug. 2005, 53 consecutive patients with NPC treated in Cancer Center of Sun Yat-sen University were enrolled. Contrast enhanced CT, MRI, and (18)FDG-PET-CT were performed on bilateral RLNs of the patients within 10 days before treatment. The diagnosis criteria of RLN metastasis for CT and MRI included single node with maximum diameter of > or =4 mm, and the presence of central necrosis or ringed enhancement; the criteria for (18)FDG-PET-CT included abnormal accumulation of irradiation in the RLN with the standard uptake value (SUV) of > or =2.5. Considering the response of RLN to clinical treatment, the differences of CT, MRI, and PET-CT in detecting RLN metastasis were compared.

Results: Among the 106 intropharyngeal spaces of the 53 patients, the positive rate of RLN metastasis was significantly higher detected by CT and MRI than by PET-CT (39.6% and 45.3% vs. 20.8%, P<0.01), but the difference between CT and MRI was not significant (P>0.05). CT detected 27 cases of RLN metastasis with the maximum diameter of 0.4-0.9 cm, MRI detected 31 cases, and PET-CT only detected 5 cases. Especially, MRI detected 7 cases of RLN metastasis with the maximum diameter of < or =0.5 cm, CT detected 4 cases, but PET-CT detected none. CT detected 12 cases of RLN metastasis with the maximum diameter of > or =1.0 cm, MRI detected 14 cases, and PET-CT detected 13 cases. All patients were followed-up after treatment. One case of RLN metastasis with necrosis and the maximum diameter of 1.0 cm, and 1 case with the maximum diameter of 1.5 cm were eliminated within 6 months after treatment; the rest were eliminated within 3 months.

Conclusions: In detecting RLN metastasis in NPC, MRI has no obvious advantage over CT, while PET-CT is markedly inferior to CT and MRI, especially in detecting the cases with the maximum diameter of <1.0 cm. Furthermore, the metastatic RLNs with necrosis or the maximum diameter of < or =5 mm can't be detected by PET-CT. Setting the maximum diameter of > or =4 mm as a diagnosis criterion of RLN metastasis for CT or MRI is reasonable and feasible.