Partial volume correction for equivocal retropharyngeal nodal metastases of nasopharyngeal carcinoma with fluorodeoxyglucose positron emission tomography-computed tomography

The objective of this study is to determine the size range where the recovery coefficient (RC) method of fluorodeoxyglucose (FDG) positron emission tomography- computed tomography (PET-CT) is helpful in detecting lateral retropharyngeal lymph (LRPL) nodal metastases of nasopharyngeal carcinoma (NPC) patients previously treated with radiation therapy. A total of 142 LRPL nodes assessed by magnetic resonance imaging (MRI) in 71 NPC patients were chosen for investigation. LRPL nodes with central necrosis, extracapsular invasion, or asymmetric grouping or those ascertained on follow-up MRI scans were considered positive for metastases. The criterion for positive diagnosis of nodal metastasis on FDG PET-CT scans was defined as maximal standard uptake value (SUV_max) ≥ 2.5. Nodes not separated from main tumors were excluded. An established RC method, the sphere-tobackground ratio, was employed. Nodes were further categorized into three groups of minimal axial diameters: below 6, 6-7 mm, and above 7 mm. A total of 88 nodes were examined by FDG PET. Thirty-five nodes were positive and 53 nodes were negative. The RC method significantly improved sensitivity (from 20 to 100 %) and accuracy (from 14 to 71 %) for nodes sized 6-7 mm. In LRPL nodes above 7 mm, the RC method also provided slight improvement, with sensitivity and accuracy both increasing from 92 to 96 %. However, the nodal sizes below 6 mm were too small for valid comparisons. In conclusion, partial volume correction in FDG PET-CT enhances the accuracy of detecting nodes in the equivocal size range of 6-7 mm for LRPL nodal metastases of NPC.