Clinical value of 18F-FDG PET/CT for detecting residual metastatic lymph nodes of DTC before initial radioiodine therapy

To evaluate the adjunctive value of 18 F-FDG PET/CT for assessing residual metastatic lymph nodes (MLN) before initial radioiodine therapy in postoperative DTC patients with high clinical suspicion of residual nodal disease. This retrospective study included 835 postoperative patients with DTC who underwent 18 F-FDG PET/CT before initial 131 I therapy because of elevated stimulated thyroglobulin (sTg) and/or suspicious conventional neck imaging. MLN status was determined using a composite reference standard. Patient-level logistic regression was used to evaluate associated factors, and receiver operating characteristic (ROC) analysis was performed to assess discrimination and determine optimal cutoffs. A sensitivity analysis reassessed diagnostic performance using a reference assessment that excluded PET/CT information. Of the 835 patients, 452 (54.1%) were classified as having residual MLN. Under the composite reference standard, PET/CT showed a sensitivity of 84.5% and a specificity of 73.4%. After PET/CT information was excluded from the reference assessment, 104 patients were classified as indeterminate; among the remaining 731 patients, sensitivity remained similar at 85.9%, whereas specificity decreased to 40.1%. PET/CT contributed to N-stage revision in 149/835 (17.8%) patients, including a joint contribution with conventional imaging in 129 patients and an independent incremental contribution in 20 patients. Among patients with residual MLN, 275/452 (60.8%) exhibited an FDG-avid/ 131 I-nonavid pattern. In TgAb-negative patients, multivariable analysis showed that nodal maximum standardized uptake value (SUV max ) and sTg were independently associated with residual MLN (both P < 0.001). The corresponding ROC-derived cutoffs were 4.6 and 14.58 µg/L, respectively. In postoperative DTC patients with a high clinical suspicion of residual nodal disease, 18 F-FDG PET/CT may complement conventional imaging and biochemical assessment in identifying residual MLN and refining N-stage assessment.