Diagnostic Value Of ¹⁸F-FDG PET/CT For Lymph Node Metastasis In Tg-positive ¹³¹I-Rx-WBS-negative DTC Patients

Objective:This study investigated the diagnostic efficacy of ¹⁸F-FDG PET/CT in Tg-positive ¹³¹I-Rx-WBS-negative DTC patients with lymph node metastases and compared it with neck ultrasound.In addition,this study evaluated the cut-off value of SUVmax for FDG uptake in metastatic lymph nodes of DTC patients.Methods:Retrospective analysis of patients admitted to our hospital from March 2017 to March 2021,who were all serum Tg positive（Tg>1μg/L）and Tg Ab negative（Tg Ab<115μg/L）after successful nail clearing treatment and negative ¹³¹I scans after systemic treatment was performed in a total of 26 patients,10 males and 16 females,aged（26-69）years old,with a mean（47.5±21.5）years old.All patients underwent lymph node surgery or lymph node aspiration biopsy,and the final pathology was used as the"gold standard"for diagnosis.The results were compared with the final pathology.The location of each lymph node was accurately marked and recorded intraoperatively,and corresponded to the location of the lymph nodes shown on the neck ultrasound and ¹⁸F-FDG PET/CT images.Results:1.A total of 85 pathologically confirmed lymph nodes were detected in 26 DTC patients with positive Tg and negative systemic ¹³¹I scans.The sensitivity、specificity、accuracy、positive predictive value and negative predictive value of neck ultrasound for the diagnosis of lymph node metastasis in patients with DTC were 77.36%（41/53）、71.88%（23/32）、75.29%（64/85）、82.00%（41/50）and 65.71%（23/35）,respectively.¹⁸F-FDG PET/CT imaging for the diagnosis of the sensitivity、specificity、accuracy、positive predictive value and negative predictive value of lymph node metastasis in patients with DTC were 92.45%（49/53）、81.25%（26/32）、88.24%（75/85）、89.09%（49/55）and 86.67%（26/30）,respectively.2.The best SUVmax cut-off value of FDG uptake in metastatic lymph nodes in patients with DTC diagnosed by ¹⁸F-FDG PET/CT was 1.985.Using SUVmax=1.985 as the cut-off value,the sensitivity、specificity、accuracy、positive predictive value and negative predictive value of¹⁸F-FDG PET/CT imaging for the diagnosis of lymph node metastasis in patients with DTC were 96.23%（51/53）、96.88%（31/32）、96.47%（82/85）、98.08%（51/52）and 93.94%（31/33）.The diagnostic efficacy of both visual analysis and semi-quantitative analysis for lymph node metastasis in patients with DTC was good,and the agreement with the clinical diagnosis.3.There were significant differences in the sensitivity and accuracy of ¹⁸F-FDG PET/CT and neck ultrasound for the diagnosis of lymph node metastasis in DTC patients with positive Tg and negative systemic ¹³¹I scan,and no significant difference in specificity.4.Among 26 DTC patients with positive Tg and negative whole-body¹³¹I scans,one patient was unexpectedly found to have nodal shadow uptake FDG in the lung by ¹⁸F-FDG PET/CT imaging,and the lung metastases from DTC were confirmed by puncture pathology.Conclusion:¹⁸F-FDG PET/CT has better diagnostic ability for lymph node metastasis in Tg-positive ¹³¹I-Rx-WBS-negative DTC patients,and the sensitivity、specificity、accuracy、positive predictive value and negative predictive value of ¹⁸F-FDG PET/CT imaging are better for diagnosing lymph node metastasis in DTC patients when SUVmax=1.985 is used as the cut-off value;and it can compensate for the shortage of neck ultrasound,detecting deep lymph nodes that cannot be detected by neck US,as well as early and timely detection of suspected distant metastases.