FDG-PET zur Lymphknoten-Diagnostik des Lungenkarzinoms: Welche SUV-Schwelle ist sinnvoll?

Abstract

AIM: 18 F-FDG PET is the most accurate noninvasive modality for staging mediastinal lymph nodes in lung cancer. Beside using visual image interpretation, some institutions use standardized uptake value (SUV) measurements in lymph nodes. Mostly, an SUV of 2.5 is used as the cut-off, but this choice was never deduced from respective studies. Receiver operating characteristic (ROC) analyses demonstrated that SUV thresholds of more than 4 resulted in the highest accuracy. But these high cutoffs imply high false-negative rates (FNRs). The aim of our evaluation was to determine an optimal SUV threshold and to compare its diagnostic performance with the results of visual interpretation. METHODS: This retrospective study included 95 patients with suspected lung cancer who underwent mediastinoscopy / mediastinal lymphadenectomy after 18 F-FDG PET imaging (90-150 min after 250 MBq of 18 F-FDG). Maximum SUV was measured in 371 LN regions biopsied afterwards and visually interpreted using a 6-level score (--- through +++). Diagnostic performance was assessed by ROC analysis. FNR and false-positive rate (FPR), the sum of both error rates (FNR + FPR), and diagnostic accuracy were plotted against a hypothetical SUV threshold to determine the optimum SUV threshold. RESULTS: SUVs in metastatic lymph nodes were higher (mean ± SD, 7.1 ± 4.5; range, 1.4-26.9; n = 70) than in tumor-free lymph node stations (2.4 ± 1.7; range, 0.6-14.9; n = 301; p < 0.01). Inflammatory lymph nodes exhibited slightly increased SUVs (2.7 ± 2.0; range, 0.8-14.9; n = 146). The plot of error rates featured a minimum of the sum FNR + FPR for an SUV of 2.5. With increasing SUV threshold, the FPR decreased most prominently up to that value whereas a continuous rise of FNR was noticed. Highest diagnostic accuracy was achieved with an SUV of 4.5. The areas under the ROC curves demonstrated that visual interpretation tends to be more accurate than SUV quantification (visual, 0.930 ± 0.022; SUV, 0.899 ± 0.025; p = 0.241). Using an SUV of 2.5 as the threshold, the resulting sensitivity, specificity, and negative predictive value were 89, 84, and 96 %, respectively. CONCLUSIONS: For mediastinal staging, the choice of an SUV of 2.5 as the threshold is justified because FNR + FPR is minimized. The resulting high negative predictive value of 96 % allows the omission of mediastinoscopy in patients with negative mediastinal findings on 18 F-FDG PET images. For the experienced observer, visual analysis should be relied on primarily, with calculation of the SUV used, at most, as a secondary aid. For the less experienced observer, the SUV may be of greater value.