355 Quantitative classification and radiomics of [18F]FDG-PET/CT 5 corresponded to entropy of the intensity histogram, nodule size, high intensity on PET, variance in area size, total lesions glycolysis and small areas with low grey levels. KMOs in all folds were excellent (≥0.927). Subgroup analyses in non-Hürthle cell and Hürthle cell nodules resulted in test set AUCs for the PET models of 0.519 and 0.694, respectively (Table 6). The performances of the PET/ CT models were similar, indicating limited diagnostic accuracy (Table 6, Figure 5). Subgroup analysis for nodules meeting the minimal size recommendation of 64 voxels per VOI (n=66) demonstrated a similar performance with an AUC of 0.421 for the PET model (Table 6) [517]. Discussion The EfFECTS trial showed that visual assessment of [18F]FDG PET/CT had a practice-changing rule-out ability in Bethesda III/IV nodules. In non-Hürthle cell nodules, [18F]FDG-PET/CT-driven management accurately avoided nearly half of the futile diagnostic surgeries for benign nodules. It was not contributing, however, in the nearly exclusively strongly [18F]FDG-positive Hürthle cell nodules [501]. In the current side study of this randomised controlled trial, we showed that quantitative [18F]FDGPET/CT analysis accurately ruled-out malignancy in both non-Hürthle and Hürthle cell nodules, provided that different SUV cut-offs were chosen for these two groups. In Hürthle cell nodules, relatively high SUV cut-offs resulted in an excellent sensitivity and benign call rates up to 24%. Consequently, a maximum 35% (7 of 20) of diagnostic surgeries could have been avoided for benign Hürthle cell nodules in the current cohort. Even though the reported SUV cut-offs require external validation in future prospective studies prior to implementation in clinical practice, quantitative assessment thus appears to have a major advantage over visual assessment in Hürthle cell nodules. In non-Hürthle cell nodules, an excellent rule-out ability was demonstrated at relatively low SUV cut-offs with moderate (11%) to excellent (35%) benign call rates. As a result, a maximum of 46% (32 of 70) surgeries for benign nodules could have been avoided in AUS/FLUS and FN/SFN nodules. These results were similar to our previous findings regarding the visual interpretation of [18F]FDGPET/CT [501]. Unfortunately, specificity was similarly limited, too: many benign nodules were still considered false-positive on quantitative assessment. Additional subgroup analysis showed that quantitative [18F]FDG-PET/CT assessment lacked discriminative capacity in visually [18F]FDG-positive non-Hürthle cell nodules. In contrast to Hürthle cell nodules, there were no separate (higher) SUV cut-offs that contributed to a better differentiation for this subgroup. As such, quantitative [18F]FDGPET/CT assessment appeared to have no additional diagnostic value over visual assessment in nonHürthle cell nodules, and is likely best applied to support the visual assessment [501]. Radiomic analysis on PET/CT did not contribute to the differentiation of [18F]FDG-positive non-Hürthle cell and/or Hürthle cell nodules, with AUCs ranging from 0.445 to 0.694. Based on the results of our previous and the current study, we suggest a diagnostic algorithm for the [18F]FDG-PET/CT-driven workup of Bethesda III and IV thyroid nodules (Figure 6). If externally validated, this workup could
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