347 Quantitative classification and radiomics of [18F]FDG-PET/CT 5 Radiomic classifier Radiomic analysis was performed in Python (version 3.6.10) and R (version 3.6.0; R Foundation for Statistical Computing, Vienna, Austria). In Python, an elastic net regression classifier was trained and evaluated in a 20-times repeated random split, in which the dataset was split in 80% training and 20% test data. Since the number of extracted features exceeded the number of patients in the dataset, dimensionality reduction incorporating redundancy filtering and factor analysis of radiomic features was performed for each split on the training set using FMradio (Factor Modelling for Radiomics Data) R-package (version 1.1.1) [515]. One factor was selected for every ten subjects in the training set (Details provided in Supplementary data, Table 2) [506]. Factors for the training and test set were calculated. The factors of the training set were used as input for the elastic net regression classifier. The predictive performance of the model is expressed as the mean AUC of the ROC curve over the 20 splits for the test sets. The 95% CIs were constructed using a corrected resampled t-test [516]. Classification models were trained on PET features and subsequently on PET and ldCT features. The definitions of the factors in the model were determined based on the underlying clusters of features in the different folds. Subgroup analysis was performed for nodules meeting the minimal size recommendation for radiomic analysis of 64 voxels per VOI [517]. The TRIPOD statement (transparent reporting of multivariable prediction model for individual prognosis or diagnosis, version 1 October 2020) was used (IBSI reporting guidelines, Supplementary data, Table 2) [518]. Results Patients The current study included 123 patients between 1 July 2015 and 16 October 2018 (Figure 1, Table 1). Cytology was AUS/FLUS in 55 (45%), FN/SFN in 39 (32%) and HCN/SHCN in 29 (24%) patients. One hundred (81%) patients underwent diagnostic surgery and 23 (19%) underwent active surveillance, including 20 (16%) with visually [18F]FDG-negative nodules. To date (29 September 2021), the median follow-up is 29 months (IQR 24-45) and all nodules have remained unchanged on ultrasound: they are considered benign. All patients completed all study-related procedures. No patients were lost to follow-up.
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