340 chapter 5 In addition to the traditional quantitative PET features such as the SUVmax, PET/CT images harbour an abundance of information inside the myriad of voxels that could be identified using radiomics [505]. In radiomics, large amounts of quantitative features are extracted from medical images, aiming to find stable and clinically relevant image-derived biomarkers that may provide new insights in tumour biology and guide patient management [506]. After a number of studies suggested that radiomic analysis could contribute to the differentiation of [18F]FDG-positive thyroid incidentalomas, one study recently also indicated its potential in the diagnosis of indeterminate nodules [373, 481, 482, 507-509]. In the current study, we sought to optimize the [18F]FDG-PET/CT-driven differentiation of indeterminate thyroid nodules through quantitative [18F]FDG-PET/CT assessment including radiomic analysis, with particular attention for the separate assessment of non-Hürthle and Hürthle cell nodules. We aimed to rule out malignancy and decrease the false-positive rate as compared to visual [18F]FDG-PET/CT assessment. We ultimately aimed to further prevent futile surgeries for benign, [18F]FDG-positive indeterminate nodules. Methods Study design and patient selection All patients who participated in a randomised controlled multicentre trial (ClinicalTrials.gov NCT02208544) on the efficacy of [18F]FDG-PET/CT in cytologically indeterminate thyroid nodules (EfFECTS) were assessed for eligibility for the current study. The EfFECTS trial was conducted in eight academic and seven community hospitals in the Netherlands with a high level of experience in the diagnosis and treatment of thyroid nodules and differentiated thyroid carcinoma. [18F]FDGPET/CT was performed in 132 patients with a solitary nodule or dominant nodule in multinodular disease from which indeterminate cytology was obtained, defined as at least two Bethesda III or one Bethesda IV cytology result (confirmed on central review). Based on cytology, clinical characteristics, and ultrasound features, diagnostic thyroid surgery was scheduled in all patients, in accordance with current international guidelines [17]. Further inclusion and exclusion criteria of the EfFECTS trial, and its comprehensive study procedures were previously reported [501]. Patients from the original study were only eligible for inclusion in the current study if their [18F]FDG-PET/CT scan was acquired with strict adherence to the EANM guidelines, including a patient fasting time of at least 4 hours and an acquisition time between 55-75 minutes (Figure 1) [468]. Written informed consent was obtained from all participants prior to any study activity. The trial was approved by the Medical Research Ethics Committee on Research Involving Human Subjects region Arnhem-Nijmegen, Nijmegen, the
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