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In all, 20% of fine-needle aspiration (FNA) biopsies of thyroid nodules have an indeterminate diagnosis; of these, 80% are found to be benign after thyroidectomy. Some previous reports indicate that positron emission tomography (PET) with 18F-fluorodeoxyglucose (FDG) imaging may predict malignancy status. We now report results on the first 51 patients in the largest prospective study of FDG-PET in patients with an indeterminate thyroid nodule FNA. Eligible patients had a dominant thyroid nodule that was palpable or ≥1 cm in greatest dimension as seen by ultrasonography, and indeterminate histology of the FNA biopsy specimen. Participants underwent preoperative neck FDG-PET alone or FDG-PET with computed tomography (FDG-PET/CT). Images were evaluated qualitatively and semiquantitatively using the maximum standardized uptake value (SUVmax). Final diagnosis was determined by histopathologic analysis after thyroidectomy. Descriptive statistical analysis was performed. A total of 51 patients underwent preoperative FDG-PET or FDG-PET/CT. Studies without focally increased uptake localized to the lesion were considered negative. For all lesions (10 malignant, 41 benign), the sensitivity, specificity, positive-predictive value (PPV), and negative-predictive value (NPV) were 80%, 61%, 33%, and 93%, respectively. Postoperatively, two malignant and six benign lesions were found to be <1 cm by pathology examination; one lesion was not measured. When these lesions were excluded, the sensitivity, specificity, PPV, and NPV were 100%, 59%, 36%, and 100%, respectively. Based on these preliminary data, FDG-PET may have a role in excluding malignancy in thyroid nodules with an indeterminate FNA biopsy. This finding justifies ongoing accrual to our target population of 125 participants. [ABSTRACT FROM AUTHOR]