The Usefulness of Standardized Uptake Value in Differentiation between Benign and Malignant Thyroid Lesions Detected Incidentally in (18)F-FDG PET/CT Examination

INTRODUCTION: In the last decade, (18)F-fluorodeoxyglucose ((18)F-FDG) positron emission tomography (PET and PET/CT) has become one of the major diagnostic tools used in oncology. A significant number of patients who undergo this procedure, due to non-thyroidal reasons, present incidental uptake of ((18)F-FDG) in the thyroid. The aim of the study was to compare the SUV(max) (standardized uptake value) of thyroid focal lesions, which were incidentally found on PET/CT, in relation to the results of thyroid fine-needle aspiration biopsy (FNAB) and/or histopathological evaluation. MATERIALS AND METHODS: Patients referred for PET/CT examination, due to non-thyroidal illness, presented focal (18)F-FDG uptake in the thyroid and were advised to undergo ultrasonography (US), hormonal evaluation, FNAB and/or total thyroidectomy at our institution. RESULTS: 6614 PET/CT examinations performed in 5520 patients were analyzed. Of the 122 patients with focal thyroid (18)F-FDG activity, 82 patients (67.2%) underwent further thyroid evaluation using FNAB. Benign lesions were diagnosed in 46 patients, malignant - in 19 patients (confirmed by post-surgical histopathology), while 17 patients had inconclusive results of cytological assessment. Mean SUV(max) of benign lesions was 3.2±2.8 (median = 2.4), while the mean SUV(max) value for malignant lesions was 7.1±8.2 (median = 3.5). The risk of malignancy was 16.7% for lesions with a SUV(max) under 3, 43.8% for lesions with a SUV(max) between 3 and 6, and 54.6% for lesions with a SUV(max) over 6. In the group of malignant lesions, a positive correlation between the lesion’s diameter and SUV(max) was observed (p = 0.03, r = 0.57). CONCLUSIONS: Subjects with incidental focal uptake of (18)F-FDG in thyroid are at a high risk of thyroid malignancy. A high value of SUV(max) further increases the risk of malignancy, indicating the necessity for further cytological or histological evaluation. However, as SUV(max) correlated with the diameter of malignant lesions, small lesions with focal uptake of (18)F-FDG should be interpreted cautiously.