Risk of Malignancy in Thyroid Incidentalomas Identified by Fluorodeoxyglucose-Positron Emission Tomography

Background: Thyroid incidentalomas detected by 2-deoxy-2- 18 F-fluoro-D-glucose positron emission tomography/computed to- mography ( 18 F-FDG PET/CT) have been reported in 1% to 4% of the population, with a risk of malignancy of 27.8% to 74%. We performed a retrospective review of FDG-avid thyroid incidentalomas in cancer screening subjects and patients with nonthyroid cancer. The risk of malignancy in thyroid incidentaloma and its association with the maximal standardized uptake value (SUVmax) in 18 F-FDG PET/CT were evaluated to define the predictor variables in assessing risk of malignancy. Methods: A total of 2,584 subjects underwent 18 F-FDG PET/CT for metastatic evaluation or cancer screening from January 2005 to January 2010. Among them, 36 subjects with FDG-avid thyroid incidentalomas underwent further diagnostic evaluation (thy- roid ultrasonography-guided fine needle aspiration cytology (FNAC) or surgical resection). We retrospectively reviewed the da- tabase of these subjects. Results: Of the 2,584 subjects who underwent 18 F-FDG PET/CT (319 for cancer screening and 2,265 for metastatic evaluation), 52 (2.0%) were identified as having FDG-avid thyroid incidentaloma and cytologic diagnosis was obtained by FNAC in 36 sub- jects. Of the subjects, 15 were proven to have malignant disease: 13 by FNAC and two by surgical resection. The positive predic- tive value of malignancy in FDG-avid thyroid incidentaloma was 41.7%. Median SUVmax was higher in malignancy than in be- nign lesions (4.7 (interquartile range (IQR), 3.4 to 6.0) vs. 2.8 (IQR, 2.6 to 4.0), P=0.001). Conclusion: Thyroid incidentalomas found on 18 F-FDG PET/CT have a high risk of malignancy, with a positive predictive value of 41.7%. FDG-avid thyroid incidentalomas with higher SUVmax tended to be malignant.