ORIGINAL RESEARCH Incidence of thyroid carcinoma in fluorodeoxyglucose positron emission tomography-positive thyroid incidentalomas Deanne L. King, MD, PhD, Brendan C. Stack, Jr, MD, Paul M. Spring, MD, MS, Ronald Walker, MD, and Donald L. Bodenner, MD, PhD, Little Rock, AR OBJECTIVE: Fluorodeoxyglucose (FDG) whole body positron emission tomography (PET) scan may show clinically occult sec- ond lesions. Such lesions in the thyroid are increasingly common. There are several recent reports of a high probability of malig- nancy in these lesions ranging from 14% to 63%. STUDY DESIGN AND SETTING: This is a retrospective re- view of 15,711 PET scans at a multi-disciplinary thyroid clinic at a tertiary care university medical center. Twenty-two patients were referred with thyroid PET “incidentalomas.” The review included 18 FDG-PET scans, ultrasound guided fine needle aspiration biopsies, and thyroid surgery pathology. Aspiration cytology or pathology were the main outcome measures. RESULTS: Three patients had malignancy of the PET-positive thyroid lesions. Papillary thyroid micro carcinomas were detected in four of the specimens that showed a benign pathology of the dominant nodule. CONCLUSION: Our experience shows a 14% malignancy rate for the dominant (imaged) nodule and a total malignancy rate of 32% when the incidental micro carcinomas are included. Both of these rates are significantly lower than results published previ- ously. © 2007 American Academy of Otolaryngology–Head and Neck Surgery Foundation. All rights reserved. F luorodeoxyglucose whole body positron emission to- mography (PET) has become a standard clinical imag- ing modality for evaluation of a variety of malignancies. PET is useful in identifying sites of metastatic disease as well as evaluating the primary malignancy. It has also been beneficial in the evaluation of suspicious findings found via other imaging modalities. It contributes valuable informa- tion in the staging of primary cancer and in the detection of recurrent disease. Combined PET-computed tomography (CT) improves lesion localization and accuracy as com- pared to PET alone. 1-3 This imaging may show clinically occult lesions. Occasionally incidental intense FDG uptake is observed in the thyroid gland on whole-body PET studies carried out for other indications. These thyroid “incidenta- lomas” were defined as a thyroid lesion seen initially on PET in a patient without a history of thyroid disease. 4 These thyroid lesions have become increasingly common with the increased use of PET scans and combined PET-CT scans. 5 Determining the most appropriate management of these “incidentalomas” has presented a dilemma. By the age of 50, approximately 40% to 50% of females will have a thyroid nodule. 6 In the case of ultrasound- or CT-identified incidentalomas, the likelihood of malignancy is approxi- mately 5%, similar to that in clinically identified nodules. 6 In contrast, thyroid carcinomas often exhibit increased met- abolic activity, as exhibited by enhanced glucose uptake on PET. 2,7,8 This is evidenced by the use of PET scans to detect cancer foci in patients with clinical or serologic evidence of recurrent or metastatic papillary or follicular thyroid carci- noma but negative I-131 whole body scan. 9,10 PET scans show metastatic disease in up to 90% of these patients. 1 In patients with medullary thyroid cancer with elevated calci- tonin levels after thyroidectomy, PET has a sensitivity of 70% to 75% for localizing metastatic disease. 11 Initial find- ings show that benign thyroid lesions seem to be far less metabolically active than thyroid malignancies. 7 An inci- dental finding of a PET-positive lesion in the thyroid in almost all series reported to date is found to be malignant over 30% of the time, and in some series approaches 100%. Although not published as a formal recommendation, if the malignancy rate is that high in PET-positive thyroid lesions, an argument could be made to proceed directly to surgery on all PET-positive thyroid lesions. We report our experience with PET-positive thyroid in- cidentalomas and show that it departs significantly from many of the published series. These data may be significant, however, because they come from the second busiest PET-CT center in the United States. PET-CT can be ac- cepted most generally as a superior modality to stand alone PET for oncology imaging. Additionally, about 60% of our PET-CT volume comes from our multiple myeloma service. Received January 18, 2007; accepted February 26, 2007. Otolaryngology–Head and Neck Surgery (2007) 137, 400-404 0194-5998/$32.00 © 2007 American Academy of Otolaryngology–Head and Neck Surgery Foundation. All rights reserved. doi:10.1016/j.otohns.2007.02.037