Evaluation of thyroid FDG uptake incidentally identified on FDG-PET/CT imaging.

OBJECTIVES: To characterize the prevalence and malignancy of thyroid F-flurodeoxyglucose (FDG) uptake incidentally identified on FDG-PET/computed tomography (CT) scan in a relatively large population.

METHODS: Two thousand five hundred and ninety-four cases of FDG-PET/CT performed at our institute in the past 1 year and a half were retrospectively reviewed. Images with incidental focal or diffuse thyroid FDG uptake were identified. Data of the PET findings, thyroid functional assay, and pathological diagnosis were collected and analyzed. Incidental thyroid FDG uptake was defined as a new thyroid lesion initially identified on PET scan in a patient without a previous known history of thyroid disease.

RESULTS: The prevalence of incidental thyroid FDG uptake (including both focal and diffuse lesions) was 3.8% (99 of 2594) on FDG-PET/CT, of which 1.8% (46 of 2594) were diffuse and 2.0% (53/2594) were focal. Of the 46 cases with diffuse uptake, 21 had thyroid functional assay and/or ultrasound study, and a diagnosis of chronic thyroiditis was made in all of the 21 cases. Eleven of the 53 patients with focal uptake had fine-needle aspiration or postsurgical pathological diagnosis, four benign lesions (four of 11=36.4%: two thyroid adenomas and two hyperplastic lesions); seven malignancies (seven of 11=63.6%: three papillary carcinomas, two follicular carcinoma, and two metastases). There was overlapping of the lesion SUVmax between the benign and malignant cases, with no statistical difference of the mean SUVmax between the two groups.

CONCLUSION: Thyroid FDG uptake incidentally identified on FDG-PET/CT occurred at a frequency of 3.8%, with about half of focal and half of diffuse lesions. The risk of thyroid malignancy was 63.6% in lesions with focal uptake, whereas the majority of diffuse uptake cases represents chronic thyroiditis. More data are needed to elucidate the role of SUV in the differentiation of benign and malignant lesions.