The importance of FDG avidity in incidental thyroid nodules on FDG PET/CT

The aim of this study was to determine the relationship between fluoro 2-deoxy-d-glucose (FDG) uptake patterns and malignancy potential in thyroid incidentalomas detected on FDG positron emission tomography/computed tomography (PET/CT).

A retrospective review was conducted on 11,591 patients who underwent FDG PET/CT. Of these, 1611 patients with diffuse, focal, or mixed FDG uptake in the thyroid gland were included in the study. Histopathological data were obtained for 214 (13 %) of these patients.

The mean age of the 1611 patients was 62 ± 13 years (age range: 2-96). Of the patients, 874 (54 %) were female, and 737 (46 %) were male. Diffuse involvement was observed in 227 (14 %) cases, focal involvement in 1339 (83 %), and mixed involvement in 45 (3 %) cases. In the malignancy group, 20 (61 %) had papillary carcinoma, 2 (6 %) had follicular carcinoma, 2 (6 %) had anaplastic carcinoma, 2 (6 %) had metastasis, 1 (3 %) had medullary carcinoma, and 6 (18 %) were classified as AUS (Atypia of Undetermined Significance). While the median thyroid SUVmax (IQR) was 5.3 (6.5) in the group with malignancy and AUS, it was 2.2 (2.5) in the benign group, with a significant difference between them (p < 0.001).

Malignant potential (including malignant and AUS cases) was present in 42 % of patients with FDG uptake above the threshold value of 3.5 for SUVmax. Contrary to the classical understanding that "well-differentiated thyroid cancers show low FDG uptake," the rate of both well-differentiated and other thyroid malignancies significantly increases in thyroid incidentalomas with high FDG uptake.