Characteristics of malignant thyroid lesions on [18F] fluorodeoxyglucose (FDG)-Positron emission tomography (PET)/Computed tomography (CT)

Rare images of an intense 18F-FDG PET-scan uptake in an incidentally discovered thyroid lesion: A case report

Thyroid carcinoma is a type of cancer that starts in the cells of the thyroid gland, which plays a crucial role in regulating the body's metabolism. Thyroid cancer can manifest in various forms. We report the case of an 89 year-old patient presenting an incidental intense Fluorine-18-fluorodeoxyglucose (18F-FDG) PET-computed tomography (PET-CT) uptake within a thyroid lesion and the role of 18F-FDG PET-CT guiding the anatomopathological examination. The prevalence of FDG-avid thyroid incidentaloma ranges between 0.2% and 8.9%. Higher risks of cancer seems to be related to focal or unilateral uptake of 18F-FDG. Lesions with higher standardized uptake values or suspicious CT are more likely to be cancers. The diagnosis is clinically challenging. Nuclear and radiological images can guide practitioners.

The role of PET/CT in thyroid autoimmune diseases

Autoimmune thyroid diseases (AITD) are a heterogeneous group of disorders. They include, in particular, Graves' disease and Hashimoto's thyroiditis with a wide range of different functional status ranging from subclinical biochemical abnormalities to severe hyperthyroidism or severe hypothyroidism respectively. Furthermore, other conditions more frequently infectious or drug related can cause an immune reaction in the thyroid tissue. In AITDs, positron emission tomography/computed tomography (PET/CT) does not play a primary role for disease diagnosis or management, but accidental findings can occur in both symptomatic and asymptomatic patients, and they should be recognized and well interpreted. A comprehensive literature search of the PubMed databases was conducted to identify papers (systematic review, prospective and retrospective study, case report) evaluating the role of PET/CT in thyroid autoimmune diseases. Thyroid diffuse uptake of 18F-fluoro-2-deoxy-2-d-glucose ([18F]FDG) has been shown to be frequently associated with AITDs, but also with immune-induced thyroid disorders related to SARS-CoV-2 or immunotherapy, while malignant lesions more often have a focal aspect. Other radiopharmaceuticals as [68Ga]-DOTA-peptides, [68Ga]-fibroblast activation protein inhibitors (FAPIs) and [68Ga]-prostate specific membrane antigen ([68Ga]-PSMA) showed similar findings. In conclusion, PET/CT scan in AITDs does not play a primary role in the diagnosis, but the occasional finding of a thyroid uptake must always be described in the report and possibly investigated for a better patient's management.