Incidental focal uptake of 18F-fluorodeoxyglucose (FDG) in the thyroid on positron emission tomography (PET/CT) is rare but often associated with malignancy, according to the research

Thyroid incidentalomas are common in imaging, but the risk of an underlying malignancy varies greatly depending on the imaging modality used and the proportion of thyroid nodules in the background population.

A notable number of studies and systematic reviews have investigated the frequency and malignancy rate of FDG-avid thyroid incidentalomas in PET/CT. A total of 32 original retrospective reports showed a median frequency of 2.3% of FDG-avid lesions in the populations studied, with thyroid cancer in nearly one in every three patients with focal FDG-avid thyroid lesions with follow up.

Thyroid incidentalomas

The epidemiology of thyroid incidentalomas has only to some extent been described in countries with iodine deficiency. Here we report data from Denmark, a country with known iodine deficiency and wide access to PET/CT. All FDG PET/CT comprising the head and neck region, during 2014, were retrospectively reviewed, and patients with focal FDG uptake in the thyroid gland were identified.

A total of 2451 patients had an FDG PET/CT of which 59 (2.4%) patients presented with FDG-avid focal lesions in the thyroid gland. Among the 59 patients with FDG-avid lesions, 33 patients (56%) received work up with ultrasound, thyroid technetium scintigraphy, fine needle aspiration, and/or histology of which 20 patients had a conclusive pathology report.

Ten patients with FDG-avid lesions were identified with thyroid malignancy. The risk of thyroid malignancy was 16.9% among patient with incidental FDG-avid thyroid lesions.

The key findings in this report were the demonstration of FDG-avid lesions in 2.4% of the total population and the malignancy rates, which were 17% in patients with FDG-avid lesions, 30% among patients with focal FDG-avid lesions and follow-up, and 50% in patients who had a conclusive cytology/pathology examination. These figures are very similar to data presented in recent systematic reviews on thyroid incidental findings by FDG PET/CT.

Prior reports seldom present data among patients with a final pathology examination. These data are available in this report. Absence of a final cytological and/or pathological reference, as seen in approximately one-third of the patients, was likely caused by ultrasound and/or thyroid scintigraphy findings without suspected nodules.

Their findings indicated a similar frequency of FDG thyroid incidentalomas and malignancy rates in an iodine deficient population compared to summary data from prior studies, studies mostly performed in geographical areas of normal or excess iodine supplementation.