Thyroid incidentalomas on FDG PET/CT in patients with non-thyroid cancer - a large retrospective monocentric study.

Background: The purpose of this study was to compare the incidence of thyroid cancer among patients with primary non-thyroid cancer, who showed focal thyroid uptake in (18)F-fluoro-2-deoxy-d-glucose (FDG) positron emission tomography/computed tomography (PET/CT).

Material and methods: We reviewed a total of 22,674 FDG PET/CTs performed at our institution between March 2005 and June 2011. A retrospective review was conducted on 433 non-thyroid cancer patients (male: n = 90, female: n = 343) who had thyroid incidentaloma on FDG PET/CT. In 286 patients, diagnostic confirmation was done by ultrasound-guided fine needle aspiration biopsy (FNAB).

Results: Among 22,674 FDG PET/CT scans, 483 subjects (2.1%) showed focal thyroid uptake. Among the 286 patients who underwent FNAB, 280 were included in the study. Of those, 68 patients (24.3%) demonstrated papillary thyroid carcinoma on the final pathologic findings. We divided patients into 7 groups depending on the primary cancer.

Conclusion: In patients with cancer of non-thyroid origin, incidental FDG uptake in the thyroid gland was observed in 2.1% and associated with a 24.3% risk for well-differentiated thyroid carcinoma. However, there was no statistically significant difference in the malignant risk of focal FDG uptake of the thyroid gland according to the underlying primary non-thyroid cancer type.