Clinical Meaning of Diffuse 18F-FDG PET Thyroid Uptake

Kim, Hee Kyung; Yoon, Jee Hee; Kim, Soo Jeong; Kang, Ho Cheol

Abstract

BACKGROUND/AIMS
Thyroid abnormalities are frequently detected in patients undergoing 18F-fludeoxyglucose positron emission tomography (FDG-PET). The clinical importance of diffuse thyroid FDG uptake (DU) has been controversial, although focal thyroid FDG uptake (FU) indicates a high probability for thyroid cancer.
METHODS
In total, 3,303 subjects underwent PET for cancer staging (n = 1,744) or a health screening (n = 1,559) between August 2004 and March 2006. Fifty-six (1.7%) subjects showed FU (n = 34) or DU (n = 22). Serum thyroid simulating hormone (TSH) and thyroid autoantibodies were measured with high-resolution thyroid ultrasonography (US). US-guided fine-needle aspiration cytology was performed when a nodular lesion was found.
RESULTS
Twelve of 34 (35.3%) patients with FU and 3 of 22 (13.6%) patients with DU had a malignancy. This difference in malignancy frequency was not significant between the two groups (p = 0.07). The US findings were consistent with chronic thyroiditis in 17 of 22 (77.3%) patients with DU. Three cases of DU had an apparently normal thyroid gland. In the three patients diagnosed with a malignancy in the DU group, the tumors included one papillary thyroid cancer, one primary thyroid lymphoma, and one metastatic squamous cell carcinoma from lung cancer. No correlation between TSH and SUVmax (p = 0.086) was observed in the DU group, and the SUVmax in patients with a malignancy was not different from that in the benign group.
CONCLUSIONS
Most patients with DU had chronic autoimmune thyroiditis, but the frequency of malignancy was not different from that of the FU group. Further clinical examinations, including thyroid function tests, thyroid autoantibodies, and thyroid US are needed to diagnose chronic autoimmune thyroiditis and possible thyroid malignancy in subjects with DU.