Relationship among FDG uptake, thyroglobulin doubling time, and clinical behavior in postsurgical papillary thyroid cancer

Objectives Although various scoring systems have been proposed to predict prognosis of thyroid cancer, prognostic factor for thyroid cancer after total thyroidectomy has not been optimized. The aim of the study was to compare FDG uptake in postsurgical papillary thyroid cancer (PCA) with Thyroglobulin- doubling time (Tg-DT) and clinical aggressiveness of tumors.

Methods 84 patients with PCA, who had previously underwent total thyroidectomy and radioiodine ablation and were free from anti-Tg Ab, underwent FDG-PET/CT. Serum Tg levels were measured at least 5 or more time after imaging of FDG-PET/CT. Serum TSH levels at PET/CT and Tg measurement were below the normal lower limit in all. Uptake of FDG was assessed both visually and semi-quantitatively (SUVmax). If a patient had multiple tumors, average of SUVmax in the 2 largest tumors were used for evaluation. Tg-DT was determined as previously described (Thyroid 2011; 21: 707- 716). Base on Tg-DT, patients were classified into 3 subgroups: A) those with Tg-DT < 1yr (n=11), B) those with 1≤Tg-DT < 3yrs(n=32), and C) those with Tg-DT>3 yrs. or others. Patients were follower up for 25-76months (median 41). Changes in the tumor size were evaluated based upon RECIST1.1

Results SUVmax of FDG in group A, B, and C were 9.5±5.1*, 7.4±4.6, and 4.1±3.3*, respectively (*: p<0.01). Frequencies of progressive disease (%) in group A, B, and C was 50**, 29, and 14**, respectively (**:p<0.01). Tumors were more metabolically active and clinically aggressive in patients with Tg-DT of 1yr > than those with Tg-DT of >3 yrs. or negative. There was an inverse correlation between SUVmax and Tg-DT in patients with visually discernible FDG uptake (r=-0.59).

Conclusions Higher FDG uptake in postsurgical PCA indicates shorter Tg-DT and higher risk of progressive disease. Combination of FDG-PET/CT and Tg-DT may be helpful in characterizing prognosis of postsurgical PCA.