Impact of TSH stimulation on ۲-[۱۸F]FDG PET/CT results in patients with papillary thyroid carcinoma presented with elevated serum thyroglobulin level and negative diagnostic iodine-۱۳۱ whole-body scan

Introduction: ۲-[۱۸F]fluoro-۲-deoxy-D-glucose-positron emission tomography/computed tomography (۲-[۱۸F]FDG-PET/CT) is implemented in papillary thyroid cancer (PTC) patients with elevated Thyroglobulin (Tg) and negative Iodine-۱۳۱ whole-body scan (۱۳۱I-WBS). Here, we evaluated the impact of TSH stimulation after levothyroxine withdrawal on the detection rate of ۲-[۱۸F]FDG-PET/CT.Methods: A prospective study was performed on ۶۰ PTC patients, presented with negative ۱۳۱I-WBS and elevated or unjustifiably high Tg. ۲-[۱۸F]FDG-PET/CT was performed in ۳۰ patients while they were on levothyroxine therapy (unstimulated-TSH [uns-TSH]) and after Levothyroxine withdrawal in the other ۳۰ patients (stimulated-TSH [s-TSH]). Results of the two groups were compared using nonparametric tests. Receiver operating characteristic curve was used to find Tg cutoff values for predicting positive scan results.Results: Overall, ۲-[۱۸F]FDG-PET/CT was positive in ۶۳.۳% of the patients, ۸۰% (۲۴/۳۰) in s-TSH and ۴۶.۷% (۱۴/۳۰) in uns-TSH group. The detection rate was higher in s-TSH group (p=۰.۰۰۷). It was still significant in multiple regression analysis (p=۰.۰۴۱). In uns-TSH group, ۲-[۱۸F]FDG-PET/CT was more often positive in patients with higher uns-Tg level (p=۰.۰۰۲). An uns-Tg level of ≥۱۹.۰۰ ng/mL predicted positive results with the sensitivity of ۰.۷۸۶ and specificity of ۰.۷۵۰ (area under curve=۰.۸۱۹). Although statistically insignificant (p=۰.۰۵۵), s-Tg was higher in patients with positive ۲-[۱۸F]FDG-PET/CT studies in the s-TSH group. No relation was demonstrated between TSH and anti-Tg-antibody levels and ۲-[۱۸F]FDG-PET/CT positivity.Conclusion: TSH-stimulation after levothyroxine withdrawal might enhance the detection rate of ۲-[۱۸F]FDG-PET/CT in PTC patients. Additionally, ۲-[۱۸F]FDG-PET/CT is more often positive in patients with higher Tg levels.

Keywords:

Differentiated thyroid cancer , Levothyroxine withdrawal , ۲-[۱۸F]FDG PET/CT , Elevated thyroglobulin , Negative radioiodine scan