Background: The aim of this study is to evaluate the role of ¹⁸F-FDG PET/CT in the detection of recurrence or distant metastasis in patients with differentiated thyroid carcinoma (DTC) with elevated serum thyroglobulin (Tg) and negative ¹³¹I whole-body scan.
Material and methods: The study included 19 patients (13 female, 6 male, average age 64 years) with DTC after total thyroidectomy and ¹³¹I ablation therapy that had elevated stimulated Tg and negative whole-body radioiodine scan. In all patients, standard imaging methods showed no suspicious changes. ¹⁸F-FDG PET/CT was performed after TSH stimulation with rhTSH or withdrawal of thyroid hormone. An evaluation of the dependence of the result of ¹⁸F-FDG PET/CT on the stimulated Tg levels was made accordingly. The statistical analysis was performed using Kruskal-Wallis test and ROC curves.
Results: Based on the results of the study ¹⁸F-FDG PET/CT in 6 patients, the suspicion of metastasis involved: the cervical lymph nodes (3 patients, ~16%) and lungs (3 patients, ~16%). The patients underwent surgery. The histopathology confirmed metastatic thyroid cancer in all cases. High levels of TSH-stimulated Tg (Tg from 32 to >300 ng/ml, median of 59.7 ng/ml) in patients were reported. The group of remaining 13 patients (~68%) with negative 18F-FDG PET/CT had low levels of TSH-stimulated Tg (Tg of from 1.76 to 10.2 ng/ml, median of 4.0 ng/ml). A particular correlation was observed between ¹⁸F-FDG PET positivity and stimulated Tg levels. The receiver operating characteristic curve (ROC) analysis demonstrated a stimulated Tg cut-off of 28.5 ng/ml with 100% sensitivity and specificity. Stimulated Tg has a large and statistically significant (p<0.0001) accuracy in the detection of recurrence/metastasis.
Conclusion: 1. ¹⁸F-FDG PET/CT is useful in the diagnosis of radioiodine-negative DTC in patients with high levels of stimulated Tg. 2. The sensitivity of ¹⁸F-FDG PET/CT increases with stimulated Tg levels. At stimulated Tg > 28.5 ng/ml, the sensitivity of the study reaches 100%.