The study objective was to investigate the role of PET/CT as an indirect determination of the differentiation status of metastases and for the prediction of radioactive iodine (RAI) therapy response in patients with metastatic DTC. 

Materials and methods. 

The 40 metastatic DTC patients were enrolled in the study that underwent both post-therapeutic radioiodine scan and PET/CT at the same period. 

Results. 

12 patients (30%) responded to RAI therapy. The remaining 28 patients (70%) not responded to RAI therapy showed stabilization or progression. The accumulation of radioiodine by metastases positively correlated with the total response rate (RR), while the FDG avidity is negative. Significant direct correlation with RR was observed in the group with only radioiodine uptake. However, this correlation was not observed in the patients with both tracers uptake. The patients with FDG-positive metastases showed poor response to RAI therapy, regardless of the degree of radioiodine uptake. 

Conclusion. 

The FDG uptake by metastases is a predictor of a poor response to RAI therapy, even in the presence of RAI uptake. The use of 18F-FDG PET/CT in patients with metastatic DTC can be recommended at the beginning of RAI therapy to clarify the prognosis and provide a personalized approach to the treatment and observation of the most difficult category of patients