Background: Fluorine-18 fluorodeoxyglucose positron emission tomography (FDG-PET) is a new method employed in the management of differentiated thyroid cancer (DTC). The integrated FDG-PET plus computed tomography (PET/CT) fusion imaging system seems able to provide some additional advantages over PET alone, mainly related to a better anatomical localisation of the hypermetabolic metastatic lesions. The influence of serum TSH levels on 18F-FDG uptake by recurrences or metastases of DTC has not been clarified yet.
Aim: To evaluate the clinical use of PET/CT during the follow-up of patients with DTC; moreover, to ascertain whether the administration of recombinant human thyrotropin (rhTSH) can increase the sensibility and specificity of PET/CT.
Patients and methods: We selected 12 pts with positive or equivocal thyroglobulin (Tg) levels and negative or equivocal 131I scintigraphy and/or conventional morphological imaging techniques (ultrasound, MRI, etc); they underwent 18F-FDG PET/CT during TSH suppression (<0.05 IU/L) and after rhTSH administration (>30 IU/L).
Results: For 4 pts both basal and rhTSH-stimulated PET/CT scans were positive: in 3 cases tumour foci were detected (confirmed also by histology in 2 cases) whereas 1 of them was false positive results (due to lymph nodes inflammation). PET/CT was completely negative in 8 pts: 6 results were true negative while 2 were false negative, since scanning following rhTSH identified metastatic lesions.
Therefore, PET/CT was able to identify the metastatic foci very efficiently and to localise previously unknown tumour relapse; moreover, in 2 out of 12 patients, rhTSH administration resulted in detection of new lesions.
Conclusions: Our data confirm that PET/CT is a valuable tool in detecting residual disease in DTC patients and suggest a potential role for rhTSH in enhancing the diagnostic accuracy of this method