18F-FDG PET/CT concurrent with first radioiodine post-therapeutic scan in high risk differentiated thyroid cancer: a useful tool or just an expensive diversion?

Background: Aim of the present study was to evaluate the clinical impact of fluorine- ¹⁸ F-fluorodeoxyglucose PET/CT ( ¹⁸ F-FDG-PET/CT) concurrent with post-therapeutic whole-body radioiodine scan (TxWBS) after first radioiodine (RAI) treatment in patients with high-risk differentiated thyroid carcinoma (DTC).
Methods: This was a retrospective, single-center study including 39 patients with DTC (22 females, 17 males, median age 54; IQR: 35-60 years, 87% papillary thyroid cancer, 13% follicular thyroid cancer). All patients underwent ¹⁸ F-FDG-PET/CT and RAI treatment, both performed off L-T4 about 3 months after total thyroidectomy. TxWBS was obtained 3 days afterwards using planar technique and SPECT/CT of neck and thorax regions. Semiquantitative analysis was performed on positive ¹⁸ F-FDG-PET/CT scans to assess SUV<inf>max</inf>, SUV<inf>ratio</inf>, MTV and TLG values in target lesions (hottest ¹⁸ F-FDG-positive lesion present in each patient). Receiver operating characteristics (ROC) curve analysis was obtained to establish a cut-off point for SUV<inf>max</inf> able to predict the presence of RAI nonavid lesions. Univariate and multivariate analyses were executed to find out predictive factors for abnormal ¹⁸ F-FDG-PET/CT imaging.
Results: In 11 (28%) patients 18F-FDG-PET/CT and TxWBS were both negative and in 9 (23%) both positive, showing loco-regional or distant metastases. In 14 patients (36%) ¹⁸ F-FDG-PET/CT showed more lesions than TxWBS, while in 5 (13%) patients more lesions were present at TxWBS than 18F-FDG-PET/CT. Overall, 23 patients (59%) showed ¹⁸ F-FDG avid lesions and ¹⁸ F-FDG-PET/TC changed the management in 14 (36%), including the choice to perform RAI therapy with higher activities than expected, lymph-node dissection for loco-regional metastases, direct therapy for solitary bone metastases. Through ROC curve analysis, a value superior to 7.25 of SUV<inf>max</inf> was able to predict the presence of RAI non-avid lesion at TxWBS. Serum stimulated thyroglobulin and extranodal invasion resulted to be risk factors for abnormal ¹⁸ F-FDG-PET/CT imaging. However, only extranodal invasion turned out to be an independent risk factor for abnormal ¹⁸ F-FDG-PET/CT.
Conclusions: The present study demonstrated the clinical value of RAI-concurrent ¹⁸ F-FDG-PET/CT in patients with high-risk DTC. However, some questions remain open, including the pretherapeutic thyroglobulin level to use as indication to ¹⁸ F-FDG-PET/CT and the predictive value of ¹⁸ F-FDG-PET/CT semiquantitative parameters.