Role of 18F-FDG-PET and PET/CT imaging in thyroid cancer.

In patients affected by differentiated thyroid cancer (DTC), the lacking of 131Iodine trapping by metastatic tissue does not allow 131Iodine whole body scintigraphy to visualize matastatic spread as well as the use of 131Iodine therapy to cure such metastatic spread. Prognosis of 131Iodine-negative DTC metastasis, so-called non-functioning metastasis, is significantly worst. In these patients an early diagnosis of non-functioning metastasis and their surgical extirpation remains the optimal therapeutic approach. In this view, a high sensitive localizing imaging different form 131Iodine whole body scintigraphy is required. Ultrasonography is characterized by a relatively high sensitivity in these patients but it is highly operator-dependent and, moreover, it can be used to explore neck alone. Computed tomography (CT) scan and magnetic resonance (MR) imaging are characterized by a relatively low sensitivity even if they are useful to provide the surgeon with anatomical information of the operating basin. Various tumor-seeking radiotracers have been proposed, mainly using SPECT as 201Thallium, 99mTc-Sestamibi and 99mTc-Tetrofosmin with good results. Even more favorable results have been reported with some positron radiotracers, mainly the 18F-FDG with PET and more recently with PET/CT tomographs. The typical indication to performing with examination is the DTC patient previously treated by total thyroidectomy and 131Iodine ablative therapy, with increased serum thyroglobulin (Tg) or anti-thyroglobulin (TgAb) antibodies during follow-up but with negative 131Iodine whole body scintigraphy even obtained after high, therapeutic 131Iodine doses. Several studies in literature have reported high sensitivity (up to 85%) and specificity (up to 95%) of FDG-PET in metastatic DTC patients. The integrated PET/CT fusion imaging systems, seem able to provide some additional advantages over PET alone, mainly related to a better anatomical localization of the hypermetabolic metastatic lesions. A change in the management of DTC patients affected by non-functioning metastatic spread not visualized by other imaging techniques has been reported in 30% of patients. Lastly, the role of PET and PET/CT fusion imaging systems seem to be promising also in patients affected by medullary thyroid carcinoma (MTC), especially for the detection of neck and mediastinal lesions, with a sensitivity superior to the other currently available imaging methods, however the data reported on medullary cancer are little and further studies are needed to elucidate the preliminary promising results.