Current status of PET imaging of differentiated thyroid cancer with second generation radiopharmaceuticals.

Although the prognosis of differentiated thyroid cancer (DTC) is favorable, some histotypes show worst clinical outcome and higher risk of recurrence. Serum thyroglobulin (Tg) levels and 131I-whole-body-scan (WBS), together with neck ultrasound (US), represent the golden standard for DTC follow-up. Nevertheless, the relatively high frequency of patients with high Tg levels and negative WBS requires further investigations by using new imaging modalities. The availability of whole body positron emission tomography (PET) methods, in parallel with the advances in radiochemistry, offer a wide substrate for many solutions. To this day ¹8F-fluoro-deoxy-glucose (¹8F-FDG) PET/CT still represents the imaging of choice in follow-up of patients with high serum Tg and negative ¹³¹I-WBS but in the last decades the research has focused on finding "second generation" radiopharmaceuticals for PET imaging, with both diagnostic and prognostic purposes, aiming to change the way to image thyroid cancer. Moreover, the use of various PET radiopharmaceuticals, that offer the possibility to explore different pathways involved in thyroid cancer, could find important applications in the near future for clinical decision making in order to program tailored treatments and follow-up. It would be desirable to use the same radiopharmaceutical for both imaging and dosimetric purpose to achieve a tailored therapy. Many efforts are focused in this direction and ¹²4I-PET/CT is now emerging as a valid tool in restaging and therapy management of DTC with promising results. Although the preliminary data available in literature require a confirmation in larger studies with longer follow-up, we think that in next future ¹²4I-PET/CT could gain an important role for management of DTC. The aim of this review was to perform a systematic analysis of literature describing the state of art of "second generation" PET-radiopharmaceuticals for imaging DTC. Discussion is focused on the utility of ¹²4I-PET/CT, but we also mention the pathways explored by 68Gallium-somatostatin analogues, ¹8F-FLT and ¹¹C-MET and their applications in this clinical setting.