PET/CT in the management of differentiated thyroid cancer.

The standard treatment of differentiated thyroid cancer (DTC) consists of surgery followed by iodine-131 (131I) administration. Although the majority of DTC has a very good prognosis, more aggressive histologic subtypes convey a worse prognosis. Follow-up consists of periodically measurements of serum thyroglobulin, thyroglobulin antibodies and neck ultrasound and 123I/131I whole-body scan. However, undifferentiated thyroid tumors have a lower avidity for radioiodine and the ability of DTC to concentrate 131I may be lost in metastatic disease. Positron emission tomography (PET)/computed tomography (CT) has been introduced in the evaluation of patients with thyroid tumors and the 2-[18F]-fluoro-2-deoxyd-glucose (18F-FDG) has been largely validated as marker of cell's metabolism. According to the 2015 American Thyroid Association guidelines, 18F-FDG PET/CT is recommended in the follow-up of high-risk patients with elevated serum thyroglobulin and negative 131I imaging, in the assessment of metastatic patients, for lesion detection and risk stratification and in predicting the response to therapy. It should be considered that well-differentiated iodine avid lesions could not concentrate 18F-FDG, and a reciprocal pattern of iodine and 18F-FDG uptake has been observed. Beyond 18F-FDG, other tracers are available for PET imaging of thyroid tumors, such as Iodine-124 (124I), 18F-tetrafluoroborate and Gallium-68 prostate-specific membrane antigen. Moreover, the recent introduction of PET/MRI, offers now several opportunities in the field of patients with DTC. This review summarizes the evidences on the role of PET/CT in management of patients with DTC, focusing on potential applications and on elucidating some still debating points.