Thyroid cancer--indications and opportunities for positron emission tomography/computed tomography imaging.

Although thyroid cancer is a comparatively rare malignancy, it represents the vast majority of endocrine cancers and its incidence is increasing. Most differentiated thyroid cancers have an excellent prognosis if diagnosed early and treated appropriately. Aggressive histologic subtypes and variants carry a worse prognosis. During the last 2 decades positron emission tomography (PET) and PET/computed tomography (CT), mostly with fluorodeoxyglucose (FDG), has been used increasingly in patients with thyroid cancers. Currently, the most valuable role FDG-PET/CT exists in the work-up of patients with differentiated thyroid cancer status post thyroidectomy who present with increasing thyroglobulin levels and a negative (131)I whole-body scan. FDG-PET/CT is also useful in the initial (post thyroidectomy) staging of high-risk patients with less differentiated (and thus less iodine-avid and clinically more aggressive) subtypes, such as tall cell variant and Hürthle cell carcinoma, but in particular poorly differentiated and anaplastic carcinoma. FDG-PET/CT may help in defining the extent of disease in some patients with medullary thyroid carcinoma and rising postoperative calcitonin levels. However, FDOPA has emerged as an alternate and more promising radiotracer in this setting. In aggressive cancers that are less amenable to treatment with (131)iodine, FDG-PET/CT may help in radiotherapy planning, and in assessing the response to radiotherapy, embolization, or experimental systemic treatments. (124)Iodine PET/CT may serve a role in obtaining lesional dosimetry for better and more rationale planning of treatment with (131)iodine. Thyroid cancer is not a monolithic disease, and different stages and histologic entities require different approaches in imaging and individualized therapy.