Clinical thyroid cancer is an uncommon disease, representing only about 1% of all cancers. Microscopic papillary carcinomas, however, are very common, with an incidence reported as high as 35.6%. The most common site for persistent disease, recurrence, and lymph node metastases is the neck. Interpretation of PET images of the neck region presents a particular challenge. The region shows substantial variations in normal uptake that can present difficulties in the identification of pathology. PET-CT or PET fused with a diagnostic CT is of great help to differentiate between physiologic muscle uptake and pathologic uptake in tumor or metastatic lymph nodes. FDG PET-CT is an expensive procedure and is probably most useful in patients with more aggressive thyroid cancer before planned surgery or radiation therapy and to monitor such patients. Neck ultrasound, neck and chest CT, and 131- or 123-I scanning will normally be the first line imaging modalities. In most studies evaluating PET as a diagnostic tool in thyroid cancer, the PET examinations have been performed with PET only without integrated CT. Combined PET-CT is shown to be more accurate than PET alone in the detection and anatomic localization, leading to improved diagnostic accuracy in most cancers, including suspected recurrent or metastatic well-differentiated thyroid cancer. The role of FDG PET in the diagnosis and follow-up medullary thyroid cancer is not well defined.
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