Selected patients with thyroid cancer can benefit from the use of PET imaging with FDG or with I-124. The PET scan impacts on management by providing (1) more accurate information about staging of patients in terms of extent of tumor for better treatment planning, especially in patients who do not concentration radioactive I-131; (2) the relationship of tumor involvement to vital structures, especially in the neck and central nervous system; and (3) prognostic information (an SUV > 10 and extensive PET + disease connotes a poor prognosis in advanced patients). In the occasional patient, surgically respectable disease has been identified on PET with the result that the patient has been rendered no evident disease with treatment. PET has also been used in the follow-up of patients who have been treated for thyroid cancer, to assess response. PET may also be useful for lesion specific dosimetry, with I-124. The combination of PET and CT in the same gantry facilitates localization of thyroid cancer PET scan abnormalities in relationship to critical organs and structures.