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