Positron emission tomography of the thyroid, with an emphasis on thyroid cancer

The role of Positron Emission Tomography (PET) using F-18-fluorodeoxyglucos e (FDG) in the management of thyroid cancer is discussed. It is important t o ensure that patients are relaxed because uptake of FDG in tense or active muscles in the neck and larynx can be misinterpreted as metastases. The ma jor role for PET is in patients where the stage of disease is uncertain, us ually the result of discordant negative I-131 scan and a positive serum thy roglobulin (Tg) values. PET identifies the source of Tg production in 50-80 % of patients. PET scan can be negative in well differentiated cancers whic h retain the ability to trap iodine. This can result in a 'flip/flop', with negative PET, positive radio-iodine scan, or positive PET, negative radioi odine scan. PET is also valuable in identifying the source of calcitonin pr oduction in patients with medullary thyroid cancer. When focal uptake is se en in the thyroid of patients who are scanned for non thyroidal reasons, th e likelihood of primary thyroid cancer is high. Ln contrast diffuse uptake of FDG in the thyroid is usually the result of auto-immune thyroid disorder s. ((C) 2001 Lippincott Williams & Wilkins).