Imaging of Metastases of Thyroid Cancer with Fluorine 18 Fluorodeoxyglucose

FDG is a D-glucose analog, which is converted in cells to FDG-6-phosphate by hexokinase. FDG-6-phosphate is metabolically trapped and accumulates in tissue where glucose-6-phosphatase is lacking. Metabolic trapping is the key factor responsible for the biodistribution of 18F-2-deoxyglucose.109 Because other enzymes that act on glucose-6-phosphate have only a negligible affinity for FDG-6-phosphate and membrane permeability is low, the rate of accumulation of FDG-6-phosphate is proportional to the phosphorylation rate of exogenous glucose and D-glucose utilization of the tissue. I8F has a half-life of about 109 minutes, so that the patient is exposed to a tolerable amount of radiation."0

FDG-PET is primarily used to localize recurrent differentiated and poorly differentiated thyroid cancers, especially in patients who are serum thyroglobulin positive and 13II WBS negative. Serum thyroglobulin determination and diagnostic l3'I WBS provide the diagnosis of recurrent disease. Recurrent differentiated thyroid cancer may or may not take up radioiodine. '"A patient whose recurrent tumor is detected by radioiodine scanning has a significantly better prognosis than does a patient whose tumor does not take up l3'I."2 FDG-PET can be positive in the same site as a WBS-positive site or WBS-negative site, or both can be present in the same patient. Grunwald and associates"3 reported that FDG-PET was particularly useful in WBS-negative patients, showing a high sensitivity of 85%. Patients with poorly differentiated thyroid cancers were more likely to be WBS negative and FDG-PET positive. Those patients also have a worse prognosis—FDG-PET helps stage the disease and guide treatment strategy. Possible therapeutic strategies include surgery, external-beam radiation, and redifferentiation therapy.114"117 Patients who are WBS positive and FDG-PET negative usually respond better to treatment with 13'I. The result obtained from FDG-PET scanning may influence therapy; for example, the removal of mediastinum lymph node metastases usually should be omitted when additional distant metastases are detected by FDG-PET scan in l3lI scan-negative patients. The benefit of further radioiodine therapy in thyroglobulin-positive l3lI scan-negative patients is controversial.

Correlation between the grade of differentiation and the rate of detectability with FDG-PET and MIBI or WBS exists. Since glucose metabolism is increased particularly in poorly differentiated tumors, a higher sensitivity of FDG-PET can be expected in these tumors, in association with a low sensitivity of WBS.118"123 Metastases showing high FDG uptake but low 13'I uptake often grow more rapidly and are generally more aggressive.118 A higher mortality rate in patients who have a negative WBS (with or without elevated thyroglobulin levels) in spite of proven metastases has also been reported.30 Grunwald and coworkers124 reported a higher rate of positive FDG-PET scans in patients with high-risk patients by TNM staging. Thus, positive FDG-PET scans were observed in 2 of 14 patients (14%) with pTl/pT2 tumor stage versus 8 of 17 patients (47%) with pT3/pT4 tumors. Higher tumor stage correlates with a poorer prognosis. Positive FDG-PET scans also occur in patients with sarcoidosis,30 granulomas,125 parathyroid tumors,126 Hurthle cell adenomas,127 follicular adenomas, and adenomatous goiters.128 FDG-PET scanning can detect metastases in lymph nodes less than 1 cm in diameter.123 Small pulmonary metastases (<1 cm), with or without radioiodine uptake, were not detected by FDG-PET but were detected by spiral CT scanning.123124

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