Despite the relatively good overall prognosis, approximately 1400 patients are expected to die of thyroid cancer in the United States alone in 2003 . More than 70% of the deaths occur in individuals whose initial histology was well-differentiated thyroid carcinoma. The current prognostic scoring systems (e.g. MACIS, TNM, and AGES), which utilize clinical and pathological information obtained at the time of initial diagnosis, have served us well and have stood the test of time. The major limitation is their inability to predict the outcome of patients when, years later, they are discovered to harbor metastases. This is particularly important as it is well known that there is a highly variable trajectory of disease in such patients. Some will die within a year, where as others can survive for more than 30 years with stable metastases. The recent use of FDG-PET scanning for evaluating patients with DTC has added a new dimension to our understanding of this protean disease. It appears that metastatic lesions that contain predominantly well-differentiated clones have functional NIS and transport RAI. The less well-differentiated clones cannot effectively trap iodine but appear to have much higher metabolic activity and have high FDG-avidity. This characteristic enables us to detect small deposits of the more metabolically active disease when the RAI scans are negative. Furthermore, it now appears, as expected, that the more metabolically active disease is associated with a poor prognosis. Preliminary reports suggest that the volume of FDG disease or the maximum SUV may be able to predict disease trajectory. It is conceivable that the FDG scan results may enable us to achieve real-time prognosis in patients after they are found to harbor residual or recurrent metastatic thyroid carcinoma. However, FDG scanning is not perfect. It is expensive; it has poor specificity due to the high glycolytic rate of infectious or inflammatory processes; and, it has poor sensitivity at detecting well-differentiated clones of thyroid cancer cells, such as those commonly seen in cervical lymph nodes. The recent development of hybrid scanners that merge PET technology with computed tomography (PET/CT) or with magnetic resonance (PET/MRI) may reveal even more insights into the biology of thyroid cancer metastases, as we further integrate metabolism with detailed anatomy.
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