Abstract: Poorly differentiated thyroid cancer lesions often lose the ability to concentrate radioactive ¹³¹I iodine and exhibit increased metabolic activity, as evidenced by enhanced glucose uptake.¹⁸F-FDG PET was able to localize residual and metastatic thyroid cancer lesions in patients with negative diagnostic ¹³¹IWBS and elevated serum thyroglobulin levels, although it uas not sensitve enough to detect mivimal metastatic disea in cervical nodes.multivariate analysis demonstrated that the single strongest predictor of survial was the volume of FDG-avid disease.Subjects with a FDG volume greater than 125mL or with SUV greater than 10g/mL have significantly reduced short survival.