[F-18]-2-fluoro-2-deoxy-D-glucose positron emission tomography localizes residual thyroid cancer in patients with negative diagnostic I-131 whole body scans and elevated serum thyroglobulin levels

Progressive dedifferentiation of thyroid cancer cells leads to a loss of io dine-concentrating ability, with resultant false negative, whole body radio active iodine scans in approximately 20% of an differentiated metastatic th yroid cancer lesions. We tested the hypothesis that all metastatic thyroid cancer lesions that did not concentrate iodine, but did produce thyroglobul in (Tg), could be localized by [F-18]2-fluoro-2-deoxy-D-glucose (FDG)-posit ron emission tomography (PET). We performed FDG-PET on 37 patients with dif ferentiated thyroid cancer after surgery and radioiodine ablation who had n egative diagnostic I-131 whole body scans during routine follow-up. Serum T g, Tg autoantibodies, neck ultrasounds, and other clinically indicated imag ing procedures were performed to detect residual disease. In those with ele vated Tg levels, FDG-PET localized occult disease in 71%, was false positiv e in one, and was false negative in five patients. The majority of false ne gative FDG-PET occurred in patients with minimal cervical adenopathy. Surgi cal resections, biopsies, I-131 therapy, and differentiation therapy were p erformed based on the PET results. The FDG-PET result changed the clinical management in 19 of the 37 patients. In patients with elevated Tg levels, F DG-PET had a positive predictive value of 92%. In patients with low Tg leve ls, FDG-PET had a negative predictive value of 93%. No FDG-PET scans were p ositive in stage I patients; however, they were always positive in stage IV patients with elevated Tg levels. An elevated TSH level (i.e. hypothyroidi sm) did not increase the ability to detect lesions. FDG-PET is able to loca lize residual thyroid cancer lesions in patients who have negative diagnost ic I-131 whole body scans and elevated Tg levels, although it was not sensi tive enough to detect minimal residual disease in cervical nodes.