Value of FDG PET in papillary thyroid carcinoma with negative I-131 whole-body scan

The management of metastatic thyroid carcinoma patients with a negative I-1 31 scan presents considerable problems. Fifty-four athyrotic papillary thyr oid carcinoma patients whose I-131 whole-body scans were negative underwent F-18-fluorodeoxyglucose (FDG) PET; the purpose was to determine whether th is procedure could localize metastatic sites. We also assessed its usefulne ss in the management of these patients. Methods: Whole-body emission scan w as performed 60 min after the injection of 370-555 MBq F-18-FDG, and additi onal regional attenuation-corrected scans were obtained. Metastasis was pat hologically confirmed in 12 patients and was confirmed in other patients by overall clinical evaluation of the findings of other imaging studies and o f the subsequent clinical course. Results: In 33 patients, tumor had metast asized, whereas 21 patients were in remission. FDG PET revealed metastases in 31 patients (sensitivity 93.9%), whereas thyroglobulin levels were eleva ted in 18 patients (sensitivity 54.5%). FDG PET was positive in 14 of 15 me tastatic cancer patients with normal thyroglobulin levels. In 20 of 21 pati ents in remission, FDG PET was negative (specificity 95.2%), whereas thyrog lobulin levels were normal in 16 patients (specificity 76.1%). The sensitiv ity and specificity of FDG PET were significantly higher than those of seru m thyroglobulin. In patients with negative I-131 scans, FDG PET detected ce rvical lymph node metastasis in 87.9%, lung metastasis in 27.3%, mediastina l metastasis in 33.3% and bone metastasis in 9.1%. In contrast, among 117 p atients with I-131 scan-positive functional metastases, I-131 scan detected cervical lymph node metastasis in 61.5%, lung metastasis in 56.4%, mediast inal metastasis in 22.2% and bone metastasis: in 16.2%. In all 5 patients i n whom thyroglobulin was false-negative with negative antithyroglobulin ant ibody, PET showed increased F-18-FDG uptake in cervical lymph nodes, medias tinal lymph nodes, or both. Among patients with increased F-18-FDG uptake o nly in the cervical lymph nodes, the nodes were dissected in 11. Metastasis was confirmed in all, even in normal-sized lymph nodes. Conclusion: FDG PE T scan localized metastatic sites in I-131 scan-negative thyroid carcinoma patients with high accuracy. In particular, it was superior to I-131 whole- body scan and serum thyroglobulin measurement for detecting metastases to c ervical lymph nodes. FDG PET was helpful for determining the surgical manag ement of these patients.