Fluorine-18 fluorodeoxyglucose positron emission tomography and iodine-131 whole-body scintigraphy in the follow-up of differentiated thyroid cancer

Metastases of differentiated thyroid cancer may show different uptake patterns for fluorine-18 fluorodeoxyglucose and [I-131]NaI. FDG positron emission tomography (PET), iodine-131 whole-body scintigraphy (I-131 WBS) and magnetic resonance imaging were performed in 58 unselected patients, and spiral computed tomography (CT) of the lung in 25 patients. Thirty-eight patients presented with papillary carcinomas, 15 patients with follicular carcinomas and five patients with variants of follicular carcinoma. Primary tumour stage (pT) was pT1 in 3, pT2 in 19, pT3 in 11 and pT4 in 25 cases. For the detection of metastases, FDG PET was found to have a sensitivity of 50%, I-131 WBS a sensitivity of 61%, and the two methods combined a sensitivity of 86%. When FDG PET was limited to patients with elevated thyroglobulin (Tg) levels and negative I-131 WBS, the sensitivity of this algorithm was 82%. Of the 21 patients with lymph node metastases. seven presented with FDG uptake but no iodine uptake. In four of them, a second FDG hot spot appeared in a lymph node metastasis of normal size. Five of the seven patients underwent surgery. None of the eight patients with pulmonary metastases smaller than 1 cm exhibited FDG uptake, while five of them had iodine uptake, All had positive results on spiral CT. In conclusion, FDG PET cannot be substituted for I-131 WBS. If the Tg level is elevated and I-131 WBS is negative, FDG PET can be used to detect lymph node metastases and complements anatomical imaging. A spiral CT of the lung is useful to exclude pulmonary metastases before planning a dissection of iodine-negative lymph node metastases.