Positron emission tomography for detecting iodine-131 nonvisualized metastasis of well-differentiated thyroid carcinoma: two case reports.

To investigate the usefulness of positron emission tomography (PET) in detecting I-131 nonvisualized metastatic foci of well-differentiated thyroid carcinoma (WDTC), 2 patients with papillary and follicular thyroid carcinoma respectively, were studied with I-131 total body scan, thallium-201 scan, Tc99m bone scan and [18F]-2-deoxy-2-fluoro-D-glucose (FDG) PET. Case 1 showed no metastatic lesion in I-131 (up to 150 mCi) total body scan, 1 anterior mass in thallium-201 scan, none in Tc99m bone scan and 7 including the main anterior mediastinal mass in FDG-PET. Case 2 showed 2 metastatic lesions in I-131 (150 mCi) total body scan, 11 bony metastatic lesions in Tc99m bone scan and 13 in FDG-PET. However, lower extremities were not scanned in FDG-PET. Tumor/background ratio of 1.5 or above is needed to be visualized grossly. The FDG-PET tumor/background ratios are higher than those of thallium-201 except in one site. In conclusion, I-131 scintigraphy is still the first line method to use in detecting WDTC recurrence and metastasis, as I-131 has the advantage of being both a therapeutic and imaging agent. For I-131 nonvisualized metastasis of WDTC, thallium scintigraphy and FDG-PET may be considered. Even though FDG-PET has better sensitivity, resolution imaging and spatial localization, this has to be balanced with its higher cost when compared with thallium scintigraphy.