Mismatch of F-18 Fluorodeoxyglucose (FDG) Positron Emission Tomography (PET) and Tc-99m Pertechnetate Single Photon Emission Computed Tomography (SPECT) in a Euthyroid Multinodular Goiter

Imaging results of F-18 fluorodeoxyglucose (FDG) PET/CT scanning and Tc-99m pertechnetate scintigraphy of the thyroid gland are described and compared with pathology in a patient who was followed after left nephrectomy for renal cell carcinoma diagnosed 10 years earlier. On F-18 FDG PET/CT scanning, a multinodular struma with increased localized F-18 FDG uptake in 4 nodules was seen. Two nodules with increased glucose metabolism appeared normal on Tc-99m pertechnetate scintigraphy. Pathology indicated hyperplastic nodules. High focal F-18 FDG uptake was also seen in a lesion that corresponded with a "cold" nodule on Tc-99m pertechnetate scintigraphy, suggesting malignant disease. However, pathology revealed hyperplastic nodules with a background of aspecific lymphocytic thyroiditis. A fourth nodule with increased F-18 FDG uptake appeared mixed ("cold"/"hot") on Tc-99m pertechnetate scintigraphy. On pathology, a well-differentiated follicular carcinoma was found. These findings, in a single patient, illustrate the wide spectrum of matched and mismatched F-18 FDG and Tc-99m pertechnetate thyroid uptake along with their variable pathologic correlates.