Incidental Focal Thyroid Uptake on FDG Positron Emission Tomographic Scans May Represent a Second Primary Tumor

Purpose This article discusses the value of incidental thyroid uptake of fluorodeoxyglucose (FDG) on whole-body positron emission tomography (PET) scans performed in patients with cancer. This issue has been reported diversely in the literature. Materials and Methods Whole-body PET scans of four patients with cancer (two of them women; age, 49 to 78 years) in whom focal thyroid uptake was visualized and subsequently correlated with thyroid carcinoma based on cytologic or histopathologic data were reviewed. The clinical outcomes of these patients were analyzed retrospectively. Maximum and average standardized uptake values (SUVs) of thyroid accumulation were recalculated in all patients. Results Two of the four patients referred for FDG PET scans had lung adenocarcinomas, one had prostrate carcinoma, and one had an unknown primary tumor. Focal thyroid uptake was visualized, with maximum and average SUVs ranging from 3.7 and 2.3 to 53 and 34, respectively. These findings were correlated with cytologic (two patients) or histopathologic data (two patients) that indicated thyroid carcinoma. In two patients, their treatment was changed and total thyroidectomy was performed; in one of them the SUVs of the focal thyroid accumulation (maximum and average values of 7.9 and 4.8, respectively) were less than the cutoff values for thyroid cancer noted in the literature. The clinical condition of the other two patients did not permit additional investigation or treatment for thyroid carcinoma. Conclusion Increased focal thyroid uptake on whole-body FDG PET scans should not be overlooked, even when it is not marked, and should prompt further investigation to rule out cancer.