An intrapericardial ectopic thyroid mimicking metastasis in a patient with papillary thyroid cancer: Localization, differential diagnosis by 18F-FDG PET/CT and ablation by 131I.

We report a very rare case of incidental intrapericardial thyroid in a papillary thyroid cancer patient. Post ablation scan revealed iodine-131 (131I) uptake in the mid-chest. Fluorine-18-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) was performed and showed a 18F-FDG avid lesion between the right atrium and the ascending aorta, (which was) shown to be an ectopic thyroid and not metastasis. The lesion disappeared on a 6 month follow-up 123I whole body scan while serum thyroglobulin was negative. Although intrapericardial ectopic thyroid is reported to show high iodine uptake, low 18F-FDG avidity of the lesion could be helpful in the exclusion of metastases.