Thymic extension in the superior mediastinum in patients with thymic hyperplasia: potential cause of false-positive findings on 18F-FDG PET/CT.

OBJECTIVE: Although 18F-FDG PET/CT is now well established as an accurate method for the staging and restaging of various cancers, it is also well recognized that many false-positive results can occur. One such false-positive is activity within the superior extent of the thymus in the superior mediastinum.

CONCLUSION: We reviewed all PET/CT examinations performed in children and young adults under the age of 20 years at our institution over a 2-month period. In 11 (12%) of the 93 examinations, activity was identified in the superior mediastinum of similar intensity to the thymus, which is consistent with activity within the superior thymic extension. In light of the increasing clinical use of FDG PET/CT for cancer imaging, it is important that nuclear medicine physicians and radiologists be aware of this phenomenon to avoid misdiagnosis or overstaging of disease.