An unusual false-positive scan in a patient with pericardial effusion.

Uptake of I-131 in the pericardial area is described in an asymptomatic patient who underwent total body scan for recurrent papillary thyroid cancer. Ultrasonography demonstrated a small pericardial effusion that persisted after I-thyroxine therapy was reinstituted. Two I-131 therapeutic doses were given, and follow-up total body scans were performed during the next 6 years. Although tracer accumulation in the neck was eliminated and the serum thyroglobulin level was not elevated, I-131 uptake persisted in pericardial effusion. Despite diligent study, no neoplastic, infectious, or autoimmune etiology could be demonstrated, and we thus classified it as idiopathic pericardial effusion. This phenomenon should be considered when interpreting I-131 scans that show I-131 uptake in the region of the heart.