Background: After thyroidectomy for differentiated thyroid carcinoma, extracervical uptake of iodine 131 is suggestive of metastasic dissemination. False positives can however occur.
Case report: Differentiated thyroid carcinoma was found in a female patient with a non-functional nodule. Two years after subtotal thyroidectomy and an ablative dose of iodine 131, the whole body scan showed abdominal mediastinal uptake with low serum thyroglobulin level. Considering the possibility of tumor recurrence or lymph node metastasis, the mass was excised. The histology diagnosis was mediastinal bronchogenic cyst.
Discussion: To date, iodine uptake in a bronchogenic cyst has not been reported among other false-positives previously described. The pathophysiology of this iodine 131 uptake in a bronchogenic cyst is still unknown: the presence of NIS symporter or a protein which can organify iodine in the mucus cells of the cyst remains to be proven.