Could a Bronchogenic Cyst Present as an I-131-avid, 99mTc-Non-Avid Lesion and Mimic Metastatic Thyroid Cancer?

A 54-year-old man with papillary thyroid carcinoma after thyroidectomy underwent a 99mTc thyroid scan showing no thyroid remnants but a 3.5-cm left lung lesion with faint 99mTc uptake on SPECT/CT, suggesting metastasis. Post-I-131 therapy scans on days 3 and 7 revealed intense I-131 avidity in the lung lesion. Surgical excision confirmed a benign bronchogenic cyst, negative for thyroglobulin, TTF-1, BRAF V600E, and PAX-8. I-131 uptake was likely NIS-mediated, but independent of PAX-8 or due to mucinous content. This case underscores that bronchogenic cysts should be considered in the differential diagnosis of I-131-avid lesions, with tissue confirmation essential to avoid misdiagnosis.