Bronchogenic cyst mimicking metastatic disease on post-radioiodine scintigraphy in a patient with papillary thyroid carcinoma.

This case describes a 16-year-old female with papillary thyroid carcinoma who demonstrated persistent iodine uptake in the mediastinum and a pulmonary lesion on post-therapeutic scintigraphy despite an excellent biochemical response. Although fluorine-18 fluorodeoxyglucose positron emission tomography/computed tomography ([¹⁸F]FDG PET/CT) suggested a benign process, persistent uptake and rising anti-thyroglobulin antibody levels prompted repeat radioiodine therapy and thoracotomy. Histopathological examination revealed a bronchogenic cyst and a vascular malformation, with no evidence of metastatic disease. This case highlights the diagnostic challenge posed by benign thoracic lesions exhibiting iodine uptake. Recognition of such false-positive findings is essential to avoid overtreatment in patients with thyroid cancer.