Pulmonary sequestration: a rare cause of false-positive uptake in Iodine-131 whole-body scan

Background: High-dose radioactive iodine-131 (I-131) is generally used post-total or subtotal thyroidectomy in differentiated thyroid cancer (DTC) patients. I-131 whole-body scan (WBS) is performed to localize I-131 accumulation in the body and for restaging after therapy. Nonetheless, there are numerous causes of false-positive uptake in I-131 WBS in the absence of residual thyroid tissue or thyroid cancer metastasis. Case presentation: We present a case of 51-year-old lady with false-positive uptake in I-131 WBS mimicking lung metastasis. Patient underwent total thyroidectomy, and histopathological examination showed well-differentiated papillary thyroid microcarcinoma. Diagnostic I-131 WBS followed by two subsequent high-dose radioiodine ablation showed persistent I-131uptake in the left lower lung field. However, the serum thyroglobulin (Tg) is incessantly undetectable (< 0.04 mu g/ml) with negative anti-thyroglobulin antibody (anti-TgAb). Three serial CT scans over the course of treatment coupled with MRI of the thoracolumbar region revealed stable lung lesion in the posterior segment of left lower lobe which signifies pulmonary sequestration. Based on the clinical assessment, imaging findings and blood investigations, patient was no longer subjected to I-131 therapy and is maintained on TSH suppression with L-thyroxine. After due consideration of the pros and cons of surgical intervention, the patient opted against surgical removal of the pulmonary sequestration. Conclusions: Meticulous assessment of patient with suspected disease recurrence or metastasis in thyroid carcinoma is essential to avoid unnecessary I-131 ablation or surgical intervention. In this case, the positive I-131 imaging is associated with undetectable serum Tg (with negative anti-TgAb). Albeit uncommon, pulmonary sequestration should be taken into consideration as one of the differentials in false-positive findings of an I-131 WBS.