False-Positive 131I Uptake in a Benign Bone Lesion on Post-therapy Scan.

A 56-year-old woman underwent near-total thyroidectomy and papillary thyroid carcinoma without extrathyroidal extension was diagnosed. The serum thyroglobulin (Tg) level was 2.4 µg/L, and anti-Tg was negative when serum thyroid-stimulating hormone level was 85 µIU/mL. She received 100 mCi (3.7 GBq) of 131I. Besides the residual thyroid tissue, a focal uptake in the left clavicular bone was seen on posttherapy 131I images. Then, CT and MRI were performed to diagnosis. All imaging findings suggested that it was a benign bone cyst. At 6-month follow-up, the serum Tg level was undetectable with the thyroid-stimulating hormone level of more than 150 µIU/mL.