Eight months after initial I-131 ablative therapy for papillary carcinoma in a 22-year-old woman, a second I-131 treatment was performed and a post-therapeutic study demonstrated increased activity in the upper mediastinum. Mediastinal uptake was not thought to represent a metastasis because of the low thyroglobulin level. Thymic origin was suspected on dual isotope SPECT I-131 studies and Tc-99m MDP and confirmed on a CT scan. Visualization of the thymus is not common during I-131 post-therapeutic imaging and dual-isotope SPECT is a useful tool to identify uptake at this site.
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