Presentation
History of thyroidectomy for high-risk papillary thyroid cancer 2 months prior. Underwent adjuvant therapy with 149 mCi of I-131 and presents 5 days after for diagnostic whole-body scan.
Patient Data
Whole-body I-131 scintigraphy
Whole-body planar images show increased radiotracer activity in the midline neck and mediastinum. Physiologic activity noted in the transverse and descending colon as well as bladder and nasal/oral cavity. No osseous or pulmonary radiotracer activity.
SPECT-CT demonstrates focal intense activity in the anterior midline infrahyoid neck compatible with occult residual thyroid tissue and/or thyroglossal duct cyst. Additionally, moderate diffuse radioactive iodine uptake in the anterior-superior mediastinum is present. This demonstrates an inverted v-shaped configuration typical of the thymus. On CT, the thymus shows normal anatomic appearance.
Case Discussion
While relatively rare, diffuse uptake of radioactive iodine by the thymus is a well reported normal variant. It is typically seen in younger individuals (such as in this case) as a sail or an inverted-v shape in the anterior superior mediastinum. It is believed that the iodine accumulates in the Hassall bodies of the thymus due to their similarity to thyroid follicles. If necessary, SPECT/CT imaging can be useful to confirm and exclude mediastinal lymph node metastases.
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