Incidental Popliteal Bursa Localization on I-131 Whole-Body Scintigraphy

I-131 scintigraphy can be performed after I-131 radioactive iodine (RAI) treatment in appropriate patients for dynamic risk stratification. Sodium iodide symporter (NIS) expression is a mechanism responsible for uptake of I-131 in thyroid tissue and thyroid cancer. There are other mechanisms that iodine engages in the body, leading to possible false-positive findings. A popliteal bursa, or Baker's cyst, is a bursa that lies in the popliteal region. As a bursa, it is a potential space and is lined by synovial membrane. It is not normally visualized on imaging. However, passive diffusion of radioactive iodine or pathological changes occur with local irritation. Presented herein is a 75-year-old woman with a history of poorly differentiated thyroid carcinoma. Her postoperative American Thyroid Association risk stratification was high. A diagnostic iodine whole body scintigraphy for dynamic risk stratification following RAI treatment performed. Planar imaging on posterior-view showed mildly intense iodine uptake in the left knee, differentials included atypical location of osseous metastasis, and false-positive pathologies. Regional SPECT/CT localized the false-positive iodine uptake in the Baker's cyst. The teaching point for this case is to be vigilant of uncommon false-positive pathologies on iodine scintigraphy. Diagnostic confidence is improved by adding SPECT/CT imaging.