I-131 Avid Tumor Thrombus in a Case of Poorly Differentiated Thyroid Cancer.

Intravenous tumor extension is a well-recognized phenomenon occurring in various malignancies but is a relatively rare entity in thyroid carcinoma. In patients with poorly differentiated thyroid cancer (pDTC), I-131 avid superior vena cava tumor (SVC) thrombus at initial presentation is infrequent and potential life threatening. Tumor thrombus can form either due to direct vascular extension of the primary mass or by hematogenous spread. Hybrid nuclear imaging can differentiate the two entities, which can impact the treatment plan of the patient. We present images of an interesting case of evolution of SVC thrombus in a 46-year-old woman with diagnosed pDTC over the span of two years.