Background
Radioactive iodine whole-body scintigraphy (RAI WBS) is important in the diagnostic workup and treatment of patients with differentiated thyroid cancer. Accurate interpretation of these studies includes a comprehensive knowledge of clinical history, histopathologic correlation, serum thyroglobulin levels, and available additional imaging findings. Numerous false-positive findings on RAI WBS have been described. Concurrent SPECT imaging helps in cases of false-positive conclusions by accurate anatomic localization and characterization. Correlation with prior available imaging also helps in correct interpretation, especially in discordant findings of low serum thyroglobulin level but positive RAI WBS.
Educational Goals / Teaching Points
Review false-positive uptake on radioactive iodine whole-body scintigraphy studies, causes and mechanisms of false positive radioactive iodine uptake, and false positive uptake with hemangiomas on radioactive iodine whole-body scintigraphy.
Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
A 29 year old woman with history of known multinodular goiter and status post partial thyroid resection several years ago in another country, presented with recurrent compressive symptoms. Ultrasound thyroid showed multiple nodules involving bilateral thyroid lobes and isthmus measuring up to 3.5 cm in size. The patient subsequently underwent complete thyroidectomy, and histopathology showed encapsulated follicular variant of papillary thyroid carcinoma. Staging (AJCC 7th Ed) was pT2 (m) N0, at least stage I. Molecular studies were negative for BRAF mutation. Her postoperative thyroglobulin level was 44.1 (normal reference range: 1.60 - 50.00 ng/mL) with undetectable antibody. Neck ultrasound examination did not reveal any sonographically abnormal lymph nodes, and chest CT was negative for metastatic disease. I -123 pretherapy WBS images showed RAI uptake of 2.41% at 24 hours in the thyroid remnant in the neck. Intense foci of RAI uptake were also noted in the right calvarium, thoracic spine and sacrum. On correlation with SPECT/CT and prior head CT and spine MRI, these lesions were highly likely suggestive of hemangiomas. She received 150 mCi adjuvant radioactive iodine therapy. I -131 posttherapy WBS images showed similar radiotracer uptake in the neck and in the known hemangiomas.
Conclusion
Accurate interpretation of RAI WBS is important for appropriate follow-up, treatment, and management of patients with differentiated thyroid cancer. It is essential to be aware of various causes and mechanisms for false-positive radioiodine uptake on RAI WBS images for accurate interpretation, especially in discordant studies with thyroglobulin and positive RAI WBS. Correlation and exact localization of radioiodine uptake using hybrid imaging, SPECT/CT, or available additional imaging, helps to avoid pitfalls and false-positive interpretations of RAI WBS.
