Multifocal Nonmetastatic Radioactive Iodine Avidity on Whole Body Scan After Thyroidectomy for Thyroid Cancer

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Background/Objective: Non-metastatic radioactive iodine (RAI) uptake can complicate the interpretation of whole-body scan (WBS) for differentiated thyroid carcinoma (DTC) post-thyroidectomy. We present a patient with DTC whose follow-up WBS showed nonmetastatic multifocal avidity in skeletal tissue, an uncommonly reported site of RAI uptake. Case report: A 42-year-old woman underwent a right hemithyroidectomy, followed by completion thyroidectomy and RAI remnant ablation therapy, for a 4.8 cm thyroid tumor consistent with stage pT3aNxMx follicular thyroid cancer. Follow-up WBS showed intense activity in the thyroid bed, right breast, left medial subcortical acetabulum, and several vertebral bodies. Her biochemical and clinical findings were not suggestive of cancer recurrence. Further workup with SPECT/CT and MRI showed no focal vertebral lesions and identified the left femoral lesion as a benign peripheral nerve sheath. Diagnostic mammography and ultrasound showed no evidence of suspicious breast lesions. Neck ultrasound was clear with no suspicious masses or pathologic lymphadenopathy. She remained in remission on continued active surveillance. Discussion: Nonmetastatic RAI uptake on WBS has many causes, including functional sodium-iodide symporter expression in nonthyroidal tissues, radioiodine accumulation in tissues and bodily fluids, and benign tumors. False-positive uptake can decrease the utility of post-treatment WBS in low-risk patients. Careful clinical examination, biochemical and radiologic follow-up, and close active surveillance can help distinguish false-positive uptake from metastatic or recurrent disease. Conclusion: We describe an uncommon case of RAI uptake in skeletal tissues after thyroidectomy for DTC, and we outline the steps taken to rule out underlying metastases.

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