(6.06) Intracranial Meningioma with Radioactive Iodine Uptake Mimicking Brain Metastasis from Papillary Thyroid Carcinoma

Brain metastasis from papillary thyroid cancer (PTC) is rare, and usually indicates poor prognosis. Meningiomas can exhibit iodine avidity leading to diagnostic uncertainty in patients with co-existing PTC.

Case(s) Description :

A 58-year-old woman with T3bN1b classic PTC who recently underwent total thyroidectomy and lymph node dissection presented to the hospital with syncope and resultant head injury one day after administration of 1.95 mCi of radioactive iodine (RAI) for whole body scanning (WBS) to assess disease extent under Thyrogen stimulation. Evaluation with head CT revealed a 1.7 cm right frontal lobe mass with a large area of vasogenic edema. The patient underwent the planned RAI WBS which showed residual thyroid bed and lateral neck disease, in addition to an iodine-avid right frontal intracranial lesion corresponding to the lesion seen on CT head, concerning for iodine avid brain metastasis. A subsequent brain MRI again showed this 1.7 cm dural-based right frontal mass with associated calcification and underlying bony hyperostosis corresponding to the area of iodine avidity. Given her locally advanced PTC (with extrathyroidal extension, lymph node metastases) and iodine avid frontal lobe lesion, there was high concern for a metastatic intracranial lesion. However, the underlying bony hyperostosis with associated calcification raised the possibility of meningioma. After careful consideration of the two possible diagnoses, the decision was made to pursue surgical resection of the right frontal intracranial mass. Histopathology confirmed a meningioma, with negative immunostaining for thyroid transcription factor-1 (TTF-1) further confirming the non-thyroid origin. The patient later underwent adjuvant treatment with 146 mCi of RAI, and post-therapy WBS no longer showed iodine uptake in the brain when compared to the initial staging scans.

Discussion :

Meningioma can mimic intracranial metastasis from thyroid cancer by exhibiting RAI avidity. The mechanism of RAI uptake in meningiomas is thought to be through brain edema or high vascularity, resulting in stasis of radioiodinated blood. Awareness of non-thyroidal RAI uptake such as in this case of PTC with intracranial meningioma is crucial to accurately interpret RAI uptake scan results and guide staging and clinical management of patients with differentiated thyroid cancer. Surgical resection and stereotactic radiosurgery are both considered effective therapies for brain metastases from thyroid cancer, as well as for meningiomas. In this case, the possibility of the two differential diagnoses (metastatic brain lesion from PTC vs. meningioma) was carefully considered and in light of the significant brain edema, the decision was made to pursue surgical resection to establish a histopathologic diagnosis and for definitive treatment of the lesion.