Follicular Thyroid Carcinoma with Cavernous Sinus Metastasis and Perineural Spread: A Rare Manifestation of Radioiodine-refractory Disease.

Cavernous sinus metastasis from follicular thyroid carcinoma (FTC) is exceedingly rare and typically indicates advanced, radioiodine-refractory disease. We report a 37-year-old man with minimally invasive FTC who was treated with total thyroidectomy, neck dissection, and cumulative radioiodine therapy totaling 720 mCi for persistent iodine-avid disease. Four years later, he developed progressive left-sided palsies of cranial nerves III, IV, and VI. Imaging revealed a cavernous sinus mass with orbital apex extension and perineural spread. Serum thyroglobulin was markedly elevated (>500 ng/mL), whereas diagnostic radioiodine scintigraphy demonstrated no uptake, a finding consistent with dedifferentiated disease. Biopsy confirmed metastatic FTC. The patient received external beam radiotherapy and systemic therapy with lenvatinib; however, the disease progressed, and he died 16 months after the diagnosis of cavernous sinus metastasis. This case highlights the aggressive potential of FTC, the limitations of radioiodine imaging in dedifferentiated disease, and the poor prognosis associated with skull-base metastases despite multimodal therapy.