Isolated cerebellar metastasis as the initial presentation of follicular thyroid carcinoma: A rare neurosurgical encounter

Background: Brain metastases from follicular thyroid carcinoma (FTC) are exceptionally rare, especially in the infratentorial region. Even rarer is when such a lesion precedes the diagnosis of the primary tumor. This case highlights the importance of maintaining diagnostic vigilance for metastatic disease, even in patients with seemingly benign thyroid pathology. Case Description: A 55-year-old female presented with progressive imbalance and headache. Magnetic resonance imaging revealed a solid-cystic lesion in the cerebellar vermis compressing the fourth ventricle. She had a prior diagnosis of colloid goiter based on fine-needle aspiration cytology (FNAC). Intraoperatively, the lesion mimicked a hemangioblastoma; frozen section supported this diagnosis. However, final histopathology and immunohistochemistry confirmed metastatic FTC. Subsequent positron emission tomography-computed tomography and repeat thyroid FNAC confirmed the primary tumor, leading to total thyroidectomy and radioactive iodine therapy. Conclusion: This case is likely the first reported instance of isolated cerebellar metastasis from FTC, masquerading as a primary central nervous system tumor. Surgeons should maintain a high index of suspicion when encountering posterior fossa tumors with atypical features, even with a prior benign thyroid FNAC.