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 Presentation: A 55-year-old female presented with progressive imbalance and headache. MRI 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 PET-CT and repeat thyroid FNAC confirmed the primary tumor, leading to total thyroidectomy and radioactive iodine (RAI) therapy. Conclusion This case is likely the first reported instance of isolated cerebellar metastasis from FTC, masquerading as a primary CNS tumor. Surgeons should maintain a high index of suspicion when encountering posterior fossa tumors with atypical features, even with a prior benign thyroid FNAC.