Follicular carcinoma of the thyroid presenting as metastasis in the wall of an arachnoid cyst.

The brain is an uncommon site for metastases of differentiated thyroid carcinoma with the most common location being cerebral hemispheres, followed by cerebellum and pituitary gland. Metastasis in the wall of an arachnoid cyst is exceedingly rare with single case report available in the published literature. Arachnoid cyst metastasis from an extraneuraxial malignancy has not been published until. We present a unique case of thyroid carcinoma metastasizing to the wall of an intracranial arachnoid cyst and the most interesting fact is that it was the first clinical manifestation of her malignancy.