Biopsy proven secondary CNS Lymphoma. 

Secondary CNS Lymphoma is commonly a non-Hodgkin's lymphoma and is secondary to lymphoma in other parts of the body.

Leptomeningeal enhancement is commoner in secondary lymphomas, as in this case (as compared to the parenchymal disease).

Lymphomas are usually hypercellular and display areas of restricted diffusion.

Differential diagnosis includes:  glioma, abscess, demyelinating lesions, metastasis, ependymitis and sarcoidosis.