Central nervous system lymphoma is a rare non-Hodgkin lymphoma in which malignant (cancer) cells from lymph tissue form in the brain and/or spinal cord (primary CNS) or spread from other parts of the body to the brain and/or spinal cord (secondary CNS). Because the eye is so close to the brain, primary CNS lymphoma can also start in the eye (called ocular lymphoma). The cancer can also involve the spinal fluid that bathes the spinal cord and brain. This is called leptomeningeal lymphoma. See Primary Central Nervous System Lymphoma for signs, symptoms and treatments.