Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Primary central nervous system lymphoma ?
    ### Response:
    Primary central nervous system lymphoma (primary CNS lymphoma) is a rare form of non-Hodgkin lymphoma in which cancerous cells develop in the lymph tissue of the brain and/or spinal cord. Because the eye is so close to the brain, primary CNS lymphoma can also start in the eye (called ocular lymphoma).
    