Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Primary CNS Lymphoma ?
    ### Response:
    Key Points
                    - Primary central nervous system (CNS) lymphoma is a disease in which malignant (cancer) cells form in the lymph tissue of the brain and/or spinal cord.    - Having a weakened immune system may increase the risk of developing primary CNS lymphoma.
    