A type of treatment that uses intrathecal chemotherapy (chemotherapy injected directly into the fluid-filled space between the thin layers of tissue that cover the brain and spinal cord) or high-dose chemotherapy injected into a vein to kill cancer cells in the central nervous system (CNS). Radiation therapy is also sometimes used. CNS prophylaxis is given when the cancer is diagnosed because standard chemotherapy cannot get into the CNS and kill cancer cells that may be hiding in there. It is used to lessen the chance cancer will come back in the CNS after the first treatment. CNS prophylaxis is used with other therapy to treat certain types of leukemia and lymphoma that may spread to the CNS. Also called central nervous system prophylaxis.