The next phase is the Central Nervous System (CNS) phase, and is usually given in the outpatient setting. Participants may need to be admitted to the hospital during this phase of treatment if a complication develops, such as infection. This phase of therapy begins immediately after the Consolidation I phase and lasts about 3 weeks. Treatment involves a series of lumbar punctures with anti-leukemia drugs given intrathecally over a two week period. Anti-leukemia drugs will also be given by mouth and by vein during this phase as well. Some participants may receive radiation therapy during this phase, although most do not. The decision to treat with radiation or not is based on characteristics of the cancer at diagnosis and whether or not cancer cells were seen in spinal fluid at that time. Radiation therapy is a painless procedure, the purpose of which is to prevent leukemia from coming back in the brain. For participants who receive radiation therapy, it will be given in either 8 or 10 daily treatments, depending on how many leukemia cells were seen in the spinal fluid under the microscope at diagnosis.