- Concurrent chemotherapy to control systemic disease or bulk CNS disease allowed if the systemic chemotherapy is not a phase I study agent that significantly penetrates the CSF (e.g., high-dose systemic methotrexate [greater than 1 g/m^2], thiotepa, high-dose cytarabine, temozolomide, IV mercaptopurine, nitrosourea, or topotecan) or an agent known to have serious unpredictable CNS side effects