A 24-year-old man presents with a 3-week history of progressively enlarging jaw swelling, drenching night sweats, and a 10-kg unintentional weight loss. Physical examination reveals a firm, non-tender mass of the left mandible and multiple enlarged cervical lymph nodes. Laboratory studies show LDH of 1,200 U/L (normal 140–280), uric acid of 9.2 mg/dL (normal 3.5–7.2), and normal renal function. Excisional lymph node biopsy demonstrates a “starry sky” pattern of medium-sized B cells with 100% Ki-67 positivity. Staging imaging reveals bulky intra-abdominal lymphadenopathy but no overt central nervous system (CNS) lesions. Given the high proliferation index and risk of CNS involvement, which of the following is the most appropriate prophylactic strategy to reduce CNS relapse risk in this patient?