R-MA-PD-1 Versus R-MA in Treatment-naive Primary CNS Lymphoma

Primary central nervous system lymphoma is a rare, aggressive non-Hodgkin lymphoma, predominantly diffuse large B-cell lymphoma. High-dose methotrexate-based chemotherapy is the standard induction, but the optimal combination remains to be defined. Building on a prior single-arm phase II study of penpulimab plus R-MA (NCT05347641), this randomized controlled study evaluates whether adding the PD-1 inhibitor penpulimab to R-MA improves efficacy in treatment-naive PCNSL. Induction (both arms, every 21 days x 6 cycles): * Rituximab 375 mg/m2 (Day 0) * Methotrexate 3.5 g/m2 (Day 1) * Cytarabine (Ara-C): age \<60 or ECOG ≤2: 2 g/m2 q12h Day 2-3; age ≥60 or ECOG \>2: 1 g/m2 q12h Day 2 (from cycle 2) * Experimental arm only: Penpulimab 200 mg (Day 5) Consolidation (response- and age-adapted): patients \<60 years achieving PR/CR proceed to autologous stem cell transplantation (thiotepa + busulfan); patients ≥60 years or ASCT-ineligible receive penpulimab maintenance (CR) or whole-brain radiotherapy 36 Gy plus penpulimab maintenance (PR). Experimental-arm responders receive 8 cycles of penpulimab maintenance. Sample size: 58 participants (29 vs 29), based on 2-year PFS of 39% (R-MA) versus 70% (R-MA-PD-1), two-sided alpha 0.05, power 80%, 18-month enrollment, 30-month follow-up, 10% dropout (PASS 15.0).