Sintilimab Plus Rituximab Followed by R-CHOP Regimen in Untreated PMBL

First strategy of targeted immunotherapy followed by chemotherapy for PMBL. Participants will first receive sintilimab(200mg) plus rituximab(375mg/m2) for 2 cycles. If patients achieve CR or CMR, they will continue to receive 2 courses of sintilimab plus rituximab, followed by 4 cycles of R-CHOP. Patients will receive 6 courses of R-CHOP if they do not achieve CR or CMR after 2 cycles of sintilimab plus rituximab. Patients will undergo PET-CT assessment and will receive 6 cycles of maintenance treatment with nintedanib if CR/CMR/PR is achieved and will be withdrawn from the study if SD/PD occurs. Total treatment cycles will not exceed 14 cycles.