RecruitingPhase 2Drug trialSintilimab Plus Rituximab Followed by R-CHOP Regimen in Untreated PMBLThe goal of this clinical study is to evaluate the efficacy and safety of sintilimab combined with rituximab followed by R-CHOP regimen in treatment-naïve patients with primary mediastinal diffuse large B-cell lymphoma. The main questions it aims to answer are: 1. Objective response rate of sintilimab combined with rituximab 2. Objective response rate after R-CHOP regimen