This is an open-label, multi-center, prospective, single arm phase 2 trial of the combination of bendamustine and rituximab in patients with PTLD, monomorphic cluster of differentiation antigen 20(CD20) positive DLBCL. The investigators want to investigate the efficacy and safety of the combination of bendamustine and rituximab in patients with previously untreated PTLD, monomorphic CD20 (+) diffuse large B-cell lymphoma.