In this study, the investigators propose that the addition of rituximab will lower the risk of systemic and local relapses in patients with localized PBL. Patients will be treated with 6 cycles of RCHOP-14 or RCHOP-21. The administration of radiotherapy following chemotherapy is strongly recommended, based on the findings of the retrospective IELSG study, but will be at the discretion of the treating center.