Most patients being diagnosed with primary diffuse large B-cell lymphoma of the central nervous system (PCNSL) are 60 years or older. Elderly patients with PCNSL have a poor prognosis and there is a great medical need to improve outcome for this vulnerable population. In Germany and many international centres, there are currently two widely used strategies to treat elderly PCNSL patients who are eligible for high-dose methotrexate (HD-MTX) treatment, which have not yet been compared head-to-head. The R-MP regimen has been established by the Cooperative PCNSL Study Group as a "conventional" immunochemotherapy standard treatment for elderly patients with newly diagnosed disease and consists of Rituximab, HD-MTX and Procarbazine followed by maintenance therapy with Procarbazine. In contrast, another recently established protocol also includes HD-MTX-based induction therapy, but followed by consolidating high-dose chemotherapy and autologous stem cell transplantation (HCT-ASCT). This is an overall more intensive, but substantially shorter treatment approach, feasible for elderly patients being considered eligible for a more intensive treatment. The PRIMA-CNS trial aims to compare these two treatment approaches with respect to survival, response rates and toxicity.