To Determine the Dose of BI 836826-GemOx and the Efficacy of BI 836826-GemOx Versus R-GemOx in Patients With Relapsed/Refractory DLBCL

Inclusion criteria: * Age 18 years or older * Patients with histologically confirmed, relapsed/refractory, diffuse large B-cell lymphoma (including transformed follicular lymphoma) who have received an anti-CD20-supplemented, anthracycline-containing chemotherapy and are not eligible for high dose therapy followed by an autologous stem cell transplant, or have relapsed/progressed after autologous/allogenic stem cell transplant. Allogenic stem cell transplant performed at least 6 months prior to study entry is allowed if patients do not require immunosuppressive treatment and have no evidence of active graft-versus-host disease. * Patient has not received anti-lymphoma treatment prior to the first dose of trial medication: within past 14 days or within time that is shorter or equal to 5 half-lives of the drug if the last anti-lymphoma treatment contained an investigational agent * Screening computer tomography (CT) scan with involvement of at least 1 bi-dimensional lesion/node \>1.5 cm * Screening \[18F\] fluorodeoxyglucose (FDG)- positron emission tomography (PET) scans must demonstrate positive lesion compatible with computer tomography (CT) defined anatomical tumor sites * Eastern Cooperative Oncology Group (ECOG) performance status 0, 1, 2 * Written signed informed consent consistent with International Conference on Harmonization (ICH) Good Clinical Practice (GCP) and local legislation * Patients must have an acceptable organ function * Women of childbearing potential mus…