Primary Mediastinal Lymphoma

This compares the effects of nivolumab at a fixed dose of 40 mg with chemo-immunotherapy versus chemo-immunotherapy alone in treating patients with newly diagnosed primary mediastinal B-cell lymphoma (PMBCL). Immunotherapy with monoclonal antibodies, such as nivolumab, may help the body's immune system attack the cancer, and may interfere with the ability of cancer cells to grow and spread. Treatment for PMBCL involves chemotherapy combined with an immunotherapy called rituximab. Chemotherapy drugs work in different ways to stop the growth of cancer cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Rituximab is a monoclonal antibody. It binds to a protein called CD20, which is found on B cells (a type of white blood cell) and some types of cancer cells. This may help the immune system kill cancer cells. Giving nivolumab with chemo-immunotherapy may help treat patients with PMBCL.

The aim of the trial is to evaluate the molecular characteristics and MDD/MRD of B-NHL in pediatric patients in order to identify on the one hand the very high risk group and to prescribe them more intensive treatment on the other hand to identify those patients who don't need very aggressive therapy. One more study question is to evaluate the role of PET/CT in assessment of the completeness of remission. The following primary study questions are going to be analyzed: * the effectiveness (event-free survival) in pediatric patients with very limited mature B-NHL (R1 - stage I and II R) of substituting anthracyclines and vincristine by the rituximab without compromising survival rates. * the effectiveness (event-free survival) in pediatric patients with limited mature B-NHL (R2 - stage I and II NR) of substituting anthracyclines by the rituximab without compromising survival rates. * the effectiveness (event-free survival) in pediatric patients with advanced VHR mature B-NHL (R4 - stages with unfavourable genetics of substituting standard chemotherapy by "second-line" block VICI in order to improve results Secondary study questions will address * additional parameters for immune reconstitution, lymphocyte subpopulations, immunoglobulin levels, vaccination titers and infection rates * kinetics of immune reconstitution after treatment