Treatment for Primary Mediastinal (Thymic) Large B-Cell Lymphoma (PMBCL) involves chemotherapy combined with an immunotherapy called rituximab. This combination is also known as the chemo-immunotherapy backbone. There are two types of chemo-immunotherapy backbone treatments for PMBCL: either R-CHOP or DA-EPOCH-R.
   
These drugs effectively treat PMBCL, but are known to cause long-term side effects at high doses. Side effects are unintended and unwanted results of treatment. Study doctors are interested in finding an effective treatment that improves the outcome of PMBCL.
   
Nivolumab is a type of cancer treatment which uses the immune system to fight cancer. We will be using nivolumab with chemo-immunotherapy and comparing the results with only chemo-immunotherapy in newly diagnosed PMBCL. By doing this study, we are hoping that we can figure out the best way to treat PMBCL while also improving outcomes.
   
This study looks at how well the addition of nivolumab to chemo-immunotherapy works when given to children, adolescents, and adults with PMBCL. Nivolumab is experimental because it has not been proven to work in people with PMBCL.
   
Other goals of this study include understanding the biology and treatment of PMBCL.