Phase 1/2 single-arm pediatric trial adds nivolumab to DA‑EPOCH‑R for primary mediastinal (thymic) large B‑cell lymphoma. Drugs/mechanisms: Nivolumab (human IgG4 monoclonal antibody, PD‑1 immune checkpoint inhibitor) restores T‑cell antitumor activity, Rituximab (chimeric monoclonal antibody) targets CD20 to deplete malignant B cells, Etoposide (topoisomerase II inhibitor), Doxorubicin (anthracycline/topo II inhibitor, ROS generation), Vincristine (vinca alkaloid, microtubule inhibitor), Cyclophosphamide (alkylating DNA crosslinker), Prednisolone (glucocorticoid, lympholytic). Cells/pathways targeted: PD‑1/PD‑L1 axis on T cells/tumor microenvironment, CD20+ B‑cell lymphoma cells, DNA replication/repair and mitotic spindle machinery in rapidly dividing tumor cells, glucocorticoid receptor–mediated apoptosis of lymphoid cells.