Study of the Adverse Events and Change in Disease State of Pediatric Participants (and Young Adults Between the Ages of 18-25) With Relapsed/Refractory Aggressive Mature B-cell Neoplasms Receiving Subcutaneous (SC) Injections of Epcoritamab

The most common types of mature B-cell lymphomas (MBLs) in children are Burkitt lymphoma (BL) and diffuse large B-cell lymphoma (DLBCL). Initial treatment cures 90% - 95% of children with these malignancies, leaving a very small population of relapsed/refractory disease with a poor prognosis. The purpose of this study is to assess the safety and tolerability of epcoritamab in pediatric participants with relapsed/refractory aggressive mature B-cell neoplasms and young adult participants with Burkitt's or Burkitt-like lymphoma/leukemia. Adverse events and change in disease activity will be assessed. Epcoritamab is an investigational drug being developed for the treatment of relapsed/refractory aggressive mature B-cell neoplasms. Participants will receive subcutaneous (SC) of epcoritamab. Approximately 15 pediatric participants with a diagnosis of relapsed/refractory aggressive mature B-cell neoplasms and and young adult participants, ages of 18-25, with a diagnosis of Burkitt's or Burkitt-like lymphoma/leukemia will be enrolled at 50 sites globally. Participants will receive subcutaneous epcoritamab in 28-day cycles. Participants will be followed for a minimum of 3 years after enrollment. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at an approved institution (hospital or clinic). The effect of the treatment will be frequently checked by medical assessments, blood tests, questionnaires and side effects.

Inclusion Criteria: * Participants \>= 1 and \< 18 years old at time of primary diagnosis with Burkitt's or Burkitt-like lymphoma/leukemia, diffuse large B-cell lymphoma (DLBCL), or other aggressive mature (CD20+) B-cell lymphomas. Participants up to 25 years of age with Burkitt's or Burkitt-like lymphoma/leukemia are also eligible. * Disease pathologically confirmed (tumor tissue) by local testing. * Relapsed or primary refractory disease meeting any of the following criteria: * Progressive disease at any time during second-line chemoimmunotherapy (CIT). * Best response of stable disease (SD) after a minimum of 2 cycles of second-line CIT. * Best response of partial response (PR) after a minimum of 3 cycles of second-line CIT. * Complete Response (CR) after a minimum of 3 cycles of second-line CIT therapy but unfit or ineligible for consolidation with cell therapy. * Not in CR and unable to initiate or tolerate (i.e., must discontinue) second-line CIT. * Have received cell therapy (allogeneic or autologous transplant or chimeric antigen receptor T-cell (CAR-T) therapy) as consolidation but have not obtained or maintained a CR. * Recovery from toxic effects of prior chemoimmunotherapy. * Performance status by Lansky (\< 16 years old at evaluation) or Karnofsky (\>= 16 years old at evaluation) score \>= 50 or Eastern Cooperative Oncology Group (ECOG) score \