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

Inclusion Criteria:

1. 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.
2. Disease pathologically confirmed (tumor tissue) by local testing.

Relapsed or primary refractory disease meeting any of the following criteria:

1. Progressive disease at any time during second-line chemoimmunotherapy (CIT).
2. Best response of stable disease (SD) after a minimum of 2 cycles of second-line CIT.
3. Best response of partial response (PR) after a minimum of 3 cycles of second-line CIT.
4. Complete Response (CR) after a minimum of 3 cycles of second-line CIT therapy but unfit or ineligible for consolidation with cell therapy.
5. Not in CR and unable to initiate or tolerate (i.e., must discontinue) second-line CIT.
6. 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.
7. Recovery from toxic effects of prior chemoimmunotherapy.
8. 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 <= 2 .
9. Adequate bone marrow, hepatic, and renal function.

Exclusion Criteria:

1. Known central nervous system (CNS) involvement by lymphoma at screening as confirmed by screening magnetic resonance imaging (MRI)/computed tomography (CT)/positron emission tomography (PET) brain scans (participants with evidence of CNS disease only in the cerebrospinal fluid (CSF) will be eligible).
2. Other malignancy requiring therapy.
3. Currently receiving anti-cancer therapy, including chemotherapy (excluding intrathecal therapy), radiotherapy, small molecules, monoclonal antibodies, cell therapy, or other investigational agents.