Primary Mediastinal B-Cell Lymphoma in Children and Young Adults.

Primary mediastinal B-cell lymphoma (PMBCL) is a rare but aggressive mature B-cell lymphoma that arises from thymic B cells and most commonly affects adolescents and young adults. PMBCL is now recognized by the WHO as a distinct entity from diffuse large B-cell lymphoma (DLBCL), not otherwise specified, with a unique clinical presentation and distinct morphologic features and molecular alterations. Similar to classic Hodgkin lymphoma, PMBCL tumors are characterized by alterations in the nuclear factor-κB and JAK/STAT pathways. These tumors also exhibit an immune evasion phenotype marked by upregulation of PD-L1 and loss of B2M. Historic data indicates that outcomes for pediatric patients with PMBCL are inferior compared with pediatric patients with DLBCL treated on the same protocols, and there is no current standard approach to initial treatment. Common regimens used for children with PMBCL include multiagent chemotherapy regimens designed for Burkitt lymphoma, such as Lymphomes Malins B (LMB)-based or Berlin-Frankfurt-Münster (BFM)-based chemotherapy ± rituximab. Based on initial data in adults showing excellent outcomes with the use of DA-EPOCH-R regimens, these regimens have also been adopted in pediatrics, although with mixed results. Novel agents are currently being studied in PMBCL with the goal of improving outcomes and reducing reliance on radiation and/or high-dose chemotherapy. Immune checkpoint blockade with PD-1 inhibition is of particular interest given the upregulation of PD-L1 in PMBCL and the known efficacy of these agents in the relapsed setting. Future efforts in PMBCL will also seek to determine the role of FDG-PET in evaluating response to therapy and the role of biomarkers in risk stratification.