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The European Intergroup for Childhood Non-Hodgkin Lymphoma (EICNHL) was established 25 years ago with the goal to facilitate clinical trials and research collaborations in the field both within Europe and worldwide. Since its inception, much progress has been made whereby major improvements in outcomes have been achieved. In this Review, we describe the different diagnostic entities of non-Hodgkin lymphoma in children and young adults describing key features of each entity and outlining clinical achievements made in the context of the EICNHL framework. Furthermore, we provide an overview of advances in biopathology with an emphasis on the role of biological studies and how they have shaped available treatments. Finally, for each entity, we describe future goals, upcoming clinical trials, and highlight areas of research that require our focus going forward., Competing Interests: Declaration of interests LB has sat on advisory boards for the BrigaPed study for Takeda and has received support from Bristol Myers Squibb for the nivoALCL clinical trial. GAAB has received institutional payments for consultancies from Novartis, Roche, Takeda, straZeneca, and Janssen. AA has sat on a board for MSD. All other authors have no competing interests., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

The majority of lymphoblastic (precursor cell) neoplasms presents as leukemias. Consequently, the guidelines for lineage determination and subtyping of precursor cell neoplasms were primarily established for flow cytometry methods. Large-scale studies of nonleukemic lymphoblastic lymphomas are lacking so far. We analyzed a large series of pediatric patients with lymphoblastic lymphoma treated within a prospective randomized trial (the Euro-LB 02 study). Among 193 lymphomas, in which a detailed immunohistochemical analysis was carried out, there were several unusual and diagnostically challenging morphologic and immunophenotypical variants. These included 11 lymphomas with mixed phenotypes expressing markers of at least 2 hematopoietic lineages, 7 terminal deoxynucleotide transferase-negative lymphoblastic lymphomas, and 3 undifferentiated hematopoietic neoplasms that could not be assigned to any lineage with certainty. Our data indicate that World Health Organization guidelines for lineage determination and subtyping of precursor cell leukemia need to be adapted before they can be applied to immunohistochemical diagnosis of lymphoma. Using the experience from this cohort we suggest a resource-saving diagnostic staining panel for the immunohistochemical analysis of precursor cell neoplasms in formalin-fixed paraffin-embedded tissue.