Fifth edition WHO classification: precursor lymphoid neoplasms, acute leukaemias of mixed or ambiguous lineage, myeloid/lymphoid neoplasms, and histiocytic and dendritic cell neoplasms, including strategies for application in resource-limited settings.

Abstract

The fifth edition of the WHO classification of haematolymphoid tumours (WHO-HEM5) introduces significant updates to the classification of acute lymphoblastic leukaemia, ALAL (including mixed phenotype acute leukaemia (MPAL)), myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions (MLN-TK), and histiocytic and dendritic cell neoplasms, reflecting the advances in the understanding of the genetic basis of these diseases. This review provides an overview of these changes, highlighting a shift to a more refined molecular-genetic approach. The incorporation of newly recognised genetic subtypes into the classification scheme underscores the evolving landscape of these entities. Challenges in diagnosing ALAL/MPAL and MLN-TK are discussed, along with recent insights into histiocytic and dendritic cell neoplasms, including newly defined entities such as ALK-positive histiocytosis.The review also explores the practical implications of WHO-HEM5, particularly in resource-limited settings, where comprehensive molecular testing may be unavailable. While morphology and immunohistochemistry remain essential diagnostic tools, strategic use of flow cytometry and targeted fluorescence in situ hybridisation can facilitate risk-adapted classification and improve survival in regions with limited resources and therapeutic options. Future large-scale studies are necessary to establish the diagnostic and prognostic value of these newly genetically defined entities for diverse healthcare environments, and to standardise guidelines in refining disease classification and optimising patient outcomes.