The 5th edition of the World Health Organization Classification of Haematolymphoid Tumours: Myeloid and Histiocytic/ Dendritic Neoplasms

Overview

This paper sets out the myeloid and histiocytic/dendritic part of the 5th edition of the World Health Organization Classification of Haematolymphoid Tumours. It is relevant to haematologists, haematopathologists and oncologists, because it changes how many myeloid neoplasms are diagnosed and named.

Main conceptual changes

The classification is organised hierarchically and relies more heavily on genetic features. It formally recognises myeloid precursor states, such as clonal haematopoiesis, as distinct from overt neoplasms. Myelodysplastic syndromes are renamed myelodysplastic neoplasms and divided into genetically defined and morphologically defined types. Acute myeloid leukaemia is separated into types defined by genetic abnormalities and types defined by differentiation. For many genetically defined types, the diagnosis no longer depends on a fixed blast count.

Other key updates

Chronic myeloid leukaemia is presented with a simplified approach to disease phases. The update refines the categories of myeloproliferative neoplasms and overlap myelodysplastic/myeloproliferative neoplasms. Myeloid and lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions form a defined group. Germline predisposition and myeloid neoplasms that follow cytotoxic therapy are addressed explicitly. Histiocytic and dendritic cell neoplasms are reorganised with reference to their cell of origin.

Implications for practice

Accurate diagnosis now depends on integrating morphology, immunophenotype, cytogenetics and molecular testing, so laboratories need access to comprehensive genetic analysis. The International Consensus Classification was published in parallel, and some terminology differs between the two systems, so clinicians should be aware which system a report uses.