We herein present an overview of the upcoming 5th edition of the World Health Organization Classification of Haematolymphoid Tumours focussing on lymphoid neoplasms. Myeloid and histiocytic neoplasms will be presented in a separate accompanying article. Besides listing the entities of the classification, we highlight and explain changes from the revised 4th edition. These include reorganization of entities by a hierarchical system as is adopted throughout the 5th edition of the WHO classification of tumours of all organ systems, modification of nomenclature for some entities, revision of diagnostic criteria or subtypes, deletion of certain entities, and introduction of new entities, as well as inclusion of tumour-like lesions, mesenchymal lesions specific to lymph node and spleen, and germline predisposition syndromes associated with the lymphoid neoplasms.

• MeSH
• Hematologic Neoplasms * MeSH
• Humans MeSH
• Lymphoma * pathology   MeSH
• World Health Organization MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Review MeSH

Recently, we described B-cell precursor acute lymphoblastic leukemia (BCP-ALL) subtype with early switch to the monocytic lineage and loss of the B-cell immunophenotype, including CD19 expression. Thus far, the genetic background has remained unknown. Among 726 children consecutively diagnosed with BCP-ALL, 8% patients experienced switch detectable by flow cytometry (FC). Using exome and RNA sequencing, switch was found to positively correlate with three different genetic subtypes: PAX5-P80R mutation (5 cases with switch out of 5), rearranged DUX4 (DUX4r; 30 cases of 41) and rearranged ZNF384 (ZNF384r; 4 cases of 10). Expression profiles or phenotypic patterns correlated with genotypes, but within each genotype they could not identify cases who subsequently switched. If switching was not taken into account, the B-cell-oriented FC assessment underestimated the minimal residual disease level. For patients with PAX5-P80R, a discordance between FC-determined and PCR-determined MRD was found on day 15, resulting from a rapid loss of the B-cell phenotype. Discordance on day 33 was observed in all the DUX4r, PAX5-P80R and ZNF384r subtypes. Importantly, despite the substantial phenotypic changes, possibly even challenging the appropriateness of BCP-ALL therapy, the monocytic switch was not associated with a higher incidence of relapse and poorer prognosis in patients undergoing standard ALL treatment.

• MeSH
• PAX5 Transcription Factor genetics   MeSH
• Precursor Cell Lymphoblastic Leukemia-Lymphoma * MeSH
• B-Lymphocytes MeSH
• Immunophenotyping MeSH
• Humans MeSH
• Mutation MeSH
• Precursor B-Cell Lymphoblastic Leukemia-Lymphoma * diagnosis   genetics   MeSH
• Neoplasm, Residual MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• PAX5 Transcription Factor MeSH
• PAX5 protein, human MeSH Browser