Plasmacytoid dendritic cell proliferations, malignant histiocytic (macrophage/dendritic cell) neoplasms and follicular dendritic cell neoplasms: report from the 2024 Joint CSHP/EA4HP/SH workshop.

Histiocytic and dendritic cell neoplasms of hematopoietic origin are derived from or show differentiation towards the cells of the mononuclear phagocyte system. Plasmacytoid dendritic cell proliferations/neoplasms include mature plasmacytoid dendritic cell proliferations identified in the context of a defined myeloid neoplasm and blastic plasmacytoid dendritic cell neoplasm, an aggressive tumor with a poor outcome. Malignant histiocytic (macrophage/dendritic cell) neoplasms occur de novo or in the setting of another hematological malignancy, typically "transdifferentiated" from associated lymphoid neoplasia. Recurrent alterations in the MAPK pathway have been shown to have a key role in their pathogenesis. Neoplasms derived from follicular dendritic cells and fibroblastic reticular cells originate from a mesenchymal precursor. The major entities in this group include follicular dendritic sarcoma, which has recently been shown to harbor recurrent alterations in NF-κB pathway genes, and EBV positive inflammatory follicular dendritic cell sarcoma/tumor, a rare lesion with an inflammatory pseudotumor-like morphology and association with EBV infection. At the joint Workshop of the Chinese Society of Hematopathology (CSHP), the European Association for Haematopathology (EA4HP) and the Society for Hematopathology (SH) on histiocytic/dendritic cell proliferations, neoplasms, and their mimics in Hefei, China, April 2024, there were 16 cases submitted to session 3 on plasmacytoid dendritic cell proliferations and neoplasms, and a total of 42 submissions to session 4 on malignant histiocytic and dendritic cell neoplasms, including stroma-derived neoplasms. A summary of the findings and conclusions arising from discussion of the workshop cases are presented in this report.