Primary Bone Diffuse Large B-cell Lymphoma (PB-DLBCL) – a distinct extranodal lymphoma of germinal center origin, with an EZB-like mutational profile and good prognosis

Primary bone diffuse large B-cell lymphoma (PB-DLBCL) is not recognized as a separate entity by the current classification systems. Here, we define and highlight its distinctive clinical presentation, morphology, phenotype, gene expression profile (GEP) and molecular genetics. We collected 27 respective cases and investigated their phenotype, performed gDNA panel sequencing covering 172 genes, and carried out fluorescence in situ hybridisation to evaluate MYC, BCL2 and BCL6 translocations. We attempted to genetically subclassify cases using the Two-step classifier and performed GEP for cell-of-origin subtyping and in silico comparison to uncover up- and down-regulated genes as opposed to other DLBCL. Almost all cases (n=22) were germinal center B-cell-like (GCB) by immunohistochemistry and all by GEP. Additionally, PB-DLBCL has a mutational profile similar to follicular lymphoma and nodal GCB-DLBCL, with the exception of TP53 and B2M mutations. The GEP of PB-DLBCL is unique, and the frequency of BCL2 rearrangements is lower compared to nodal GCB-DLBCL. The Two-step classifier categorised 8 of the cases as EZB, 3 as ST2 and 1 as MCD. This study comprehensively characterizes PB-DLBCL as a separate entity with distinct clinical and morpho-molecular features. These insights may aid in developing tailored therapeutic strategies and shed light on its pathogenesis.