Bone Marrow in Hodgkin Lymphoma and Mimickers

Abstract

In lymph nodes and spleen, the criteria for the diagnosis of Hodgkin lymphoma (HL) are settled since the mid sixties (1]. During the last two decades, they have undergone refinements in the Revised European American Lymphoma (REAL) Classification [2] as well as in the third and forth editions of the WHO Classification of Haematopoietic and Lymphoid Tumours [3, 4]. In particular, a clear-cut distinction has been introduced between lymphocyte predominant (LP) HL and all the remaining histotypes collectively termed classical Hodgkin lymphoma (CHL). Such distinction is based on clinical, morphologic and phenotypic findings. Thus, LPHL shows a pick in the fourth decade of life, has no relationship with Epstein Barr virus (EBV) infection, tends to relapse several years after the original diagnosis (remaining anyhow curable), can occasionally progress to diffuse large B-cell lymphoma (DLBCL) and may be preceded by, associated with or followed by progressively transformed germinal centres. The neoplastic cells display a characteristic polylobated, popcorn appearance and usually express CD20, CD79a, PAX5/BSAP, CD45, BCL6, IRF4, EMA and Immunoglobulin (Ig)-related transcription factors, while they turn negative for CD30 and CD15 (for more details see blow). On the other hand, CHL shows a bimodal age distribution (with two picks in the third and seven decade of life, respectively), is characterised by an ordered dissemination that is the base for staging procedures, and reveals variable correlation with the EBV (30–90 % of cases depending on the histotype). Four subtypes are quoted under the heading CHL: lymphocyte-rich (that in the past was included in the LP chapter), nodular sclerosing, mixed-cellularity, and lymphocyte depleted. All of them are characterised by the presence of Hodgkin and Reed-Sternberg cells (HRSC) that bear the same phenotype: CD45−, CD30+, CD15+/−, CD79a−, CD20− (partly and variably + in 20 % of cases), PAX5/BSAP+ (with a few exceptions), IRF4+, Ig-transcription factors−, BCL6−, and EMA− (for more details see below). Interestingly, neoplastic elements of both LP and CHL are related to germinal centre B-cells (GCB), LP cells residing with GC and HRSC being on the way to exit form it. This is proven by the fact that the former carry ongoing mutations of the IG@, while the latter display a high load of somatic mutations which are eventually stable. In addition, besides the Ig-encoding genes, the somatic hypermutation process does also affect C-MYC, RhoH/TTF, PIM1 and PAX5, although with different prevalence in LP and CHL. In spite of these well-defined criteria, grey-zones still exist between HL and non-Hodgkin lymphomas (NHL). Thus, for instance the borders between LPHL and Histiocyte/T-cell rich B-cell lymphoma (H/TCRBCL) are not always sharp. The same holds true for CHL and diffuse large B-cell lymphoma (DLBCL): such situation has been officially recognised in the fourth edition of the WHO Classification by the inclusion of the provisional entity “B-cell lymphoma unclassifiable with features intermediate between classical Hodgkin lymphoma and diffuse large B-cell lymphoma”.