Distinctive patterns of marrow involvement by classic Hodgkin lymphoma are clues for diagnosis and subtyping.

Classic Hodgkin lymphoma (CHL) is a lymphoid neoplasm deriving from B cells in a rich inflammatory background. There are four histological subtypes with different epidemiological features. Bone marrow involvement by CHL is infrequent, and subtyping CHL from the bone marrow is not suggested as there might be discordant histopathology between the primary tumors and bone marrow specimens. In this study, we aimed to identify the histopathological features of bone marrow involved by CHL and tried to correlate these features with their subtypes. Among the 23 recruited cases, the frequencies of mixed cellularity (MC; 48%, 11/23) and nodular sclerosis (NS; 44%, 10/23) were similar. There were two patterns of marrow involvement: pattern A (fibrous), space-occupying lesions with alternating hypo- and hypercellular areas against a fibrotic background with dilated sinusoids and pattern B (histiocyte-rich), ill-defined granuloma-like lesions in which histiocytes merged with normal hematopoietic and inflammatory cells. Pattern A was more frequent in patients with CHL-NS than CHL-MC (100% vs. 18.2%; p < 0.001). Diagnostic Hodgkin cells and Reed-Sternberg (HRS) cells were identified in all cases, while HRS variant lacunar cells were occasionally discovered, particularly in the CHL-NS subtype (NS 100% vs. MC 9%; p < 0.001). The frequency of EBV association was higher in MC (64%) than that in NS (36%) subtype, but not statistically significant. Of the two patterns of marrow involvement, pattern A was more commonly associated with the NS subtype and less frequently associated with EBV. Recognizing the patterns of marrow involvement is important for diagnosis and may contribute to the subtyping of CHL.