Primary large-cell lymphoma of the thymus: a diffuse B-cell neoplasm presenting as primary mediastinal lymphoma

Primary mediastinal nonlymphoblastic non-Hodgkin's lymphoma (NLNHL) has distinct clinical, histologic (diffuse large-cell morphology, often with sclerosis and clear cytoplasm), and immunohistochemical features (predominantly B-cell lineage, usually immunoglobulin-negative), which suggest origin from a unique B-cell population. The thymus has a resident population of B cells with a unique immunophenotype, and can be involved by primary mediastinal NLNHL, in some cases selectively. Fifteen cases of NLNHL involving the thymus were studied by paraffin-section immunohistochemistry using antibodies to formalin-resistant epitopes of B cells (4KB5 [CD45RA] and L26 [CD20]) and T cells (L60 [CD43] and UCHL1 [CD45RO]). All were diffuse large-cell or immunoblastic lymphomas with sclerosis, and were also similar to primary mediastinal NLNHL in clinical features. Neoplastic cells stained with L26 in all but one case, which stained with 4KB5 and an antibody to a leukocyte-common antigen (PD7/26 [CD45RB]), and were uniformly nonreactive with L60 (with one exception) and UCHL1. Intermingled small lymphocytes were uniformly L26-negative and positive for T-cell markers, even in one case with atypia suggesting a lymphoma of mixed morphology. These findings demonstrate that primary thymic and primary mediastinal NLNHL are similar B-lineage neoplasms, and support previous suggestions that both may originate in thymic B cells.