Mediastinal lymphomas: Primary Mediastinal (Thymic) Large B-cell Lymphoma versus classical Hodgkin lymphoma, histopathologic dilemma solved?

Primary Mediastinal (Thymic) Large B-cell Lymphoma (PMLBL) frequently demonstrates clinical, morphologic and/or immunohistochemical (IHC) features that overlap with mediastinal-classical Hodgkin lymphoma (cHL). Our study compared the immunohistochemical profile of PMLBL versus mediastinal cHL in a cohort of 18 patients, 11 of whom were diagnosed as having PMLBL, and 7 were diagnosed as having mediastinal cHL. Biopsy slides and blocks of all patients were retrieved, and further immunohistochemical workup was done on all the cases. This included staining with a panel of antibodies, including LCA, CD₂₀, CD₃, CD₃₀, CD₁₅, and IRF4/MUM1. All PMLBL cases (100%) showed positivity both for LCA and Pan B (CD₂₀), and negativity for Pan T (CD₃). All mediastinal cHL cases (100%) showed negativity both for LCA and Pan T (CD₃). Both entities showed variable positivity for IRF4/MUM1, i.e., 73% in PMLBL versus 100% in cHL. CD₃₀ was expressed in 36% cases of PMLBL versus 100% cases of cHL. CD₁₅ was expressed in a single case of PMLBL versus 86% cases of cHL. We conclude that 'Leucocyte Cell Antigen' (LCA) is the only marker that was consistently positive in PMLBL and negative in cHL. Other IHC markers, though still helpful, may be expressed in either entity.