Primary mediastinal lymphomas, their morphological features and comparative evaluation

BACKGROUND: Primary mediastinal lymphoma is an uncommon tumor. Hodgkin's lymphoma (HL), primary mediastinal B-cell lymphoma (PMBCL), and T-lymphoblastic lymphoma are the most common primary mediastinal lymphomas. Key morphological and immunohistochemistry (IHC) features play a very crucial role in diagnosis as well as further categorization. MATERIALS AND METHODS: In this study, the morphological spectrum and histological features of 32 cases of primary mediastinal lymphomas diagnosed over 5 years were studied and morphological and IHC features of PMBCL versus HL were compared. Features of PMBCL were also compared against a control group of systemic diffuse large B-cell lymphoma. RESULTS: Although PMBCL and HL are known to show overlapping morphological features, it was observed that presence of clear cells and compartmentalizing fibrosis in PMBCL; and classical Reed–Sternberg cells and dense inflammatory background in HL are important morphological clues while evaluating the biopsies. PMBCL showed diffuse, strong and uniform CD20 positivity; whereas CD30 showed focal/patchy, weak to moderate and heterogeneous expression, wherever found positive. As against this, HL showed diffuse, strong and uniform CD30 positivity; and focal/patchy, weak to moderate and heterogeneous CD20 expression, if found positive. CD20, CD3, and CD30 were sufficient in most of the cases while diagnosing PMBCL and HL. CONCLUSION: This study emphasizes the critical examination of IHC markers. Only positive expression in neoplastic cells is not sufficient to make a diagnosis, equal importance should be given to percentage, intensity, pattern, and type of positivity. Apart from basic IHC described above; CD15, leukocyte common antigen and fascin played an important role in differentiating HL and PMBCL in select doubtful cases.