PMLBCL : Introduction

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Primary Mediastinal Large B-cell Lymphoma (PMLBCL) - Introduction: PMLBCL belongs to the group of aggressive diffuse large B-cell lymphomas. It is of putative thymic B-cell origin and arises in the mediastinum. It has previously been known as mediastinal large cell lymphoma with sclerosis and primary mediastinal clear cell lymphoma of B-cell type.

It was initially considered to be a subtype of diffuse large B-cell lymphoma, but has been subsequently recognized as a distinct entity in the WHO Classification.

It is relatively rare and affects mainly young adults. It shares similarities with nodular sclerosing Hodgkin lymphoma at the clinical, morphologic, immunohistochemical and molecular genetic level.

This chest CT with contrast (axial view) from a 22 y/o male shows a large mediastinal mass. A moderately-sized pericardial effusion and bilateral pleural effusions are present. Diagnosis of PMLBCL was confirmed by a CT-guided needle biopsy of the mass.

Case courtesy of Assoc Prof Frank Gaillard, Radiopaedia.org. From the case rID: 8655

About the Disease

Primary mediastinal large B-cell lymphoma (PMLBCL) is a rare, aggressive, distinct subtype of large B-cell lymphoma localized in anterosuperior mediastinum. It shares clinical, morphologic, and molecular genetic features with nodular sclerosing Hodgkin lymphoma. It makes up 2-3% of non-Hodgkin lymphomas and affects young adults with a female predilection. Patients develop compressive symptoms, such as cough and dyspnea, from the rapidly growing mediastinal mass. About 20-30% have superior vena cava syndrome. Pleural and pericardial effusion develop in 30-50%. Surrounding thoracic structures may be invaded. Bone marrow is rarely involved. Extranodal sites may be invaded in advanced stages. The tumor cells are highly variable, medium-to-large size, with irregular or multilobated nuclei and pale or clear cytoplasm in a sclerotic stroma. Reed-Sternberg-like cells may be present. Immunophenotype: positive for pan B-cell antigens (CD19, CD20, CD22, CD79a), no surface or cytoplasmic immunoglobulins, PDL1+, PDL2+, PAX5%, CD30+ (variable, heterogenous), IRF4/MUM1+, BCL2, BCL6, PU.1, OCT2; variable positivity for CD15, CD10, CD11c, CD23, MYC; negative for EBV. MAL gene (chromosome 2q) is + in 70%. Ig heavy and light chain genes are clonally rearranged. TCR genes are germline. Rearrangements of CCND1, BCL2, BCL6, and MYC are rare/absent. Three molecular pathways are implicated in the pathogenesis - NF-κB signaling pathway, JAK-STAT signaling cascade, and immune privilege. References:Dabrowska-Iwanicka A & Walewski J. Primary Mediastinal Large B-cell Lymphoma. Curr Hematol Malig Rep (2014) 9:273-283. Lees, C. et al. Biology and therapy of primary mediastinal B-cell lymphoma : current status and future directions. British Journal of Haematology, 2019, 185, 25-41. Jaffe, E. S. et al (2017). Hematopathology - Second Edition. Philadelphia, PA. Elsevier. Swerdlow, S. H. et al (2017). WHO Classification of Tumors of Hematopoietic and Lymphoid Tissues, Revised 4th Edition; IARC, Lyon, France.

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