Primary mediastinal non-Hodgkin
lymphomas (PM-NHLs) represent ∼5% of all non-Hodgkin
lymphomas (NHLs) and comprise
lymphomas of B-cell and T-cell origin. PM-NHLs are defined as involvement of mediastinal lymph nodes, thymus, and/or mediastinal organs (heart, lung, pleura, pericardium) by NHL without evidence of systemic disease at presentation. The clinical scenario is variable and depends on the
lymphoma subtype. The radiologic presentation is also variable ranging from a mediastinal mass with or without
superior vena cava syndrome, a pleural or a cardiac mass associated with effusion, or as an effusion only. The diagnosis of PM-NHLs can only be established by microscopic evaluation, and therefore, general pathologists should be aware of these
tumors and familiar with their diagnostic approach. The most common anterior mediastinal NHLs (90% to 95%) are primary mediastinal (PM) large
B-cell lymphoma and T-
lymphoblastic lymphoma. Thymic marginal zone
lymphoma and mediastinal gray zone
lymphoma are very rare. The remainder PM-NHLs involving middle or posterior mediastinum include
diffuse large B-cell lymphoma (DLBCL) and rare cases of
T-cell lymphoma, including
anaplastic large cell lymphoma and
breast implant-associated
anaplastic large cell lymphoma extending to the anterior mediastinum. Primary pleural and cardiac NHLs are mostly DLBCLs. Other rare subtypes of PM-NHLs include DLBCL associated with chronic
inflammation/
pyothorax-associated
lymphoma,
fibrin-associated DLBCL (both Epstein-Barr virus positive), and pleural and/or pericardial
primary effusion lymphoma (human herpesvirus-8 positive/Epstein-Barr virus positive). We review the historical aspects, epidemiology, clinicoradiologic features, histopathology, immunohistochemistry, differential diagnosis, and relevant cytogenetic and molecular features of the remaining mediastinal
B-cell lymphomas, including primary thymic marginal zone
lymphoma of the mucosa-associated lymphoid tissue type, other PM small
B-cell lymphomas, PM
plasmacytoma, and the most relevant PM
T-cell lymphomas.