primary mediastinal (thymic) large b cell lymphoma primary mediastinal (thymic) large b cell lymphoma primary mediastinal (thymic) large b-cell lymphoma is a distinct type of diffuse large b-cell lymphoma involving the mediastinum, recognized in the who 2008 classification.: 370–374 primary mediastinal (thymic) large b cell lymphomaother namesmediastinal large b cell lymphoma, primary mediastinal large b-cell lymphoma (pmlbcl), mediastinal large b-cell lymphomaspecialtyhematology, oncology signs and symptoms superior vena cava syndrome occurs in 30–50%, and pleural or pericardial effusions occur in about one-third. pathophysiology pmlbcl arises from a putative thymic peripheral b cell. it has several distinctive biological features. molecular analysis shows that pmlbcl is distinct from other types of diffuse large b-cell lymphomas (dlbcl). mal gene expression is seen in 70%, unlike other diffuse large b-cell lymphomas.: 370 gene expression profiling shows considerable variance from other dlbcls and similarity to hodgkin disease.: 290–293 pmlbcl is cd20 positive, expresses pan-b markers including cd79a, and has clonal immunoglobulin gene rearrangements and mrna but paradoxically does not express cytoplasmic or cell surface immunoglobulin.: 370 clinically, pmlbcl is unusual in several respects. despite 80% pmlbcl being stage i or ii, the presenting anterior mediastinal mass is often over 10 cm and is locally invasive of lung, chest wall, pleura, and pericardium. at initial presentation, pmlbcl is usually confined to mediastinum, but its bulk, rather than additional adenopathy, can sometimes be palpated at the low neck. increased ldh is seen in approximately 75%,: 370 but unlike other large cell lymphomas, no increase in beta-2 microglobulin is seen even when bulky: 370 which may relate to defective major histocompatibility complex expression.: 370 treatment multiagent chemotherapy is recommended, but the preferred regimen is controversial, as is consolidative radiotherapy. epidemiology it affects primarily young adults; the median age is 37 years.: 370 it is more common in females. grey zone lymphoma "in-between chd and nhl"