Primary mediastinal large B-cell lymphoma.

Primary mediastinal large B-cell lymphoma (PMBCL) is a separate entity in the World Health Organization's classification, based on clinicopathologic features and a distinct molecular signature that overlaps with nodular sclerosis classic Hodgkin lymphoma (cHL). Molecular classifiers can distinguish PMBCL from diffuse large B-cell lymphoma (DLBCL) using ribonucleic acid derived from paraffin-embedded tissue and are integral to future studies. However, given that ∼5% of DLBCL can have a molecular PMBCL phenotype in the absence of mediastinal involvement, clinical information remains critical for diagnosis. Studies during the past 10 to 20 years have elucidated the biologic hallmarks of PMBCL that are reminiscent of cHL, including the importance of the JAK-STAT and NF-κB signaling pathways, as well as an immune evasion phenotype through multiple converging genetic aberrations. The outcome of PMBCL has improved in the modern rituximab era; however, whether there is a single standard treatment for all patients and when to integrate radiotherapy remains controversial. Regardless of the frontline therapy, refractory disease can occur in up to 10% of patients and correlates with poor outcome. With emerging data supporting the high efficacy of PD1 inhibitors in PMBCL, studies are underway that integrate them into the up-front setting.