Primary mediastinal B‐cell lymphoma (PMBCL) represents 2–4% of non‐Hodgkin lymphomas and, whilst previously considered to be a subtype of diffuse large B‐cell lymphoma (DLBCL), gene expression profiling data has shown it to be a separate clinicopathological entity with evidence of an overlap with classic Hodgkin lymphoma (Rosenwald et al, 2003; Savage et al, 2003). Furthermore, it is immunophenotypically distinct from DLBCL with more than 80% of cases expressing CD30 (Swerdlow et al, 2008; Steidl & Gascoyne, 2011). Its origin is thought to be a B lymphocyte of thymic origin of either germinal centre or post‐germinal centre derivation. There is evidence that the pathogenesis of PMBCL involves dysregulation of the Janus Kinase‐Signal Transducer and Activator of Transcription (JAK‐STAT) and Nuclear Factor‐kB (NF‐kB) pathways (Steidl & Gascoyne, 2011). The amplification of the 9p24.1 region of chromosome 9, where the genes CD274, encoding CD274 (PD‐L1), and PDCDILG2, encoding PDCDILG2 (PD‐L2) are located, has been well described in PMBCL (Green et al, 2010) and expression of CD274/PD‐L1 has been suggested as a possible prognostic biomarker (Bledsoe et al, 2016; Menter et al, 2016).

Declaration of Interests

The BSH paid the expenses incurred during the writing of this guidance. None of the authors had conflicts of interest to declare. All authors have made a declaration of interests to the BSH and Task Force Chairs which may be viewed on request.