A clinician's guide to double hit lymphomas

Double hit lymphomas (DHL) represent a subset of highly aggressive B-cell malignancies characterized by the presence of recurrent cytogenetic rearrangements affecting MYC and either BCL2 and/or BCL6. Recent studies have expanded the concept to include MYC/BCL2 protein co-expressing lymphomas. Around 5-10% of diffuse large B-cell lymphomas are 'double hit' using the cytogenetic definition, whilst around 30-40% are MYC/BCL2 protein co-expressing. In this review, we provide a comprehensive overview of this condition written with the practicing clinician in mind, covering the definition and classification, when DHL should be suspected and how to make the diagnosis, the prognostic factors and a detailed discussion of recent evidence regarding optimal therapy. In particular, we discuss choice of induction regimen, the role of central nervous system-directed prophylaxis, stem cell transplantation and relapsing or refractory disease and provide our opinions based on the currently available evidence. Finally, we highlight some of the more exciting therapies currently in development for this highly aggressive disease.