Definition:

Peripheral T cell lymphomas (PTCLs) are a group of rare lymphomas originating from mature (i.e., post-thymic or "peripheral") T lymphocytes and NK cells. With regard to the current WHO classification, leukemic, cutaneous, nodal, and extranodal PTCL are distinguished. The most common subtypes worldwide are the nodal T cell lymphomas. In the nodal T cell lymphomas, the major subtypes are PTCL, not otherwise specified (PTCL-NOS), angioimmunoblastic T cell lymphoma (AITL), anaplastic lymphoma kinase (ALK)-positive anaplastic large cell lymphoma (ALCL) and ALK-negative ALCL. Novel approaches are gradually clarifying the molecular pathogenesis of PTCLs. ALK gene translocation and dual specificity phosphatase 22 (DUSP22) gene translocation are seen in ALK-positive ALCL and ALK-negative ALCL, respectively. Recently, gene mutations in epigenetic regulators, including Ten-Eleven Translocation 2 (TET2), DNA methyltransferase 3A (DNMT3A), and isocitrate dehydrogenase 1 /2 (IDH1/2), are discovered in hematologic malignancies. The mutation frequencies were especially high in AITL and PTCL-NOS.