Update on the Classification of and Diagnostic Approaches to Mature T-Cell Lymphomas.

[CONTEXT.—] In the 2017 revised World Health Organization classification of tumors of hematopoietic and lymphoid tissues, some mature T-cell lymphomas were reclassified and a few new provisional entities were established based on new data from clinical and laboratory studies. T follicular helper cell lymphoma is identified by T follicular helper cell markers. Anaplastic large cell lymphoma, ALK negative, is a better-defined entity based on genetic abnormalities, and breast implant-associated anaplastic large cell lymphoma is recognized as a provisional entity. The gastrointestinal T-cell lymphomas are reclassified, with addition of a new provisional entity, indolent T-cell lymphoproliferative disorder of the gastrointestinal tract, characterized by an indolent clinical course.

[OBJECTIVE.—] To review the diagnostic approaches to reclassified and newly established entities of mature T-cell lymphomas, focusing on significant immunophenotypic features and molecular genetic abnormalities. Relevant new discoveries after the publication of the 2017 World Health Organization classification are included.

[DATA SOURCES.—] Information from the literature most relevant to the 2017 World Health Organization revised classification and publications after 2016.

[CONCLUSIONS.—] Incorporating clinical, morphologic, and immunophenotypic features usually provides sufficient evidence to reach a preliminary diagnosis of mature T-cell lymphoma. Molecular genetic studies can be very helpful for the final diagnosis and classification, especially in challenging cases. Some molecular genetic features have been found in breast implant-associated anaplastic large cell lymphoma, distinct from anaplastic large cell lymphoma, ALK negative. Immunohistochemical staining of 4 markers may enable further subtyping of peripheral T-cell lymphomas.