With the advent of the application of immunophenotyping and molecular analysis to the study of lymphomas, neoplasms arising from T- and NK-cells was recognized and our understanding of these neoplasms has steadily increased. T/NK-cell lymphomas show marked geographic variation. In the Western hemisphere, approximately 12–15% of non-Hodgkin lymphomas are of T/NK-cell lineage. By contrast, in Asia the relative frequency of T-cell lymphomas is much higher, in large part because B-cell lymphomas are less common. These variations are probably related to both enviromental and genetic factors.

T/NK-cell lymphomas can have an immature or mature T/NK-cell immunophenotype. Lymphoid neoplasms of immature type arise from precursors in the thymus or bone marrow precursors and are designated as precursor lymphoblastic lymphoma/leukemia as already discussed (see Chapter 59). Lymphoid neoplasms of mature T/NK-cell type are a relatively more heterogeneous group that can be subdivided clinically into three groups: neoplasms that involve lymph nodes (as well as other sites), neoplasms that usually involve extranodal sites, and neoplasms that present as leukemia. In this section we focus on T/NK-cell neoplasms that usually involve lymph nodes and present as lymphoma. These neoplasms include adult T-cell lymphoma/leukemia (Chapter 71), peripheral T-cell lymphoma unspecified (Chapter 73, angioimmunoblastic T-cell lymphoma (Chapter 74), and anaplastic large cell lymphoma (Chapter 75). We also discuss one type of extranodal lymphoma, mycosis fungoides, because it commonly drains from the skin to lymph nodes (Chapter 72). Other extranodal and leukemic T-cell lymphomas that infrequently involve lymph nodes are not included in this section.