T-cell lymphomas account for approximately 15 percent of all NHLs in the United States. A similar lymphocyte called a natural killer (NK) cell shares many features with T-cells. When NK cells become cancerous, the cancer is called NK or NK/T-cell lymphoma and is generally grouped with other T-cell lymphomas. There are many different forms of T-cell lymphomas, some of which are extremely rare. T-cell lymphomas can be aggressive (fast-growing) or indolent (slow-growing).
Lymphomas are often, but not always, named from a description of the normal cell that leads to cancer. The general term peripheral T-cell lymphoma (PTCL) refers to the entire group of mature or "post-thymic" T-cell lymphomas (arise from mature T-cells), which distinguishes them from the immature T-cell lymphomas such as acute lymphocytic leukemia (ALL) or lymphoblastic lymphoma. Under this broad meaning, almost all types of T-cell lymphoma fall under the category of PTCL.