T-cell lymphoma: A disease in which cells in the lymphoid system called T cells (or T lymphocytes) become malignant. T-cell lymphomas account for a minority (about 15%) of non-Hodgkin lymphomas in the US and are more common in Asia.
The T-cell lymphomas are highly diverse and include lymphoblastic lymphoma (mainly in children and adolescents, where they account for about half of pediatric lymphomas); peripheral T-cell lymphoma (a heterogeneous group of generally aggressive diseases); mycosis fungoides (called Sézary syndrome if the malignant T cells circulate in blood); and some anaplastic large cell lymphoma (ALCL), both primary cutaneous and systemic ALCL.
T-cell lymphoma may involve the bone marrow but it usually comprises less than 25% of the marrow. If 25% or more of the marrow is populated by the malignant T cells, it is considered T-cell leukemia.