A neoplasm of lymphoblasts committed to the T-cell lineage, typically composed of small to medium-sized blast cells. When the neoplasm involves predominantly the bone marrow and the peripheral blood, it is called T acute lymphoblastic leukemia. When it involves nodal or extranodal sites it is called T lymphoblastic lymphoma. (WHO, 2001). The disease is T lymphoblastic leukemia/lymphoma (MONDO_0003537). Also known as: precursor T lymphoblastic leukemia/lymphoma, precursor T lymphoblastic lymphoma/leukemia, precursor T-lymphoblastic lymphoma/leukemia.