Any cell in the body can accumulate mutations that eventually allow it to evade destruction by the immune system by neither being recognized as part of you or foreign, replicate (grow), invade surrounding structures and in some cases metastasize either through the circulatory or lymph systems predominantly. T-cells are a type of white blood cell which arise from lymphoid stem cells in the bone marrow and mature in the thymus. Abnormal T-cell malignancies, therefore, most commonly present as a hematologic malignancy which is to say the problem is in one of the early stage cells in the bone marrow and would fall under the umbrella of leukemia or during later stage maturation, most often in the thymus, which presents as a mediastinal mass (chest cavity) which is called lymphoma. Despite being very different presentations, there is a lot of overlap between leukemia and lymphoma - particularly in T-cell origin cancers.

Source: pediatric oncologist