Lymphoblastic leukemias/lymphomas are predominantly diseases of childhood, where they represent almost all acute leukemias; however, they are also encountered with significant frequency in the adult population. These neoplastic processes can be of B-cell or T-cell derivation and are composed of immature precursors of either lineage. The classification of B-lymphoblastic neoplasms relies predominantly on genetic and molecular findings, whereas the same is not true for those of T-lymphoid origin. Many of these recurrent cytogenetic abnormalities have important prognostic and therapeutic implications.
Most lymphoblastic leukemias show a B-cell phenotype, whereas most lymphoblastic lymphomas are of T-lymphoid origin.
Morphologically, distinguishing between B and T lymphoblasts is virtually impossible; therefore, immunophenotypic analysis is required for diagnosis.
The classification of B-lymphoblastic processes relies heavily on genetic and/or molecular findings, unlike T-lymphoblastic processes, which are not subclassified based on such criteria.
Cytogenetic abnormalities in B-lymphoblastic leukemia have major prognostic implications; however, early response to therapy remains the most important outcome indicator.
Lymphoblastic neoplastic processes include lymphoblastic leukemias as well as lymphoblastic lymphomas (LBLs) and represent clonal proliferations of lymphoid precursors of generally B-cell or T-cell derivation even though, rarely, natural killer (NK) phenotype might be observed. In general, if a lymphoblastic neoplasm presents in the form of acute leukemia, the likelihood of B-lymphoid origin is significantly higher (80%–85%) compared with T-cell derivation (10%–15%), whereas the opposite is true in cases with lymphomatous presentation. The distinction between leukemia and lymphoma is generally based on the degree of peripheral blood and/or bone marrow involvement. If the initial diagnosis is based on peripheral blood and/or bone marrow, a threshold of 20% is recommended by the World Health Organization (WHO) and the term acute lymphoblastic leukemia (ALL) is used; however, if the malignant process shows exclusively or predominantly tissue involvement, then the term LBL is preferred. The situation becomes slightly more complicated if both tissue and bone marrow/blood involvement are present. As a rule of thumb, if the initial diagnosis is that of LBL and subsequent bone marrow analysis performed usually for staging purposes shows no or limited involvement, the diagnosis remains LBL; however, if significant involvement is present, the diagnosis becomes lymphoblastic leukemia. The threshold between limited and significant is arbitrary and is generally set at a value of 25% given that patients with LBL accompanied by bone marrow involvement that is greater than 25% do better with leukemia rather than lymphoma therapy protocols.