Acute lymphoblastic leukaemia (ALL) and lymphoblastic lymphoma (LBL) are two closely related conditions. In the French–American–British (FAB) classification, which largely predated immunophenotyping, ALL was categorized according to morphology, as L1, L2 and L3. In the World Health Organization (WHO) classification, ALL and LBL are grouped together as B lymphoblastic (B-ALL/LBL) and T lymphoblastic (T-ALL/LBL) neoplasms. B-ALL/LBL is subclassified into B-ALL/LBL not otherwise specified (NOS) and B-ALL/LBL with recurrent cytogenetic abnormalities [1,2]. In T-ALL/LBL, a distinct group designated early T-cell precursor lymphoblastic leukaemia is included in the revised WHO classification . A rare type of precursor lymphoid neoplasm designated natural killer (NK) lymphoblastic leukaemia/lymphoma is now also recognized as a provisional entity . The term B-ALL/LBL should not be used for cases of Burkitt lymphoma (see below).
In cases of ALL occurring in infants, the leukaemia often has its origin in intra-uterine life and in childhood cases also, there may be a pre-leukaemic clone already present at the time of birth [5,6]. There is an increased risk of B-ALL/LBL in children with Down syndrome.
The incidence of B-lineage ALL shows a peak between the ages of 2 and 10 years. This childhood peak is particularly characteristic of developed countries. About three-quarters of cases are B-lineage and about one-quarter T-lineage. T-lineage cases tend to be older and show a male predominance.
Clinical features differ between LBL and ALL and differ somewhat between B- and T-lineage cases. Overall, presentation as lymphoma is much less common than presentation as leukaemia. An arbitrary cut-off of >25% bone marrow infiltration by blast cells is usually applied to differentiate ALL from LBL. T-lineage cases are more likely than B-lineage to present as lymphoma.
Patients with ALL present either with clinical features of bone marrow failure (pallor and bruising) or with clinical features resulting more directly from proliferation of leukaemic cells (lymphadenopathy, splenomegaly, hepatomegaly, bone pain, testicular enlargement and, in the case of T-ALL, respiratory difficulty resulting from thymic enlargement). Occasional patients present with abdominal masses resulting from massive renal infiltration.
In patients presenting with lymphoma rather than leukaemia, there may be thymic disease without involvement of the bone marrow and blood (T-LBL) or soft tissue involvement (T- or B-LBL).
Haematological and pathological features
The peripheral blood usually shows anaemia, thrombocytopenia and leucocytosis, the latter as a result of the presence of leukaemic blast cells in the circulation (Figure 2.1). Less often, there is anaemia and thrombocytopenia with few if any circulating blast cells.
Peripheral blood film from a patient with T-ALL showing severe thrombocytopenia and two blast cells. Romanowsky stain, ×100 objective.