Classification of acute leukaemia: the need to incorporate cytogenetic and molecular genetic information.

The purpose of any classification of leukaemia is to identify distinct biological entities which diVer in aetiology, mechanisms of leukaemogenesis, clinicopathological features, and prognosis. Since the optimal treatment for such entities diVers, their recognition is not only of scientific interest but is also essential for optimal care of patients. Acute leukaemia can be classified in many ways: (1) by morphology and cytochemistry supplemented by immunophenotyping, as proposed by the French–American–British (FAB) group; (2) by morphology, immunophenotyping and cytogenetics as proposed by the MIC groups ; (3) by immunophenotyping alone, as proposed by the European Group for the Immunological Classification of Leukemias (EGIL); (4) by antecedent events; (5) by the nature of the stem cell or progenitor cell in which the leukaemogenic mutation occurred. In addition, it has recently been proposed that acute myeloid leukaemia (AML) should be classified either as myelodysplastic syndrome (MDS) related or as true de novo AML. Finally, consideration should be given to a classification of acute leukaemia which incorporates molecular genetic information. In 1976 the FAB group published the first of an important series of papers concerning the classification of acute leukaemia. Initially classification was based on cytology and cytochemistry. Subsequently the development of immunophenotyping provided a firm basis for the diagnosis of acute lymphoblastic leukaemia and facilitated the diagnosis of acute megakaryoblastic leukaemia and acute myeloid leukaemia with minimal evidence of myeloid diVerentiation (M0 AML). Some morphological categories recognised by the FAB group, specifically acute hypergranular promyelocytic leukaemia and its variant form (M3 and M3 variant AML) and L3 acute lymphoblastic leukaemia (ALL), were subsequently shown to indeed be biological entities; application of the FAB criteria still permits their speedy diagnosis. Other FAB categories include several diVerent discrete entities and some of these overlap several FAB groups. Despite this, the FAB classifications remain of great importance in providing a widely accepted terminology and a framework into which knowledge gained from newer techniques can be incorporated. Following development of immunophenotyping techniques and the improvement of cytogenetic methods for haematological malignancies, the MIC groups 3 proposed the classification of acute leukaemia on the basis of the FAB morphological classification supplemented by immunophenotype and cytogenetics—a morphological, immunological, cytogenetic (MIC) classification. The MIC classifications defined the cytogenetic correlates of the FAB categories of M3 AML and L3 ALL. In addition, several new entities were recognised, for example subsets of B lineage ALL associated with t(1;19)(q23;p13), t(4;11)(q21;q23), and t(9;22)(q34;q11), and subsets of AML associated with t(8;21)(q22;q22) and inv(16)(p13q22). Most of the MIC categories continue to be recognised as discrete entities, although the validity of categories of ALL defined by the presence of deletion of the long arm of chromosome 6 [del(6q)] or deletion of the short arm of chromosome 9 [del(9p)] might be questioned. The MIC classifications are seen as open ended, with new categories being added once their characteristics have been clearly defined. The European Group for the Immunological Classification of Leukemias (EGIL) has proposed that acute leukaemia be classified on the basis of immunophenotype alone. This classification has the strength that it suggests standardised criteria for defining a leukaemia as myeloid, T lineage, B lineage, or biphenotypic. It also suggests criteria for distinguishing biphenotypic leukaemia from AML with aberrant expression of lymphoid antigens, and from ALL with aberrant expression of myeloid antigens. However, a purely immunological classification has the disadvantage that discrete entities may fall into one of two categories; for example some cases of AML of FAB M2 subtype associated with t(8;21)(q22;q22) would be classified as “AML of myelomonocytic lineage” while others would be classified as “AML J Clin Pathol 1998;51:420–423 420