Symposium: classification of leukemia. 1. The classification of acute leukemia.

Two main forms of acute leukemia have been recognized by the French-American-British (FAB) group: myeloid (AML) and lymphoblastic (ALL). Some types of AML can be diagnosed on well prepared bone marrow films stained with May-Grünwald-Giemsa. Poorly differentiated types, myeloblastic (M1) and monoblastic (M5PD), need confirmation by positive cytochemical reactions (Sudan Black B, myeloperoxidase and non-specific esterase). There are 2 sub-types of promyelocytic leukemia: M3 typical, hypergranular and M3 variant, microgranular. The M3 variant has a more acute course, higher WBC and may require cytochemistry to demonstrate promyelocytic differentiation. Electron microscopic cytochemistry can also help in the classification of difficult AML cases; the 'platelet-peroxidase' reaction, for example, is essential for the diagnosis of megakaryoblastic leukemia, a disorder often presenting as 'acute' myelosclerosis. Three morphological types are seen in ALL: L1, predominantly in children, L2, more frequently in adults, and the relatively rare L3 or Burkitt type. Immunological and enzyme markers (ALL and la antigens, terminal transferase, etc.) help define the cell phenotype: (1) non-B, non-T ALL with 3 forms (common, null and pre-B), (2) T-ALL, related to but distinct from T-lymphoblastic lymphoma, and (3) B-ALL, usually with L3 morphology, There is growing evidence that the FAB morphological types correlate with prognosis in ALL independently of other factors. The immunologically defined types also correlate with prognosis but not as an independent variable.