Diagnosis and characterization of acute erythroleukemia subsets by determining the percentages of myeloblasts and proerythroblasts in 69 cases

Acute erythroleukemia (FAB M6) is a rare heterogeneous disease with an increase in red cell precursors and myeloblasts. Three subsets have been described: M6A (myeloblast‐rich erythroleukemia); M6B (proerythroblast‐rich erythroleukemia); and M6C (myeloblast‐ and proerythroblast‐rich mixed variant). This study was undertaken to define and compare the clinical courses and survival outcomes among M6A, M6B, and M6C variants of erythroleukemia. Sixty‐nine cases of M6 leukemia were categorized as consisting of ≥50% erythroid of all nucleated cells and M6A with ≥30% myeloblasts/nonerythroid component; M6B with ≥30% proerythroblasts/erythroid component; and M6C with ≥30% myeloblasts and ≥30% proerythroblasts. The demographics, cell type distribution, and survival (mean ± sd) of these groups were compared. There were 32 M6A, 26 M6B, and 11 M6C patients. No significant difference was seen among the groups in age, sex, or treatment. Compared to M6A, both the M6B (P< 0.0001) and M6C (P< 0.0001) variants showed a statistically significant increase in the percentage of bone marrow erythroid cells, proerythroblasts, and the proerythroblasts/erythroid ratios. Comparing the groups for survival, M6B (3 ± 3.6 months) versus M6A (25 ± 28 months), P< 0.002, and M6C (10 ± 13 months) versus M6A, P< 0.01 had a poorer prognosis. Calculating the proerythroblasts as a component of total bone marrow erythroids provides a complimentary method for delineating the pure red cell erythroleukemia (M6B) and mixed variant (M6C), similar to that for the myeloid/erythroid (M6A) leukemia. Now that it is possible to delineate erythroleukemia subtypes, innovative treatments are indicated to target the malignant erythroid population, which is resistant to myeloid‐based therapies. Am. J. Hematol. 65:5–13, 2000. © 2000 Wiley‐Liss, Inc.