AML M6b (FAB): A Re-Evaluation Using the 2008 WHO Criteria
W Liu, RP Hasserjian, Z Liping, LJ Medeiros, SA Wang. MD Anderson Cancer Center, Houston, TX; MGH, Boston, MA; UMASS, Worcester, MA
Background: Pure Erythroid leukemia-M6b in the FAB classification defined a very rare subgroup of AML with a neoplastic proliferation of erythroblasts. Because myeloblasts are not increased, it was often challenging to distinguish from profound erythroid hyperplasia. We designed the current study to analyze the clinicopathologic and cytogenetic features of affected patients, and to reevaluate the neoplasm using the 2008 WHO classification scheme.
Design: We searched the data files of three large medical centers over a 10-year period for cases that fulfilled FAB diagnostic criteria for M6b. Patient survival was compared to 72 patients with erythroid/myeloid leukemia (M6a) and 33 patients with AML-myelodysplasia related changes (AML-MRC) from the same hospitals.
Results: Eighteen patients with a diagnosis of M6b were identified, including 12 men and 6 women, with a median age of 68 years. 5 patients had preexisiting MDS, 2 long-standing cytopenias; 1 CML; 4 received cytotoxic therapy for various cancers; and 7 had de novo disease. By the 2008 WHO, these cases were classified as: 1 CML blast crisis; 4 t-AML; and 13 AML-MRC either because of a MDS history and/or the cytogenetic findings. Bone marrows showed sheets of immature erythoblasts and dyserythropoiesis with chunky PAS+ cytoplasmic granules (9/9). The erythroblasts were variably CD117+(12/12), CD33+(5/6) and subset glycophorin+(9/10); and negative for CD34, HLADR or MPO. All cases(17/17) showed an extremely complex karyotype (median abnormalities 12, range 3-37) frequently involving chromosomes 5, 7, 17 and 19. In 5 patients with an antecedent karyotype for comparison, clonal cytogenetic evolutions were identified at the time of M6b diagnosis. The median survival of these 18 patients was 3 months, significantly inferior to M6a and AML-MRC patients either as a cohort (median 12 months, p<0.0001), or stratified by poor risk cytogenetics (median 7 months, p=0.0006). This survival was also inferior to t-AML published in the literature.
Conclusions: The entity previously designated as M6b using the FAB system harbors a complex karyotype and predicts a very short survival regardless the underlying etiology or classification. The current WHO system, by merging most of these cases into the AML-MRC category, does not fully capture their cytogenetic and prognostic features. We suggest that retaining M6b has value, and that it is important for pathologists to recognize this subtype of AML.
Wednesday, March 24, 2010 1:00 PM
Poster Session VI # 182, Wednesday Afternoon