The addition of refractory cytopenia with multilineage dysplasia (RCMD), myelodysplastic syndrome, unclassifiable (MDS-U), and the myelodysplastic syndrome associated with an isolated del(5q) chromosome abnormality completes the WHO cellular classification scheme for MDS. Finally, the WHO classification of MDS removed CMML from MDS and placed it in a new category, myelodysplastic syndromes and myeloproliferative diseases (MDS and MPD). (Refer to the PDQ summary on Myelodysplastic/ Myeloproliferative Neoplasms for more information.)
Myelodysplastic Syndromes: Comparison of the FAB and WHO Classifications
AML = Acute Myeloid Leukemia; FAB = French-American-British classification scheme; MDS = Myelodysplastic Syndromes; WHO = World Health Organization.
|FAB (1982)||WHO (1997)|
|Myelodysplastic Syndromes |
|Refractory anemia.||Refractory anemia.|
| ||Refractory cytopenia with multilineage dysplasia.|
|Refractory anemia with ringed sideroblasts.||Refractory anemia with ringed sideroblasts.|
|Refractory anemia with excess blasts.||Refractory anemia with excess blasts.|
| ||Myelodysplastic syndrome, unclassifiable.|
| ||Myelodysplastic syndrome associated with del(5q).|
| ||Reclassified from MDS to:|
|Refractory anemia with excess blasts in transformation.||Acute Myeloid Leukemia identified as AML with multilineage dysplasia following a myelodysplastic syndrome.|
|Chronic myelomonocytic leukemia. ||Myelodysplastic and Myeloproliferative Diseases |
MDS cellular types and subtypes in either cellular classification scheme have different degrees of disordered hematopoiesis, frequencies of transformation to acute leukemia, and prognoses. All WHO cellular types and subtypes and the FAB cellular type, RAEB-t, are described in detail below.
Refractory anemia (RA)
In patients with RA, the myeloid and megakaryocytic series in the bone marrow appear normal, but megaloblastoid erythroid hyperplasia is present. Dysplasia is usually minimal. Marrow blasts are less than 5%, and no peripheral blasts are present. Macrocytic anemia with reticulocytopenia is present in the blood. Transformation to acute leukemia is rare, and median survival varies from 2 years to 5 years in most series. RA accounts for 20% to 30% of all patients with MDS.
Refractory anemia with ringed sideroblasts (RARS)
In patients with RARS, the blood and marrow are identical to those in patients with RA, except that at least 15% of marrow red cell precursors are ringed sideroblasts. Approximately 10% to 12% of patients present with this type, and prognosis is identical to that of RA. Approximately 1% to 2% of RARS evolve to AML.
Refractory anemia with excess blasts (RAEB)
In patients with RAEB, there is significant evidence of disordered myelopoiesis and megakaryocytopoiesis in addition to abnormal erythropoiesis. Because of differences in prognosis related to progression to a frank AML, this cellular classification is comprised of two categories, refractory anemia with excess blasts-1 (RAEB-1) and refractory anemia with excess blasts-2 (RAEB-2). Combined, the two categories account for approximately 40% of all patients with MDS. RAEB-1 is characterized by 5% to 9% blasts in the bone marrow and less than 5% blasts in the blood. Approximately 25% of cases of RAEB-1 progress to AML. Median survival is approximately 18 months. RAEB-2 is characterized by 10% to 19% blasts in the bone marrow. Approximately 33% of cases of RAEB-2 progress to AML. Median survival for RAEB-2 is approximately 10 months.