Pathologic and Prognostic Systems for Myelodysplastic Syndromes
Myelodysplastic syndromes (MDS) are classified according to features of cellular morphology, etiology, and clinical presentation. The morphological classification of MDS is largely based on the percent of myeloblasts in the bone marrow and blood, the type and degree of myeloid dysplasia, and the presence of ring sideroblasts. The clinical classification of the MDS depends upon whether there is an identifiable etiology and whether the MDS has been treated previously.
The World Health Organization (WHO) classification  has supplanted the historic French-American-British (FAB) classification, as shown in Table 1.
Table 1. Myelodysplastic Syndromes: Comparison of the FAB and WHO Classifications
|FAB (1982)||WHO (2008)|
|AML = acute myeloid leukemia; FAB = French-American-British classification scheme; MDS = myelodysplastic syndromes; WHO = World Health Organization.|
|Refractory anemia.||Refractory anemia.|
| ||Refractory cytopenia with multilineage dysplasia. Refractory cytopenia with unilineage dysplasia.|
|Refractory anemia with ring sideroblasts.||Refractory anemia with ring sideroblasts.|
|Refractory anemia with excess blasts.||Refractory anemia with excess blasts -1 and -2.|
| ||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.
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 ring 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 ring 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 acute myeloid leukemia (AML).