Refractory anemia with excess blasts in transformation (RAEB-t)
In the FAB classification, RAEB-t represents a panmyelosis in which 20% to 30% of marrow cells are blasts, and more than 5% blasts are seen in the blood. Auer rods may be seen. Sixty percent to 75% of patients develop overt acute leukemia, and median survival is 6 months or less. Approximately 25% of patients present with RAEB-t. In the WHO classification, RAEB-t is not a distinct clinical entity; rather, it is included within the broader category, AML with multilineage dysplasia, and identified as AML with multilineage dysplasia following a myelodysplastic syndrome. (Refer to the PDQ summary on Adult Acute Myeloid Leukemia Treatment for more information.)
Refractory cytopenia with multilineage dysplasia (RCMD)
In patients with RCMD, bicytopenia or pancytopenia is present. In addition, dysplastic changes are present in 10% or more of the cells in two or more myeloid cell lines. There are less than 1% blasts in the blood and less than 5% blasts in the bone marrow. Auer rods are not present. Monocytes in the blood are less than 1 × 109. RCMD accounts for approximately 24% of cases of MDS. The frequency of evolution to acute leukemia is 11%. The overall median survival is 33 months. Refractory cytopenia with multilineage dysplasia and ringed sideroblasts (RCMD-RS) represents another category of RMDS. In RCMD-RS, features of RCMD are present, and more than 15% of erythroid precursors in the bone marrow are ringed sideroblasts. RCMD-RS accounts for approximately 15% of cases of MDS. Survival in RCMD-RS is similar to that in primary RCMD.
Unclassifiable myelodysplastic syndrome (MDS-U)
The cellular subtype, MDS-U, lacks findings appropriate for classification as RA, RARS, RCMD or RAEB. Blasts in the blood and bone marrow are not increased.
Myelodysplastic syndrome associated with an isolated del(5q) chromosome abnormality
This MDS cellular subtype, the 5q- syndrome, is associated with an isolated del(5q) cytogenetic abnormality. Blasts in both blood and bone marrow are less than 5%. This subtype is associated with a long survival. Karyotypic evolution is uncommon. Additional cytogenetic abnormalities may be associated with a more aggressive MDS cellular subtype or may evolve to acute myeloid leukemia.
The clinical classification of MDS is used to determine disease prognosis and treatment strategy, and to define entry requirements for many MDS clinical trials.
De novo myelodysplastic syndrome
Most MDS cases occur de novo with no known cause.
Secondary myelodysplastic syndrome
The risk of developing MDS may be increased by exposure to a variety of agents including:[12,13,14]
- Tobacco smoke.
- Ionizing radiation.
- Organic chemicals (e.g., benzene, toluene, xylene, and chloramphenicol).
- Heavy metals.
- Stone and cereal dusts.
- Exhaust gases.
- Nitro-organic explosives.
- Petroleum and diesel derivatives.
- Alkylating agents.
- Marrow-damaging agents used in cancer chemotherapy.