Most physicians classify myelodysplastic syndrome (MDS) according to subtype using a system called the French American British (FAB) system, which takes into account a number of features of the dysplastic cells. Under this system, physicians track the number and type of cytopenias -- shortages of specific types of blood cells; the number of blasts in the marrow and circulating blood; and the condition of the marrow. Physicians use this information to both help select the most effective treatment and to predict the course of the disease. Another classification system, the WHO system, was recently introduced, but its use is controversial among hematologists.
Classification by Subtype
Myelodysplastic syndromes comprise five main subtypes, listed below:
Refractory Anemia (RA)
- less than 5 percent blasts in the marrow; less than 1 percent in the bloodstream
- cytopenia in at least one cell line, most often red blood cells
- normal or hypercellular (filled with cells) marrow
- accounts for 20-30 percent of MDS cases
Refractory Anemia with Ringed Sideroblasts (RARS)
- less than 5 percent blasts in the marrow; less than 1 percent in the bloodstream
- cytopenia in at least one cell line, most often red blood cells
- more than 15 percent of red blood cell precursors in the marrow are of a type called ringed sideroblasts
- accounts for 2-5 percent of cases
Refractory Anemia with Excess Blasts (RAEB)
- 5-20 percent blasts in the marrow; less than 5 percent in the bloodstream
- normal or hypercellular (filled with cells) marrow
- accounts for about 30 percent of cases
Refractory Anemia with Excess Blasts in Transformation (RAEB-t)
- 21-30 percent blasts in the marrow; more than 5 percent in the bloodstream
- normal or hypercellular (filled with cells) marrow
- accounts for about 25 percent of cases
Chronic Myelomonocytic Leukemia (CMMOL)
- 5-20 percent blasts in the marrow; less than 5 percent in the bloodstream
- excessive numbers of monocytes (a type of white blood cell)
- normal or hypercellular (filled with cells) marrow
- accounts for 15-20 percent of cases.
In another subset of MDS, patients have hypoplastic bone marrow (bone marrow with decreased numbers of cells). This type resembles a disease called aplastic anemia, and physicians may choose to treat patients with this form of MDS with approaches similar to those used to treat aplastic anemia.
Classification by Chromosomal Abnormality
Physicians also classify MDS according to the genetic (chromosomal) abnormalities found in the diseased cells. Each cell in the body contains chromosomes (tightly coiled strands of DNA), which contain all the information that cells need to function normally and reproduce. In about half of patients with MDS, one or more chromosomal change, or mutation, can be identified. These mutations can include translocations (when pieces of DNA are exchanged between two chromosomes), inversions (when a piece of chromosome breaks off, turns upside down, and reattaches to the original chromosome), deletions (when a piece of a chromosome is missing), and additions (when extra pieces of the chromosome are present).
The most common chromosomal abnormalities in MDS include a translocation between chromosomes 5 and 7; an extra, third copy of chromosome 8 (trisomy 8); and a deletion of material on chromosome 20 (20q-). Any of these abnormalities may be detected in any of the forms of MDS. Patients with refractory anemia may have a syndrome known as 5q- syndrome (which requires a deletion on chromosome 5).
In primary MDS, physicians are more likely to discover a single chromosome change, while in treatment-related MDS, it is more common to find a complex array of chromosomal abnormalities.
Classification by Prognosis
Physicians classify individual cases of MDS using a prognostic scoring system called the International Prognostic Scoring System (IPSS). This system is based on three factors:
- the number of blast cells in the bone marrow
- the number of cytopenias of the blood cells
- the pattern of chromosomal abnormalities
Physicians use the scoring system to determine the extent of each patient's disease and to help determine the best treatment options and outlook for patients with MDS. Those with treatment-related MDS tend to have the poorest prognosis.
To learn more, visit the MDS Foundation's Patient Information Web site for detailed information on the IPSS system.
