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Myelodysplastic syndromes (MDS) are classified using the World Health Organization (WHO) classification system, which was most recently updated in 2016. It divides MDS into types based mainly on how the cells in the bone marrow look under the microscope, as well as some other factors:
Based on these factors, the WHO system recognizes 6 main types of MDS:
Because small differences in the way the cells look can change the diagnosis, doctors may sometimes disagree on a patient’s exact type of MDS.
This is the most common type of MDS. In the past, it was referred to as refractory cytopenia with multilineage dysplasia (RCMD).
This type of MDS is not common. It seldom, if ever, progresses to acute myeloid leukemia (AML). Patients with this type of MDS can often live a long time, even without treatment.
This was referred to as refractory cytopenia with unilineage dysplasia (RCUD) in the previous classification system. It includes refractory anemia (RA), refractory neutropenia (RN), and refractory thrombocytopenia (RT), depending on which cell type is affected.
In this type of MDS, many of the early red blood cells are ring sideroblasts. For this diagnosis, at least 15% of the early red blood cells must be ring sideroblasts (or at least 5% if the cells also have a mutation in the SF3B1 gene).
This condition is further divided into 2 types, based on how many of the cell types in the bone marrow are affected by dysplasia:
This type of MDS is not common. It rarely turns into AML, and the outcome for people with this type is generally better than for some other types of MDS. This was previously referred to as refractory anemia with ring sideroblasts (RARS).
In this type of MDS, there are more blasts than normal in the bone marrow and/or blood. The person also has low numbers of at least one type of blood cell. There may or may not be severe dysplasia in the bone marrow.
This condition is further divided into 2 types, based on how many of the cells in the bone marrow or blood are blasts:
This type accounts for about 1 in 4 cases of MDS. It is one of the types most likely to turn into AML, with the risk being higher for MDS-EB2 than for MDS-EB1. This was previously referred to as refractory anemia with excess blasts (RAEB).
In this type of MDS, the chromosomes of the bone marrow cells are missing part of chromosome number 5. (There may also be one other chromosome abnormality, as long as it isn’t a loss of part or all of chromosome 7.) The person also has low numbers of 1 or 2 types of blood cells (usually red blood cells), and there is dysplasia in at least 1 cell type in the bone marrow.
This type of MDS is not common. It occurs most often in older women. For reasons that aren’t clear, patients with this type of MDS tend to have a good prognosis (outlook). They often live a long time and rarely go on to develop AML.
This type of MDS is uncommon. For MDS-U, the findings in the blood and bone marrow don’t fit any other type of MDS. For example, the numbers of any one of the cell types may be low in the blood, but less than 10% of that type of cell looks abnormal in the bone marrow. Or the cells in the bone marrow have at least one certain chromosome abnormality that is only seen in MDS or leukemia.
This type is rare, so it has not been studied well enough to predict prognosis (outlook).
Along with the WHO classification, MDS can also be classified based on the underlying cause. This is known as a clinical classification.
Secondary MDS is often related to prior cancer treatment, or it develops in someone who already had a different bone marrow disease. This is discussed further in Risk Factors for Myelodysplastic Syndrome.
Identifying MDS as primary or secondary is important because the secondary type is much less likely to respond to treatment.
The American Cancer Society medical and editorial content team
Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.
Arber DA, Orazi A, Hasserjian R, et al. The 2016 revision to the World Health Organization classification of myeloid neoplasms and acute leukemia. Blood. 2016; 127:2391-2405.
Aster JC, Stone RM. Clinical manifestations and diagnosis of the myelodysplastic syndromes. UpToDate. 2017. Accessed at https://www.uptodate.com/contents/clinical-manifestations-and-diagnosis-of-the-myelodysplastic-syndromes on October 4, 2017.
National Comprehensive Cancer Care Network. NCCN Treatment Guidelines in Oncology: Myelodysplastic Syndromes. V.1.2018. Accessed at www.nccn.org/professionals/physician_gls/pdf/mds.pdf on October 4, 2017.
Last Revised: January 22, 2018
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