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Myelodysplastic Syndrome

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Myelodysplastic syndrome (MDS) disorders are described and classified using systems developed by medical researchers and international consensus groups. These systems have evolved over time and include classifications based on the type of cell(s) affected, suspected cause, and prognosis. Classification of a patient's MDS is intended to aid in predicting the course of disease and making treatment decisions.

Cellular Classification
The current generally-accepted World Health Organization (WHO) classification is based upon the appearance of the cells in the blood and bone marrow. The following table summarizes the different types:

MDS Classification Affected Cells Blasts Comments
Refractory cytopenia with unilineage dysplasia (RCUD) Low levels of one or two types of circulating blood cells*; significant abnormality in only one type of cell in the bone marrow No or rare blasts (<1%) in blood; no increase in blasts in the marrow Constitutes about 10% to 20% of all MDS cases; generally good prognosis. Note: When RBCs are low (<10 g/dL), the condition is called refractory anemia (RA).
Refractory anemia with ring sideroblasts (RARS) Similar to RA, but 15% or more of the immature RBCs in the bone marrow have rings of iron around their nucleus (called ring sideroblasts). No or rare blasts (<1%) in blood; no increase in blasts in the marrow Makes up about 3% to 11% of MDS cases; also generally good prognosis
Refractory cytopenia with multilineage dysplasia (RCMD) Low levels of two or more types of blood cells*; significant abnormality in two or more types of bone marrow cells No or rare blasts (<1%) in blood; no increase in blasts in the marrow blood Makes up about 30% of MDS cases; about 10% of people with RCMD will develop acute leukemia; less favorable prognosis.
Refractory anemia with excess blasts-1 (RAEB-1) One or more types of blood cells are low* and one or more types of cells look abnormal Increased number of blasts, but less than 10% in marrow (5-9%) and less than 5% of WBCs in blood (2-4%) About 25% develop acute myeloid leukemia (AML); poorer prognosis
Refractory anemia with excess blasts-2 (RAEB-2) Same as RAEB-1 More blasts in the marrow (10-19%) and/or blood (5-19%) Up to 40% may develop AML; the remainder of cases are fatal due to bone marrow failure.

Together with RAEB-1, accounts for about 40% of all MDS cases.

Myelodysplastic syndrome, unclassifiable (MDS-U) One or more cell types in blood may be low*. No or rare blasts (≤1%) in blood; no increase in blasts in the marrow Lacks findings appropriate for classification into any other MDS category
Myelodysplastic syndrome associated with isolated del(5q) Low RBCs, normal WBCs, platelets may be increased No or rare blasts (<1%) in blood; no increase in blasts in the marrow Affected cells are missing part of chromosome 5; generally good prognosis; progression to acute leukemia occurs in less than 10% of patients.

*The thresholds for MDS are hemoglobin <10 g/dL, absolute neutrophil count (ANC) <1,800/microliter, and platelets <100,000/microliter.

Clinical Classification
The clinical classification is assigned based upon the perceived cause of the MDS. This includes:

  • Primary (de novo) MDS – when there is not an identifiable cause. The majority of cases of MDS are primary.
  • Secondary MDS – an identifiable cause exists. This may be a previous drug therapy or an exposure to something that is known to be associated with MDS. This type is less likely to respond to treatment.

Increased risk for the development of secondary MDS is associated with:

  • Previous chemotherapy for various tumors/cancers
  • Previous high-dose radiation exposure
  • Smoking
  • Exposure to:
    • Herbicides, pesticides and fertilizers
    • Benzene, toluene and other organic chemicals
    • Heavy metals
    • Petroleum industry-related chemicals

Prognostic Tool
An International Prognostic Scoring System (IPSS) has been developed that assigns a low, intermediate, or high prognosis risk value to a person's MDS. It evaluates information on how many blasts are present in the bone marrow, the number of different cell types that are low, and the chromosome abnormalities present. Based on these, it assigns a prognosis risk, an estimated median survival time, and a likelihood of progressing to develop acute myeloid leukemia (AML) within a specified time.

Those at a low risk are likely to live longer and are less likely to progress to AML in the next few years, while those at the highest risk are likely to have a poorer prognosis and are more likely to progress to AML within the next few months.

It is important to note that this tool looks at likelihood and cannot determine what will exactly happen with a specific person.

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