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 outcome (prognosis). Classification of a patient's MDS is intended to aid in predicting the course of disease and making treatment decisions.
World Health Organization (WHO) Classification
The current generally-accepted 2016 WHO classification is based upon the appearance of the cells in the blood and bone marrow as well as chromosome analysis (cytogenetics). The following table summarizes the different types:
|MDS Classification||Affected Cells*||Blasts||Comments|
|MDS with single lineage dysplasia (MDS-SLD)||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 (<5%)||People with this type generally have a good prognosis. They generally live a long life.|
|MDS with ring sideroblasts (MDS-RS)||Fifteen percent or more of the immature RBCs in the bone marrow have rings of iron around their nucleus (called ring sideroblasts). If only one type of bone marrow cells appear significantly abnormal, it is called MDS-RS-SLD. If two or all three types of bone marrow cells appear significantly abnormal, it is called MDS-RS-MLD.||No or rare blasts (<1%) in blood; no increase in blasts in the marrow (<5%)||People with this type also generally have a good prognosis. If a mutation in a gene called SF3B1 is detected, the required number of ring sideroblasts is 5% or higher.|
|MDS with multilineage dysplasia (MDS-MLD)||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 (<5%)||About 10% of people with this type will develop acute leukemia. About half of these patients may die within 2 years of their diagnosis.|
|MDS with excess blasts-1 (MDS-EB-1)||One or more types of blood cells are low* with or without significant abnormalities among bone marrow cells.||Increased number of blasts, but less than 10% in marrow (5-9%) and less than 5% of WBCs in blood (2-4%); no Auer rods||About 25% of people with this type will progress to acute myeloid leukemia (AML) and most die within 2 years of their diagnosis.|
|MDS with excess blasts-2 (MDS-EB-2)||Same as MDS-EB-1||More blasts in the marrow (10-19%) and/or blood (5-19%), or blast number in MDS-EB-1 range but with Auer rods||Up to 50% of people with this type develop AML; the remainder of cases are fatal due to bone marrow failure.|
|MDS unclassifiable (MDS-U)||One or more cell types in blood may be low* with or without significant abnormality among bone marrow cells.||No or rare blasts (<1% or exactly 1%) in blood; no increase in blasts in the marrow (<5%)||This type is rare. People may have this type when results of their blood and bone marrow tests don't fit into any other MDS category (e.g., MDS with exactly 1% blasts in blood, abnormality in single type of bone marrow cell but all three types of cells are low in blood*, or none of the cell types in blood is low but certain cytogenetic abnormality is detected in bone marrow).|
|MDS 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 (<5%); no Auer rods||Affected cells are missing the long arm of chromosome 5 [called del(5q)] alone or with one additional abnormality except -7 or del(7q). People with this type generally have a good prognosis. Less than 10% progress to acute leukemia. They may respond to Lenalidomide treatment.|
*The thresholds for MDS are hemoglobin <10 g/dL, absolute neutrophil count (ANC) <1,800/microliter, and platelets <100,000/microliter.
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 intensive chemotherapy or an exposure to something that is known to be associated with MDS, such as radiation therapy. 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
- Exposure to:
- Herbicides, pesticides and fertilizers
- Benzene, toluene and other organic chemicals
- Heavy metals
- Petroleum industry-related chemicals
An International Prognostic Scoring System (IPSS) was developed in 1997 that assigns a low, intermediate, or high prognosis risk value to a person's MDS. The IPSS was revised in 2012 (known as IPSS-R) to incorporate more detailed disease factors (number of blasts, cytogenetics, hemoglobin, absolute neutrophil count, platelet count) and patient's age. The IPSS-R is more precise in predicting outcome than the original IPSS. It assigns a prognosis risk, an estimated median survival time, and a likelihood of progressing to acute myeloid leukemia (AML) within a specified time.
A World Health Organization (WHO) Prognostic Scoring System (WPSS) has also been developed. It evaluates the type of MDS based on the WHO classification, chromosome abnormalities, and whether or not the patient requires blood transfusions and assigns a prognosis risk.
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 these tools look at likelihood and cannot determine what will exactly happen with a specific person.