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Causes and Risk Factors

Most people who have MDS are diagnosed with primary MDS, also called de novo MDS. With primary MDS, there are usually no obvious causes that would explain why the disease developed.

One known MDS risk factor is repeated exposure to the chemical benzene, which damages the DNA of stem cells. Benzene is found in cigarette smoke, the most common known cause of exposure to this toxin. Benzene is also found in certain industrial settings. Strict regulations on its use, however, have resulted in decreased exposure in the workplace.

Some people with MDS are diagnosed with treatment-related MDS (also called secondary MDS). Treatment-related MDS arises after chemotherapy and radiotherapy treatment for other cancers such as lymphoma, myeloma or breast cancer. The proportion of people exposed to chemotherapy and radiotherapy who develop MDS is small.


  • You can't catch MDS from someone else.
  • Some experts think MDS and other cancers may develop in some people because their cells have a limited ability to repair themselves after exposure to certain risk factors, but this hasn't been proved.

How Does MDS Develop?

The process that leads to MDS starts in a stem cell in the bone marrow. Stem cells form blood cells (white cells, red cells and platelets).

With MDS, blood cell production in the marrow usually increases with more than the normal number of developing blood cells filling the marrow. These developing cells in the marrow die as they approach maturity, before they would normally be released into the blood. This results in a lower than normal number of circulating blood cells.

Several things can happen if MDS isn't treated:

  • Low numbers of red cells mean they can no longer supply an adequate amount of oxygen, leading to anemia.
  • The immune system can't guard against infection effectively because of a lack of neutrophils (a type of white cell), a condition called neutropenia.
  • Low numbers of platelets can cause bleeding and bruising with no apparent cause, a condition called thrombocytopenia.

However, marrow cell disturbances in MDS patients range from mild to very severe. In some patients, the MDS cells can still function and enter the blood. Red cells continue to carry oxygen, white cells (neutrophils and monocytes) ingest and kill bacteria and platelets plug up injury to blood vessels.

In more severe cases of MDS, blood cell formation is more disordered and abnormal blast cells (blasts) accumulate in marrow and blood. These cells don't mature into cells that function properly. They aren't as capable as normal cells are of maturing into red cells, neutrophils and platelets.

Normally, blasts make up less than 5 percent of all cells in the marrow. With MDS, blasts often make up more than 5 percent of cells in marrow. The number of blasts is key to determining how severe the MDS is.

Low-Risk and High-Risk MDS

Your doctor may describe your MDS as "low risk" or "high risk." Low-risk means that:

  • your disease is progressing slowly
  • the abnormal blasts that enter your bloodstream function like normal blood cells
  • the disease causes only mild to moderate anemia

High risk means that:

  • your levels of red cells, white cells and platelets are significantly low
  • your blast cells don't develop into functional red cells, white cells and platelets
  • People whose blast cells make up more than 20 percent of total cells in the marrow are diagnosed with acute myeloid leukemia (AML).

Your doctor plans your treatment according to whether your MDS is low or high risk. For more information, see Before Treatment.

Is MDS Cancer?

Many patients ask whether MDS is a type of cancer. MDS, in fact, is cancer. Cancer means that a mutation (change) to a normal cell leads to the development of cells that no longer behave normally. However, the effect of a disease on a patient's life is more important than the term used to describe the disease. The course of MDS can be slower and interfere less with quality of life than the course of some other diseases that aren't cancer.

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last updated on Friday, March 16, 2012

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