Myelodysplastic syndromes (MDS) are a group of diseases of the blood and bone marrow. They are caused by abnormal blood-forming cells of the bone marrow.

Bone marrow is the soft, inner component of bones. It is composed of blood-forming cells, fat cells and tissues that support the growth of blood cells. All forms of blood cells are produced in the bone marrow from a cell called the stem cell. Blood cells produced from stem cells in the bone marrow include:

• White blood cells (cells that fight infection). The three main types of white blood cell include:
  
  
• • Granulocytes (neutrophils, basophils and eosinophils)
    
    
  • Monocytes
    
    
  • Lymphocytes
    
    
• Red blood cells (cells that carry oxygen to tissues all over the body).
  
  
• Platelets (cells that help develop blood clots and control bleeding).

In patients with MDS, however, the bone marrow cannot effectively manufacture blood cells. Instead, many of the blood cells produced are defective and they fail to mature into normal red blood cells, white blood cells or platelets. The body usually destroys the abnormal cells in the bone marrow, or shortly after they enter the bloodstream. As a result, patients with MDS have shortages of blood cells. These shortages may lead to infection, anemia and bleeding disorders.

In the past, MDS were not considered to be cancer because MDS cells differ from cancer cells in some ways. Cancer cells, which usually live longer than normal cells, build up and crowd out normal cells. Although MDS cells may also accumulate, they do not function normally and die more easily than normal cells.

Today, most hematologists (physicians who specialize in diseases of the blood and blood-forming tissues) classify MDS as a form of cancer. It is considered to be a clonal disease, which is a large group of abnormal cells that share identical growth properties and come from a common abnormal mother cell. This characteristic is often observed in cancers where the group of abnormal cells appear to have developed from a single abnormal cell.

Another reason for the classification of MDS as cancer, according to the American Cancer Society (ACS), is that approximately 30 percent of MDS cases eventually progress into acute myeloid leukemia (a cancer of bone marrow cells). Although the diseases may never progress into leukemia, some physicians believe MDS is an early form of the disease. In fact, the diseases were known as preleukemia or smoldering leukemia in the past.

Cases of MDS are not tracked, and as a result, the exact number of MDS cases occurring each year in the United States is unknown. According to the ACS, most estimates are between 10,000 and 15,000 new cases each year. Approximately 80 to 90 percent of patients with MDS are over the age of 60. The diseases rarely occur in young adults.