Normal Anatomy and the Development of MDS

All blood cells start in the bone marrow. They begin as stem cells. These cells mature into:

• Red blood cells—Carry oxygen from the lungs to the rest of the body.
• Platelets—Helps the blood clot to stop bleeding.
• White blood cells—Helps the body fight infection and disease.

Blood cells need to be replaced often. If new ones are not made it will lead to low levels of blood cells in the body. With MDS, stem cells are made but do not fully mature. They crowd the bone marrow and make it hard for healthy cells to grow. They also move out into the blood. These early cells cannot do the work of healthy blood cells. It can lead to a wide range of symptoms.

Primary MDS is caused by a problem in the genes. It affects how the cell develops. It is not always known what causes the change. Secondary MDS is linked to prior cancer treatment. Chemotherapy and radiation therapy both cause bone marrow damage. It may lead to damaged stem cells.

Cancer Cell Growth

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Types of MDS

Blood and bone marrow cells are tested in a lab. The results help with how MDS is classified. Types of MDS based on The World Health Organization (WHO) classification include:

• Refractory cytopenia with multilineage dysplasia (RCMD)—This is the most common type of MDS. Found in about half of cases. RCMD affects at least 2 types of blood cells. In some people, RCMD can become acute myeloid leukemia (AML).
• Refractory anemia with ringed sideroblasts (RARS)—Low numbers of red blood cells, and normal numbers of white blood cells and platelets.
• Refractory cytopenia with unilineage dysplasia (RCUD)—Low numbers of one type of blood cell, and normal numbers of the 2 others. This type of MDS generally has the best outcome. RCUD turning into AML is rare.
• Refractory anemia with excess blasts-1 (RAEB-1)—One or more blood cell types are too low. Some blasts may be found in the bloodstream
• Refractory anemia with excess blasts-2 (RAEB-2)—Like RAEB-1. More blasts are found in the bone marrow.
• Myelodysplastic syndrome, unclassified (MDS-U)—The details of blood and bone marrow cells do not fit into any other type. It is not common.
• Myelodysplastic syndrome associated with isolated del (5q) abnormality —A certain gene in the bone marrow cells is missing. Red blood cell counts are low, but white cell counts are normal. There may be a higher number of platelets.