A myelodysplastic syndrome is a type of cancer in which the bone marrow does not make enough healthy blood cells and there are abnormal (blast) cells in the blood and/or bone marrow.
In a healthy person, the bone marrow makes blood stem cells (immature cells) that become mature blood cells over time.
Anatomy of the bone. The bone is made up of compact bone, spongy bone, and bone marrow. Compact bone makes up the outer layer of the bone. Spongy bone is found mostly at the ends of bones and contains red marrow. Bone marrow is found in the center of most bones and has many blood vessels. There are two types of bone marrow: red and yellow. Red marrow contains blood stem cells that can become red blood cells, white blood cells, or platelets. Yellow marrow is made mostly of fat.
A blood stem cell may become a lymphoid stem cell or a myeloid stem cell. A lymphoid stem cell becomes a white blood cell. A myeloid stem cell becomes one of three types of mature blood cells:
- Red blood cells that carry oxygen and other substances to all tissues of the body.
- Platelets that form blood clots to stop bleeding.
- White blood cells that fight infection and disease.
Blood cell development. A blood stem cell goes through several steps to become a red blood cell, platelet, or white blood cell.
In a patient with a myelodysplastic syndrome, the blood stem cells (immature cells) do not become healthy red blood cells, white blood cells, or platelets. These immature blood cells, called blasts, do not work the way they should and either die in the bone marrow or soon after they go into the blood. This leaves less room for healthy white blood cells, red blood cells, and platelets to form in the bone marrow. When there are fewer healthy blood cells, infection, anemia, or easy bleeding may occur.
The different types of myelodysplastic syndromes are diagnosed based on certain changes in the blood cells and bone marrow.
- Refractory anemia: There are too few red blood cells in the blood and the patient has anemia. The number of white blood cells and platelets is normal.
- Refractory anemia with ring sideroblasts: There are too few red blood cells in the blood and the patient has anemia. The red blood cells have too much iron inside the cell. The number of white blood cells and platelets is normal.
- Refractory anemia with excess blasts: There are too few red blood cells in the blood and the patient has anemia. Five percent to 19% of the cells in the bone marrow are blasts. There also may be changes to the white blood cells and platelets. Refractory anemia with excess blasts may progress to acute myeloid leukemia (AML). See the PDQ Adult Acute Myeloid Leukemia Treatment summary for more information.
- Refractory cytopenia with multilineage dysplasia: There are too few of at least two types of blood cells (red blood cells, platelets, or white blood cells). Less than 5% of the cells in the bone marrow are blasts and less than 1% of the cells in the blood are blasts. If red blood cells are affected, they may have extra iron. Refractory cytopenia may progress to acute myeloid leukemia (AML).
- Refractory cytopenia with unilineage dysplasia: There are too few of one type of blood cell (red blood cells, platelets, or white blood cells). There are changes in 10% or more of two other types of blood cells. Less than 5% of the cells in the bone marrow are blasts and less than 1% of the cells in the blood are blasts.
- Unclassifiable myelodysplastic syndrome: The numbers of blasts in the bone marrow and blood are normal, and the disease is not one of the other myelodysplastic syndromes.
- Myelodysplastic syndrome associated with an isolated del(5q) chromosome abnormality: There are too few red blood cells in the blood and the patient has anemia. Less than 5% of the cells in the bone marrow and blood are blasts. There is a specific change in the chromosome.
- Chronic myelomonocytic leukemia (CMML): See the PDQ summary on Myelodysplastic/ Myeloproliferative Neoplasms Treatment for more information.