Myeloid malignancies are diseases that affect the blood and bone marrow. Those that can occur during childhood include acute myeloid leukemia (AML), myelodysplastic syndromes and juvenile myelomonocytic leukemia (JMML).

Acute myeloid leukemia (AML) is a type of leukemia, or cancer of the body’s blood-forming cells. Also known as acute myelogenous leukemia, AML affects the body’s blood making system, specifically the bone marrow. Leukemia is the most common cancer in children and adolescents, with acute leukemias being the most common.

The American Cancer Society (ACS) predicts that in 2006 there will be 2,800 new cases of leukemia in children in the United States. Of these, 78 percent will be cases of acute lymphocytic leukemia (ALL), which arises from a different family of blood cells than AML.  Most of the remaining cases will be AML, which is most common during the first two years of life and is less common among older children.

AML develops in the bone marrow but typically spreads quickly into the blood. 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 to fight infection
  
• Red blood cells to carry oxygen to tissues throughout the body
  
• Platelets to help develop blood clots and control bleeding

In children with AML, the stem cells usually develop into a form of immature white blood cell called myeloblasts, or myeloid blasts. These cells are abnormal and fail to develop into healthy white blood cells. Since they are defective, these cells are unable to defend the body from disease. In other cases of AML, too many stem cells develop into abnormal red blood cells or platelets.

As the leukemia progresses, the abnormal cells (known as leukemia cells) can begin to increase, crowding out normal white blood cells, red blood cells and platelets. As a result, production of normal white blood cells is affected and the body’s ability to fight infection is impaired. In addition, the decrease in production of red blood cells and platelets results in anemia and bleeding disorders. Although AML begins in the bone marrow, it may eventually spread through the bloodstream to organs, including the liver, brain, spinal cord, ovaries, skin and testicles.

A less common type of myeloid malignancy in children is known as myelodysplastic syndrome (MDS). MDS is a group of diseases that affects the blood and bone marrow. In children with these diseases, the stem cells do not mature into healthy red blood cells, white blood cells or platelets. Some physicians consider MDS as an early form of leukemia or pre-leukemia. According to the ACS, approximately 30 percent of MDS cases eventually progress to acute leukemia. The ACS estimates that between 10,000 and 20,000 new cases of myelodysplastic syndromes are diagnosed each year. However, these syndromes are rare in young children with more than 50 percent of the cases occurring in adults over age 70.

Juvenile myelomonocytic leukemia (JMML) is a third type of children’s cancer in which the bone marrow produces too many white blood cells. A rare type of childhood cancer, JMML occurs most often in children under the age of 2. According to the National Cancer Institute (NCI), it accounts for less than 1 percent of all childhood leukemias.

As with AML, children with myelodysplastic syndromes and JMML are likely to develop anemia, infection or easy bleeding due to a decrease in normal red blood cells, white blood cells and platelets.