Cancer cells circulating in the blood are usually tumors of the blood cells themselves, known as leukemias. The World Health Organization recognizes more than 32 unique leukemias arising in children and adults. Acute leukemias usually have a more rapid onset and faster progression than chronic forms. Whether a tumor is a myeloid or lymphoid leukemia depends on the type of blood cell from which the tumor arises. The most common leukemias in adults are acute myeloid leukemia and chronic lymphocytic leukemia. The most common leukemia in children is acute lymphoblastic leukemia.
Acute Myeloid Leukemia
The majority of cases of acute myeloid leukemia (AML) arise in adults over the age of 60, with an equal incidence in men and women. Many types of AML comprise different types of myeloid cells. The tumor cells range from immature cells without differentiation toward a specific type of blood cell to tumors comprising mature forms of red blood cells or white blood cells. While the types of AML may differ in the specific myeloid cell involved, most of these tumors are classified according to their genetic profiles rather than what the tumor cells look like. Medical research has shown that several genetic mutations are associated with the development of AML. In some cases, these genetic changes allow for therapy to be specifically targeted to the tumor cells, improving the chances of successful treatment. A very small percentage (approximately 1 to 2 percent) of these tumors may be attributed to the toxic effects of viruses, ionizing radiation, certain chemotherapy and benzene. Prognosis depends upon the type of AML and its genetic profile.
Chronic Lymphocytic Leukemia
Chronic lymphocytic leukemia (CLL) arises predominantly in adults over the age of 50 and is diagnosed in men almost twice as often as in women. It is a tumor of mature B-cell lymphocytes and is rarely identified in children. While most CLL tumor cells have abnormal genetic profiles, they are less specific than those seen in AML and are less amenable to targeted therapy. The clinical progression is slow and while remissions may follow diagnosis, the disease is not considered curable. The overall median survival is less than 10 years, which may be abruptly shortened if an acute, or rapidly proliferating, phase develops.
Acute Lymphoblastic Leukemia
Acute lymphoblastic leukemia (ALL) is classified into two categories depending on the subtype of lymphoid cells present: precursor B lymphoblastic leukemia or precursor T lymphoblastic leukemia. More than 85 percent of ALL cases are of precursor B-cell type, which predominantly affects children younger than 6. Precursor T-ALL comprises only 15 percent of childhood acute lymphoblastic leukemia, and in that population is most common in adolescent males. As in AML and CLL, genetic abnormalities are present in ALL. In precursor B-ALL, however, the genetics are important, as they help determine prognosis in addition to directing treatment options. Some of the genetic findings are associated with worse outcomes than others, especially when seen in very young patients (less than age 1). Overall, precursor B-ALL is associated with a cure rate exceeding 80 percent in children. A variety of genetic findings have also been identified in T-ALL; however, unlike in B-cell disease, none has an associated clinical significance. Survival rates in both B- and T-ALL are similar.
References
- "WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues, 4th Edition"; Edited by Steven H. Swerdlow, Elias Campo, Nancy Lee Harris, Elaine S. Jaffe, Stefano A. Pileri, Harald Stein, Jurgen Thiele, James W. Vardiman; 2008
- National Cancer Institute: Adult Acute Myeloid Leukemia Treatment
