Leukemia is a cancer of the blood cells. There are many different types of leukemia, depending upon which specific blood cells are affected. Each leukemia has different disease characteristics and therefore different treatment options. Several clinical diagnostic tests are utilized in order to determine the type and extent of leukemia. In order to better understand leukemia and its treatment, a basic understanding of normal blood cell production is useful.
Normal blood is made up of fluid called plasma and three main types of blood cells. Plasma is mainly water, but contains minerals, proteins and antibodies. The three major blood cell types are white cells, red cells and platelets. Each type of blood cell has a specific function. White blood cells, also called leukocytes, help the body fight infections and other diseases. Red blood cells, also called erythrocytes, make up half the blood’s total volume. They contain hemoglobin, which picks up oxygen from the lungs and carries it to the body’s organs. Platelets, or thrombocytes, help form blood clots to control bleeding.
Blood cells are produced inside the bones in a spongy space called the bone marrow. The process of blood cell formation is called hematopoiesis. All blood cells have a common origin called a stem cell. Stem cells develop into specific mature blood cells by a process called differentiation. Early immature cells are called blasts, which grow into mature blood cells. Once the cells are matured, they are released into the blood where they circulate throughout the body and perform their respective functions. In healthy individuals, there are adequate stem cells to continuously produce new blood cells. Normal production of mature blood cells occurs in an orderly fashion.
When leukemia occurs, the body produces large numbers of abnormal or immature blood cells. Leukemia cells look different and act different than normal blood cells and are often unable to perform their intended functions. Most leukemias occur in white blood cells and are classified as either myelocytic or lymphocytic, depending upon the type of white blood cell is affected. Leukemia is further classified by how fast the disease develops. When leukemia develops quickly and is composed of immature cells that do not properly mature, it is called acute leukemia. When leukemia is referred to as chronic, the cells are more mature and the accumulation of the abnormal cells occurs less rapidly.
Although leukemia is a cancer of the blood, it may affect other organs. In acute leukemias, the abnormal cells may collect in the central nervous system, the testicles, the skin and any other organ in the body. The most common place to detect the leukemia, however, is the blood and bone marrow. The following tests may be used to diagnose leukemia:
Bone marrow aspirate and biopsy: Since all blood cells ultimately originate in the bone marrow, an examination of the bone marrow consisting of a bone marrow aspirate and biopsy, provides useful information regarding the diagnosis and management of leukemia. Bone marrow aspirates and biopsies are typically performed on the hip bone with the patient laying face down. Patients are given an anesthetic under the skin to numb the area of the biopsy. The physician places a needle through the skin into the middle of the bone, typically a hip bone, and draws out a small amount of marrow (aspiration). This is followed by a biopsy, during which time the physician removes a small amount of bone as well as bone marrow from the same place the aspirate was drawn. Patients typically feel pressure and minimal pain from the procedure. The collected cells and the bone marrow biopsy are viewed under a microscope and special tests are performed to distinguish which type of blood cell is cancerous and the aggressiveness of the cancer.
Different types of leukemias have unique proteins and/or carbohydrates called antigens found on the surface or inside of the cell. Certain antigens are correlated to specific disease characteristics, leading to further classification of leukemia to help define optimal treatment options. The detection of specific antigens is called immunophenotyping. A laboratory test called immunohistocompatibility (IHC) testing is able to test for a multitude of antigens from a sample of blood or tissue.
The detection of chromosomal abnormalities, often referred to as “cytogenetic analysis”, is the testing of cancer cells to determine if specific genetic abnormalities exist. Chromosomes contain the genetic makeup or DNA of an individual, with a full copy of DNA present in every cell. Mutations, or alterations, in DNA can be responsible for the development of cancer and attribute to specific characteristics of the cancer. Different laboratory tests, including fluorescent in-situ hybridization (FISH), polymerase chain reaction (PCR) and flow cytometry, are able to detect specific genetic mutations within a cancer cell. Results from cytogenetic tests may become key factors in determining appropriate treatment options for patients.
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