Acute Myeloid Leukemia (AML) is a disorder in which myeloid blasts (precursor of myeloid cells) start dividing without control. The abnormal blasts get crowded in the red bone marrow and peripheral blood causing a reduction in the number of normal blood cells. Pathologically AML is defined as the presence of >/=20% myeloid blasts in the bone marrow or peripheral blood. Presence of <20% of myeloid blasts is defined as myelodysplastic syndrome (MDS).
Diagnosis of Acute Myeloid Leukemia
If a person is suspected to have AML, some investigations are required to confirm the diagnosis of the disease. Further, these investigations can help in determining an appropriate treatment approach.
Following are some commonly used diagnostic tools for AML:
Blood tests provide very important information that provides direction to the diagnostic workup of AML. Following are the commonly employed blood tests for the diagnosis of the AML:
Complete Blood Cells Count (CBC)
This test provides information on the level of RBCs, WBCs, and platelets. Usually, RBCs and platelets are reduced and WBCs may be reduced or increased.
In this test, a drop of a blood sample is spread on a glass slide and this is observed under a microscope. It helps in detecting any change in the appearance and number of various blood cells. Apart from above blood tests, blood coagulation tests (disseminated intravascular coagulation panel) and some blood chemistry analyses (the level of creatinine, uric acid, potassium, calcium, phosphorus, and lactate dehydrogenase (LDH) level) may also be employed.
Bone Marrow Aspiration/Biopsy
Aspiration samples contain a small number of cells and biopsy contains a tiny piece of tissue collected from the bone with the help of a biopsy needle. The biopsy sample is then tested in a laboratory and can provide very useful information about the AML cells such as the type AML, the severity of cancerous changes involved, and the presence of specific defective genes or proteins.
In this technique, a very thin portion of biopsy sample is first attached to a microscope glass slide. The sample is then treated with a specific antibody which gets attached to a protein specific to certain types of cancer cells. Some reagents are then added to the treated sample that causes the bound antibody to change its color. The change in color of the antibody-protein complex can be observed under the microscope, which confirms the type of cancer cells.
In this technique, the aspiration sample is first treated with some fluorescent antibodies that get attached to certain specific proteins (antigens) on the surface of cells. The treated sample is then analyzed using a laser beam and a detector attached to a computer. This test can detect different types of cells along with the quantification of each type of cells.
In this technique, chromosomes are evaluated for certain defects which are common in AML. The sample cells are first grown into the culture medium and are observed under a microscope after adding certain reagents that bind only to a specific defective portion of a chromosome. This test enables detection of chromosomal abnormalities like translocation, amplification, or deletion.
Fluorescent in situ hybridization (FISH)
In this technique, a fluorescent RNA probe is used which binds to a specific portion of a chromosome in the sample cells. Then, the sample can be examined under a microscope to determine the presence of certain chromosomal abnormalities like translocation, addition, or deletion. This technique is very sensitive, fast, and accurate. Thus, this technique is preferably used for detecting chromosomal abnormalities.
Polymerase chain reaction (PCR)
This is a very sensitive diagnostic tool which can detect a very small number of leukemia cells with a specific genetic change. This technique is generally used to diagnose minimum residual disease (MRD) in patients after treatment.
Utility of imaging tests is limited for the diagnosis of AML. However, these tests can be used to detect the involvement of different body parts by leukemia.
Computed tomography (CT) scan In this technique, detailed cross-sectional images of body organs are generated using x-rays. It can be utilized for scanning neck, chest, abdomen and pelvis for the diagnosis of any abnormal lymph node or involvement of liver, spleen, or other structures.
Magnetic resonance imaging (MRI) scan This technique provides detailed images of internal body structures using radio waves, strong magnetic field, and gadolinium-based contrast material (which is used via intravenous injection to improve the clarity of the MRI images). It can be utilized for scanning neck, chest, abdomen and pelvis for the diagnosis of any abnormal lymph node or involvement of liver, spleen, or other structures. It is considered very sensitive to detect the involvement of CNS the patients with neurological symptoms.
In this procedure, a sample of cerebrospinal fluid (CSF, a biological fluid that surrounds the brain and spinal cord) is collected with the help of a needle inserted up to the space around the spinal cord through the lower part (lumbar region) of the spine. The collected sample is then analyzed in a laboratory for the presence of leukemia cells. Generally, >/=5 leukocytes cells/microliters of CSF with the presence of lymphoblasts is considered as the CNS leukemia. This technique can also be used to deliver a treatment to the CSF.
Classification of Acute Myeloid Leukemia
The diagnosis of AML also includes it’s classification and risk stratification.
The previous classification of AML by French, American, and British (FAB) system (based on AML cells morphology) has been largely replaced with WHO classification system:
AML with certain genetic abnormalities
-AML with a translocation between chromosomes 8 and 21
-AML with a translocation between chromosomes 8 and 21
-AML with a translocation between chromosomes 9 and 11
-APL with a translocation between chromosomes 15 and 17
-AML with a translocation between chromosomes 6 and 9
-AML with a translocation or inversion in chromosome 3
-AML (megakaryoblastic) with a translocation between chromosomes 1 and 22
AML not otherwise specified
-AML with minimal differentiation (M0)
-AML without maturation (M1)
-AML with maturation (M2)
-Acute myelomonocytic leukemia (M4)
-Acute monocytic leukemia (M5)
-Acute erythroid leukemia (M6)
-Acute megakaryoblastic leukemia (M7)
-Acute basophilic leukemia
-Acute panmyelosis with fibrosis AML with myelodysplasia-related changes AML related to previous chemotherapy or radiation
Myeloid sarcoma (also known as granulocytic sarcoma or chloroma) Myeloid proliferations related to Down syndrome Undifferentiated and biphenotypic acute leukemias (both lymphocytic and myeloid features are present in such cases)
Each of these types consists of different cytogenetic and molecular characteristics. Analysis of cytogenetic and molecular abnormalities play important role in the diagnosis of the disease and to estimate disease prognosis.
Risk Stratification of Acute Myeloid Leukemia
Based on the various diagnostic investigations discussed above, risk stratification of AML may be done as follows-
Risk Stratification based on Chromosomal Abnormalities
Presence of following abnormalities is generally associated with the favorable outcome: translocation between chromosomes 8 and 21; translocation and between chromosome 15 and 17; and Inversion of chromosome 16.
Similarly, the presence of following cytogenetic abnormalities is generally associated with poor outcome: deletion of part of chromosome 5 or 7; translocation or inversion of chromosome 3; translocation between chromosomes 6 and 9 and between chromosomes 9 and 22; abnormalities of chromosome 11; and complex changes involving several chromosomes.
Risk Stratification based on Genetic Abnormalities
Poor prognosis is indicated by the presence of a mutation in the FLT3 gene, while a mutation in NPM1 gene (without any other change) and CEBPA gene suggest a better outcome.
Other prognostic factors which govern the severity of AML include patient’s age, WBC count, CNS involvement, response to induction therapy, prior hematological disorder. With the progress in understanding about the disease and genetic abnormalities involved, immunophenotypic/cytogenetic characterization has become an important parameter for selecting an appropriate treatment approach.