Leukemia refers to a series of malignant neoplasms of hematopoietic stem cells, characterized by progressive and uncontrolled production of immature white blood cells (WBCs). Naturally, this leads to an increase in WBCs counts in the peripheral blood and the bone marrow. Leukemia is a misnomer, and it is commonly known as blood cancer.
Leukemias are different from lymphomas in that they don’t form aggregated masses in lymph nodes. However, lymphomas can have a leukemic phase where malignant white blood cells are spilled into the peripheral blood due to mutations and overcrowding of cells.
Blood cells are derived from multipotent hematopoietic stem cells, which give rise to myeloblastic stem cell series and lymphoblastic stem cell series. Leukemias can develop due to an abnormal maturation and differentiation process of these blast cells into lymphocytes and myelocytes. Lymphocytes give rise to T and B lymphocytes while myelocytes give rise to red blood cells, platelets, granulocytes and monocytes. The worldwide incidence of leukemia is 1 in every 10,000 per year.
Classification of leukemia
Leukemia is classified according to course of the disease into acute leukemia (symptoms appear in a matter of weeks to months) and chronic leukemia (symptoms appear after years).
Acute leukemia is further classified into acute lymphoblastic leukemia (ALL), and acute myeloblastic leukemia (AML). Chronic leukemia is also classified as chronic lymphocytic leukemia (CLL) and chronic myelocytic leukemia (CML).
The WHO has classified acute leukemias according to the immunotyping, which takes in consideration surface markers on cells. They also consider cell morphology and molecular changes these cells undergo. This classification helps to achieve an easier diagnosis and treatment, and it is also a useful prognostic tool.
According to the WHO, acute myeloid leukemias can be classified into 6 types, including AML with recurrent genetic abnormalities, AML with myelodysplasia-related changes, therapy-related AML, AML related to trisomy 21 (Down syndrome), and unspecified AML. On the other hand, ALL has 2 types which are precursor T cell ALL (more common in males between 15 to 20 years of age), and precursor B cell ALL (more common, overall).
As mentioned above, lymphomas can also have leukemic phase due to spilling of the malignant cells into the blood. Examples of this include precursor B cell lymphoma, precursor T cell lymphoma, and small lymphocytic lymphoma. Small lymphocytic lymphoma is very similar to chronic lymphocytic leukemia, and precursor T and B cell lymphomas are similar to Acute lymphoblastic leukemia. Sometimes, these names can be used interchangeably, but they are often seen as different stages of same malignancy.