The term “leukemia” is often used broadly, leading to misunderstandings about various blood cancers. This article clarifies whether polycythemia vera is a type of leukemia and explains the relationship between these distinct blood disorders.
What is Polycythemia Vera?
Polycythemia vera (PV) is a chronic myeloproliferative neoplasm (MPN), which is a type of blood cancer that originates in the bone marrow. The defining characteristic of PV is the overproduction of red blood cells, leading to an abnormally high red blood cell mass. In many instances, the bone marrow also produces too many white blood cells and platelets. This excess cellular production makes the blood thicker than normal, a condition known as hyperviscosity, which increases the risk of blood clots.
Most individuals with polycythemia vera have a specific genetic change known as the JAK2 V617F mutation in their blood-forming cells. This mutation is found in approximately 95% to 98% of PV cases and plays a significant role in the uncontrolled blood cell production. Symptoms often develop slowly and can include fatigue, headaches, dizziness, itchy skin, and an enlarged spleen. PV is a chronic condition that typically progresses over time.
What is Leukemia?
Leukemia refers to a group of cancers that affect the blood-forming tissues, most commonly the bone marrow. It is characterized by the uncontrolled growth and accumulation of abnormal white blood cells, also known as leukocytes. These abnormal cells fail to mature properly and can interfere with the bone marrow’s ability to produce healthy red blood cells, white blood cells, and platelets.
Leukemia is generally classified based on two main factors: its progression speed and the type of blood cells affected. It can be either acute, progressing rapidly and involving immature blood cells, or chronic, developing more slowly and involving more mature cells. Furthermore, leukemia is categorized as myeloid or lymphoid, depending on whether the cancer originates in myeloid cells or lymphoid cells.
How Polycythemia Vera Relates to Leukemia
While both polycythemia vera and leukemia are cancers that originate in the bone marrow, they are classified as distinct types of blood disorders. Polycythemia vera is not typically categorized as a “leukemia” in the traditional sense. Instead, it belongs to a group of conditions called Myeloproliferative Neoplasms (MPNs). MPNs involve the overproduction of one or more types of mature blood cells, such as the red blood cells predominantly seen in PV.
Leukemia, by contrast, primarily involves the uncontrolled proliferation of immature white blood cells or their early precursors. A key distinction lies in the primary cell lines affected: PV predominantly impacts red blood cell production, while leukemia primarily affects white blood cell lines. Both conditions are clonal disorders, meaning they arise from a single abnormal stem cell in the bone marrow, but their classifications differ significantly.
When Polycythemia Vera Transforms
Polycythemia vera is not initially a leukemia, but it carries a risk of evolving into other conditions over time. One potential transformation is into Acute Myeloid Leukemia (AML). This occurs when abnormal bone marrow cells become more immature and aggressive, leading to a significant increase in blast cells. The reported incidence of AML transformation from PV ranges from approximately 2% to 14% within 10 years, and 2% to 5% at 15 years.
Polycythemia vera can also progress to myelofibrosis, a condition where the bone marrow is replaced by scar tissue, impairing its ability to produce healthy blood cells. This occurs in about 10% to 15% of PV patients, with the risk increasing to around 25% after 20 years of diagnosis. These are considered transformations or progressions of the original disease, rather than its initial state, and they occur in a minority of patients.