Is essential thrombocytosis (ET) a type of blood cancer?

Essential Thrombocythemia Is a Type of Blood Cancer

Yes, essential thrombocythemia (ET) is classified as a type of blood cancer, specifically a Philadelphia chromosome-negative myeloproliferative neoplasm (MPN). 1 According to the National Comprehensive Cancer Network (NCCN) guidelines, ET belongs to a group of heterogeneous disorders of the hematopoietic system that includes polycythemia vera (PV) and myelofibrosis (MF), collectively known as myeloproliferative neoplasms. 1

Classification and Characteristics

Essential thrombocythemia is characterized by:

• Clonal proliferation primarily of the megakaryocyte lineage 1
• Sustained platelet count ≥450 × 10^9/L 1
• Presence of driver mutations in approximately 80-90% of cases:
  • JAK2 V617F (most common)
  • CALR mutations
  • MPL mutations 2

The 2016 World Health Organization (WHO) classification clearly categorizes ET as a neoplasm, requiring the following diagnostic criteria:

1. Platelet count ≥450 × 10^9/L
2. Bone marrow biopsy showing proliferation mainly of the megakaryocyte lineage with increased numbers of enlarged, mature megakaryocytes
3. Not meeting criteria for other myeloid neoplasms
4. Presence of JAK2, CALR, or MPL mutation, or in their absence, no evidence for reactive thrombocytosis 1

Disease Burden and Prognosis

The prevalence of ET in the United States is estimated to be approximately 134,000 patients. 1 While ET is generally considered an indolent form of blood cancer with better prognosis than many other malignancies, it still impacts survival:

• Median survival is approximately 18 years overall 2
• Younger patients may have survival exceeding 35 years 2
• Life expectancy is less than that of the general population 2, 3

Risk of Disease Progression

As with other blood cancers, ET carries risks of disease progression:

• Risk of transformation to myelofibrosis (post-ET MF) at 10 years is approximately 4-8% 2
• Risk of leukemic transformation (progression to acute myeloid leukemia) at 10 years is <1% 2, 3
• Certain genetic factors (spliceosome mutations, abnormal karyotype) may increase transformation risk 2

Treatment Approach

Treatment for ET follows cancer management principles with risk stratification:

• Very low-risk and low-risk patients may receive observation or aspirin therapy 1
• Higher-risk patients require cytoreductive therapy (hydroxyurea, interferon-α, busulfan) 4
• Treatment goals include preventing thrombotic/hemorrhagic complications and monitoring for disease progression 4, 2

Clinical Implications

Understanding ET as a blood cancer is important because:

1. It requires regular monitoring for disease progression and transformation
2. Treatment decisions are based on risk stratification models specific to neoplastic conditions
3. Patients need appropriate oncologic follow-up and surveillance
4. Some patients may eventually require consideration for allogeneic stem cell transplantation if disease progresses 4

While ET is generally less aggressive than many other blood cancers, its classification as a myeloproliferative neoplasm reflects its nature as a clonal hematologic malignancy with potential for disease progression and impact on mortality and quality of life.