Chronic myeloid neoplasms
Myeloproliferative neoplasms (MPN)
Polycythemia vera

Author: Erika M. Baardsen, D.O. (see Authors page)
Editor: Ling Zhang, M.D.

Revised: 30 October 2017, last major update October 2017

Copyright: (c) 2003-2017, PathologyOutlines.com, Inc.

PubMed Search: Polycythemia vera [title] Review[ptyp]
Cite this page: Baardsen, E.M. Polycythemia vera. PathologyOutlines.com website. http://pathologyoutlines.com/topic/myeloproliferativePV.html. Accessed December 15th, 2017.
Definition / general
  • Also referred to as polycythemia rubra vera, primary polycythemia and essential polycythemia
  • BCR-ABL1 negative myeloproliferative neoplasm (MPN) characterized by peripheral erythrocytosis with or without leukocytosis or thrombocytosis
Essential features
  • Relatively common MPN characterized by increased red blood cell production with elevated red blood cell count and hemoglobin level, which is not proportional to erythropoietin (EPO) production (usually have markedly decreased EPO level)
  • Diagnosis requires all 3 major criteria or 2 major and minor criterion (see Diagnosis section below, 2016 WHO classification)
  • JAK2 V617F (exon 14, valine to phenylalanine) and JAK2 exon 12 mutations are identified in > 95% and approximately 5% of patients, respectively (Blood 2006;108:1865, Hum Pathol 2006;37:1458, N Engl J Med 2007;356:459)
  • Three phases:
    • Prodromal / pre polycythemic
    • Overt polycythemic
    • Post polycythemic myelofibrosis / spent (20%)
  • < 10% in post polycythemic phase undergo blastic transformation
Epidemiology
Sites
  • Peripheral blood and bone marrow are major sites of involvement
  • Spleen and liver are common sites of extramedullary hematopoiesis in later stages
Pathophysiology
  • Clonal hematopoietic stem cell disorder
  • Point mutation in Janus kinase 2 (JAK2) gene at 9p24 results in constitutional activation of transcription factors of the STAT family, which promote growth factor independent proliferation and survival
  • The mechanisms of thrombosis and bleeding are due to increased circulating red blood cells and elevated platelets
  • Three phases:
    • Prodromal / pre polycythemic: characterized by borderline to mild erythrocytosis
    • Overt polycythemic: associated with significantly increased red cell mass
    • Post polycythemic myelofibrosis / spent: cytopenias, including anemia, are associated with ineffective hematopoiesis, bone marrow fibrosis, extramedullary hematopoiesis and hypersplenism
Etiology
  • Underlying cause is unknown
  • Genetic predisposition, ionizing radiation and occupational toxin exposure have been postulated as possible causes
Clinical features
  • Major symptoms are related to hypertension or vascular abnormalities caused by increased red cell mass
  • Plethora (characterized by excess blood and marked by turgescence and a florid complexion) is a unique feature of PV due to increased RBC mass
  • Common complaints are headache, dizziness, vertigo, visual disturbances, angina pectoris, tinnitus, paresthesias, pruritus, intermittent claudication, gout and erythromelalgia
  • 20% have documented venous thrombosis, such as hepatic portal vein thrombosis (eMedicine.com: Budd-Chiari syndrome [Accessed 18 October 2017]) or mesenteric vein thrombosis, or arterial thrombosis, myocardial ischemia, transient ischemic attack or stroke
  • May have symptoms related to altered microcirculation such as peptic ulcer, due to microthrombus formation, increased histamine release or decreased gastric pH in stomach
  • Early satiety is likely due to massive splenomegaly, seen in later phases
  • Abdominal pain may be secondary to ischemic bowel from thrombotic events
Diagnosis
  • WHO 2016 Revision - PV Criteria - diagnosis requires meeting either all 3 major criteria or the first 2 major criteria and the minor criterion (Blood 2016;127:2391, Am J Hematol 2017;92:94)
  • Major Criteria:
    • 1. Hemoglobin > 16.5 g/dL in men, > 16.0 g/dL in women, hematocrit > 49% in men, > 48% in women or increased red cell mass
      • More than 25% above mean normal predicted value
    • 2. Bone marrow biopsy showing hypercellularity for age with trilineage growth (panmyelosis) including prominent erythroid, granulocytic and megakaryocytic proliferation with pleomorphic, mature megakaryocytes (differences in size)
    • 3. Presence of JAK2 V617F or JAK2 exon 12 mutation
  • Minor Criterion:
    • Subnormal serum erythropoietin level (normal range of EPO: Adults: 4.1 - 19.5 mU/mL)
  • Major criterion 2 may not be required in cases with sustained absolute erythrocytosis:
    • Hemoglobin levels > 18.5 g/dL (hematocrit, 55.5%) in men or > 15.6 g/dL (hematocrit, 49.5%) in women, if major criterion 3 and the minor criterion are present.
    • Of note, approximately 20% of PV patients' bone marrow biopsies show myelofibrosis, which may predict a more rapid progression to overt myelofibrosis (post PV MF)
Diagnostic criteria for post polycythemic myelofibrosis (post PV MF)
  • Required Criteria (Blood 2016;127:2391, Am J Hematol 2017;92:94)
    • Documentation of a previous diagnosis of WHO defined PV
    • Bone marrow fibrosis grade 2 or 3 on a 0 to 3 grading scale (either 2008 WHO classification or European consensus on grading of bone marrow fibrosis) (Haematologica 2005;90:1128)
  • Additional criteria (2 required):
    • Anemia or sustained loss of either phlebotomy (in absence of cytoreductive therapy) or cytoreductive treatment requirement for erythrocytosis
    • Leukoerythroblastic peripheral blood picture
    • Increasing splenomegaly, defined as either an increase in palpable splenomegaly of > 5.0 cm from baseline (distance from left costal margin to the tip of the spleen) or appearance of newly palpable splenomegaly
    • At least 2 of these constitutional symptoms: >10% weight loss in 6 months, night sweats, unexplained fever (> 37.5 C)
Laboratory
    Diagnostic criteria set by the Polycythemia Vera Study Group (PVSG)
  • Requires all three Category A criteria be present or that criterion A1, A2 and any two Category B criteria are present
  • Category A criteria:
    • A1. Total red blood cell mass ≥ 36 mL/kg in males or ≥ 32 mL/kg in females
    • A2. Arterial oxygen saturation ≥ 92%%
    • A3. Splenomegaly
  • Category B criteria:
    • Thrombocytosis with platelet count > 400,000/μL
    • Leukocytosis with a white blood cell count > 12,000/μL
    • Leukocyte alkaline phosphatase (LAP) > 100 U/L in the absence of fever or infection
    • Serum vitamin B12 concentration > 900 pg/mL or binding capacity > 2,200 pg/mL

Other findings that may be present but are not necessary for diagnosis include:
  • Subnormal erythropoietin (EPO) levels
  • Endogenous erythroid colony (EEC) formation
  • Detection of JAK2 V617F or functionally similar mutation (JAK2 exon 12)
Prognostic factors
  • Leukocytosis may predict inferior survival and leukemic transformation
  • A recent study showed older age, leukocytosis, venous thrombosis and abnormal karyotype are associated with adverse clinical outcomes (Leukemia 2013;27:1874)
  • Three risk groups are proposed including low risk (0 points), intermediate risk (1 or 2 points) and high risk (3 points)
    • Adverse points are assigned to age 67 years (5 points), age 57 - 66 years (2 points), leukocyte count 15 x 109/l (1 point) and venous thrombosis (1 point) with median survival of 28 years (low risk), 19 years (intermediate risk) and 11 years (high risk) (Leukemia 2013;27:1874)
  • Next generation sequencing study indicated that ASXL1, SRSF2 and IDH2 mutations are negative prognostic parameters associated with shorter overall, leukemia free and fibrosis free survival (Am J Hematol 2017;92:94)
Case reports
Clinical outcome and treatment
  • Median survival > 10 years
  • Most patients die of thrombosis or hemorrhage (approximately 15%)
  • Approximately 20% progress to post PV MF at 10 to 15 years
    • Due to increased red blood cell production independent of mechanisms that typically regulate erythropoiesis
    • 15% develop spent phase with marrow fibrosis and marked cytopenias after 10 years, death in months if no treatment
  • Leukemic transformation rates at 20 years are estimated at < 10% for PV (Am J Hematol 2017 Jan;92:94)
    • 2% develop acute myeloid leukemia with phlebotomy treatment but 15% with alkylating agents or radioactive phosphorus (no longer used)
    • Risk of leukemic transformation (ALL) is rare, except in patients treated with cytotoxic regimens
  • Therapeutic goals:
    • Decrease thrombosis without increasing bleeding
    • Prevent progression
    • Ameliorate symptoms
  • Treatment guidelines (Ann Oncol 2015;26:v85, Blood 2014;124:3212, N Engl J Med 2004;350:114)
    • Low / intermediate risk for thrombosis:
      • First line: phlebotomy
      • Low dose aspirin
      • Intensive management of cardiovascular risk factors
    • High risk for thrombosis:
      • Cytoreductive therapy (hydroxyurea)
      • Adjunct phlebotomy
      • Low dose aspirin
    • Pregnant women:
      • First line: Phlebotomy
      • Low dose aspirin
      • Low molecular weight heparin in postpartum period
Microscopic (histologic) description
  • Even in early pre polycythemic stage, the morphologic findings are sufficient to allow distinction of PV from secondary polycythemia and other subtypes of MPN

Bone Marrow (Pre polycythemic and overt polycythemic phases) (Figures A - C)
  • Hypercellular (mean 80% cellularity, range 37 - 100%)
  • Panmyelosis with notable predominance of erythroid and megakaryocytic lineages
  • Complete and progressive maturation of all 3 hematopoietic lineages
  • Abnormal megakaryocyte morphology and architecture (PAS stain can assist in identification); variably hyperlobulated; not as pronounced as in essential thrombocythemia (ET) or as pleomorphic as in primary myelofibrosis (PMF) (see ET and PMF images)
  • Reticulin fibrosis tends to be minimal or absent or limited to a small focus
  • Iron stores are decreased, often absent

    Post PV / spent phase: (Figures D - G)
  • Leukoerythroblastosis
  • Overt bone marrow reticulin and collagen fibrosis
  • Osteosclerosis usually prominent
  • Blasts may increase but are less than 20%
  • Dilated sinuses with intrasinusoidal hematopoiesis
  • Decline in hematopoietic cells
  • Overall picture may be indistinguishable from advanced PMF; need prior documentation of PV diagnosis
Microscopic (histologic) images

Images hosted on Pathout server:

Images contributed by Erika M. Baardsen, D.O. and Ling Zhang, M.D.
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Figure A: Hypercellular bone marrow

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Figure B: Panmyelosis

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Figure C: Mild increase in reticulin fibers

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Figure D: Leukoery-throblastosis

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Figure E: Fibrosis and erythroid islands


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Figure F: Core biopsy with dense fibrosis

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Figure G: Core biopsy with extensive fibrosis

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Figures H and I: Bone marrow core biopsy with leukemic transformation (AML, M5)

Peripheral smear description
  • Peripheral blood (Pre polycythemic and overt polycythemic phases):
    • Normochromic, normocytic red blood cells; or microcytic, hypochromatic when iron deficiency is present
    • Erythrocytosis with increased hemoglobin or hematocrit
    • Red blood cell morphology is unremarkable except in cases with concomitant iron deficiency
    • Deeply basophilic reticulocytes or rare normoblasts may be present
    • Mild neutrophilic leukocytosis (WBC usually < 20k/L) with rare left shift
    • Mild basophilia may be seen
    • Thrombocytosis; may be particularly prominent in prodromal phase or exaggerated by concurrent iron deficiency
  • Post PV / spent phase: myeloid metaplasia characterized by leukoerythroblastosis (Figure D)
    • Poikilocytosis with teardrop shaped red blood cells
  • Findings of >10% blasts in the peripheral blood or bone marrow or the presence of significant myelodysplasia is unusual and most likely signals transformation to an accelerated phase or a myelodysplastic syndrome (MDS)
  • Cases in which 20% or more blasts are found are considered AML (Figures H - I)
Positive stains
Negative stains
  • c-Mpl (thrombopoietin receptor) on megakaryocytes is usually moderate to strong in normal controls and secondary erythrocytosis, but this is not used for diagnosis (Blood 2000;96:771)
Molecular / cytogenetics description
  • Most frequent somatic activating mutations of JAK2 are located in exon 14 (JAK2 V617F) (96%) followed by exon 12 (3%)
    • Mutation occurs in all myeloid lineages
    • JAK2 V617FF is not specific and is found at a lower frequency in other MPN
  • Patients with JAK2 V617F are usually elderly with higher level of hemoglobin, WBC count and lower level of platelets while patients with JAK2 exon 12 mutation are frequently younger with erythroid hyperplasia but low erythropoietin production (low EPO level) (Am J Hematol 2017;92:94)
  • Cytogenetic abnormalities are detectable in about 20% of patients and most commonly include +8, +9, del(20q), del(13q) and del(9p)
  • Chromosomal abnormalities increase in frequency with disease progression with approximately 100% of patients who develop MDS or AML having cytogenetic abnormalities
    • AML clone usually lacks JAK2 mutations
  • Next generation sequencing identified several gene mutations in PV in addition to JAK2, which include most frequently TET2, ASXL1 followed by SRSF2, IDH2 with combined prevalence of 15%
  • References: Blood 2006;108:1865, Hum Pathol 2006;37:1458, N Engl J Med 2007;356:459, Haematologica 2017;102:1511
Differential diagnosis
  • Chronic myelogenous leukemia (CML): leukocytosis (WBC ranges from 12k/L to 1,000k/L, median 100k/L), basophilia, thrombocytosis, monocytosis (often accounts for less than 2 - 3% of WBC)
    • Erythrocytes are essentially unremarkable, positive for BCR-ABL1 gene rearrangement and no JAK2 mutation detected
  • Essential thrombocythemia (ET) and early phase of primary myelofibrosis (PMF): following the adoption of the updated 2016 WHO classification of PV or when a patient presents with thrombocytosis, it is particularly important to differentiate masked / prodromal polycythemia from JAK2 positive ET by careful examination of bone marrow morphology
  • Gaisbock syndrome: associated with hypertension, obesity, anxiety or other causes; normal megakaryocytes, less cellular marrow, no increase in reticulin, more normal iron stores, normal or high erythropoietin levels (Angiology 1978;29:520)
  • Reactive diseases (e.g., leukomoid reaction): toxic features and lack basophilia
    • Rarely the WBC count may exceed 50k/L
    • Erythrocytosis is uncommon
  • Secondary erythrocytosis / polycythemia: may be compensatory (e.g., lung disease, high altitude living, cyanotic heart disease), paraneoplastic (EPO secreting tumors, including renal cell carcinoma, hepatocellular carcinoma, cerebellar hemangioblastoma), heavy smoker, carbon monoxide poisoning, body builder or humoral replacement (e.g. testolone, a selective androgen receptor modulator), hemoglobin mutants with high oxygen affinity or an inherited defect that stabilizes HIF1a (prolyl hydroxylase mutations, homozygous VHL mutations, etc.)
    • In contrast to primary polycythemia vera, EPO levels are elevated
    • Negative JAK2 mutation and normal serum EPO level make PV diagnosis unlikely
Board review question #1
    Which of the following is the most reliable in establishing the diagnosis of primary polycythemia?

  1. Detection of JAK2 exon 12 mutation
  2. Hemoglobin > 16.5 g/dL (men) or 16.0 g/dL (women)
  3. Increased of EPO level
  4. Marrow fibrosis grade 3
  5. Presence of BCR-ABL1 gene fusion product
Board review answer #1
A. JAK2 V617F and JAK2 exon 12 mutations are not only found in PV but detection is fairly sensitive and is identified in nearly all patients with PV (Am J Hematol 2017;92:94). Whereas answer choice B is a major criterion, elevated hemoglobin can be seen in other conditions and would not be the most reliable. Marrow fibrosis (answer D), especially grades 2 and 3, can represent the post polycythemic / spent phase but may also be found in other conditions, such as primary myelofibrosis (PMF) or secondary changes after direct exposure to radiation therapy. The presence of Philadelphia chromosome (BCR-ABL1 gene fusion product, answer E) is associated with CML and in some cases of AML. Increased EPO levels (answer C) are associated with secondary polycythemia or administration of EPO agonist for treatment of anemia.
Board review question #2
    A patient undergoes bone marrow biopsy. Reticulin staining of the core reveals myelofibrosis grade 3. Which of the following is the most reliable in differentiating post polycythemic / spent phase of PV from PMF?

  1. Detection of JAK2 mutation
  2. Detection of MPL mutation
  3. Differentiation is not necessary because the prognosis and treatments are similar
  4. Longstanding history of elevated hemoglobin and hematocrit levels and panmyelosis on previous bone marrow biopsies
  5. Low EPO level
Board review answer #2
D. In addition to bone marrow fibrosis grade 2 or 3, documentation of a previous diagnosis of WHO defined PV is required to diagnose post polycythemic/spent phase and would be the most reliable in differentiating the post polycythemic / spent phase from primary myelofibrosis. JAK2 mutations (answer A) are detected in > 95% of PV patients; however, they can also be detected in other conditions such as PMF (approximately 50% of cases). Detection of MPL mutations (answer B) has been reported in up to 5% of PMF cases and some cases of essential thrombocythemia (ET) but has not been reported in PV (Leukemia 2017 May 31 [Epub ahead of print). Low EPO levels (answer E) are characteristic of PV but can also be seen in several other conditions, including diseases associated with inherited EPO receptor mutations. The predictive factors, prognosis, natural disease progression and treatments are different for PV and PMF, making distinction between the two diseases important (answer C).