PVSG and WHO vs European Clinical, Molecular and Pathological Criteria for prefibrotic myeloproliferative neoplasms

Table 1 Blood and bone marrow features in one prospective study of Thrombocythemia Vera Study Group-defined essential thrombocythemia and one retrospective study of Polycythemia Vera Study Group defined essential thrombocythemia at platelet counts above the upper limit of normal

Ref.	Michielset al[11]	Lengfelderet al[30]
Type of study	Prospective 1975-1985	Retrospective 1975-1995
Diagnosis ET	TVSG criteria	PVSG criteria
Inclusion criterion	ET	ET	Tentative diagnosis
Platelet count × 109/L	> 400	> 350	WHO-ECMP
Number of ET patients	30	143
Platelets × 109/L range	420-1500	< 350-> 000
Below 600	13%	29%	Early latent ET
Between 600-1000	54%	45%	Fits with ET
Above 1000	33%	26%	Fits with ET
Leukocytes
Above 12 × 109/L	10%	51%
Hemoglobin
Below 16 g/dL	-	80%
Below 17 g/dL	-	100%
Above 16 g/dL	-	20%	Fits with PV
Splenomegaly
No	63%	56%
Yes	37%	44%
Spleen size on echogram (cm)
n < 12/12-15/> 15	2019/8/3	-
Bone marrow biopsy
Normal cellularity	17 (57%)	52%	Fits with true ET
Increased cellularity	13 (43%)	60%
Increased erythropiesis	13 (43%)	17%	Fits with early PV
Increased granulopiesis	0	45%	Fits with CMGM
Myelofibrosis	No	No

Essential thrombocythemia (ET) according to Polycythemia Vera Study Group (PVSG) criteria appears to be a spectrum of normocellular ET, prodromal polycythemia vera (PV) and ET due to megakaryocytic, granulocytic myeloproliferation or ET associated with chronic or primary megakaryocytic granulocytic myeloproliferation (CMGM/PMGM) when diagnostic World Health Organization and European Clinical, Molecular and Pathological (WHO-ECMP) bone marrow features are applied. TVSG: Thrombocythemia Vera Study Group.

Table 2 2008 World Health Organization and European Clinical, Molecular and Pathological criteria for the diagnosis and classification of JAK2^V617F mutated essential thrombocythemia into 3 stags or phenotypes: important to differentiate because natural history differs

Clinical and molecular criteria	WHO bone marrow criteria
ET stage 1	Normocellular ET
Platelet count of > 350 × 109/L and the presence of large platelets in a blood smear in all stages of ET	Predominant proliferation of enlarged megakaryocytes with hyperlobulated nuclei and mature cytoplasm, lacking conspicuous morphological abnormalities. No increase, proliferation or immaturity of granulopoiesis or erythropoiesis
Presence of JAK2V617F mutation	No progression to post-ET myelofibrosis
ET stage 2	Prodromal PV
Platelet count of ≥ 350 × 109/L and normal hematocrit: male < 51%, female < 48%	Increased cellularity with trilineage myeloproliferation (i.e., panmyelosis). Proliferation and clustering of small to giant (pleomorphic) megakaryocytes
erythrocytes < 6 × 1012/L	No pronounced inflammatory reaction (plasmacytosis, cellular debris). Absence bone marrow features consistent with congenital polycythemia and secondary erythrocytosis
Presence of JAK2V617F mutation	Progression to overt PV during follow-up
Low serum EPO level and/or increased score for leukocyte alkaline phosphatase
Spontaneous EEC
ET stage 3	ET.MGM
Platelet count of ≥ 3500 × 109/L and no signs of leuko- erythroblastosis	Increased cellularity due to MGM and normal or relative reduction of erythroid precursors with various degrees pleiomorphic loosely clustered megakaryocytes containing dysmorphic (not cloud-like) nuclei and maturation defects
Erythrocytes < 6 × 1012/L	No or slight RF (RF 0 or 1)
Presence of JAK2V617F mutation	Progression to post ET myelofibrosis
Slight splenomegaly on ultrasound and no anemia Hb > 12 g/dL
No preceding or allied of CML, PV, RARS-T or MDS

Masked myeloproliferative neoplasms: normal platelets, leukocytes and hematocrit, but slight splenomegaly on echogram with the presence of JAK2^V617F mutation and/or a World Health Organization (WHO) bone marrow is rare (rather frequent in patients with splanchnic vein thrombosis and/or Budd Chiari syndrome). ET: Essential thrombocythemia; PV: Polycythemia vera; ET.MGM: ET due to megakaryocytic, granulocytic myeloproliferation; EEC: Endogenous erythroid colony; CML: Chronic myeloid leukemia; RARS-T: Thrombocythemia associated with refractory anemia with increased ringed sideroblasts; RF: Reticulin fibrosis.

Table 3 The 2008 World Health Organization and European Clinical, Molecular and Pathological criteria for the diagnosis of polycythemia vera and diagnostic differentiation between polycythemia vera and congenital or acquired erythrocytosis

Clinical and molecular criteria	Pathological criteria (WHO)
Major PV criteria	P1. Early PV
A0. Early PV. Hematocrit in the upper limit of normal: Ht: 0.45 to 0.51 in male and 0.43 to 0.48 in female, Erythrocytes < 6 × 1012/L A1. Classical WHO defined PV: Hematocrit > 0.51/> 0.48 in male/ female, Erythrocytes > 6 × 1012/L A2. Presence of JAK2V617F mutation (sensitivity 95%) or exon 12 mutation A3. Low serum EPO level and/or spontaneous endogenous erythroid colony formation	Increased cellularity of bone marrow predominantly due to increased erythropoiesis and loose clusters of large megakaryocytes with hyperlobulated nuclei. No or slight increase of granulopoiesis and RF
	P2. Overt PV
	Hypercellular (75%-100%) bone marrow due to trilinear increase of erythropoiesis, megakaryopoiesis and granulopoiesis and clustering of small to giant (pleomorph) megakaryocytes with hyperlobulated nuclei. Absence of stainable iron
Minor MPD criteria	P3. Erythrocytosis
B1. Persistent increase of platelet count: grade I: 400-1500, grade II: > 1500 B2. Granulocytes > 10 × 109/L or Leukocytes > 12 × 109/L and/or raised LAP-score or increased PRV-1 expression in the absence of fever or infection	Selective increase of erythropoiesis, normal granulopoiesis and megakaryocytes of normal size, morphology and no clustering of megakaryocytes in primary or secondary erythrocytosis
	Grading of RF (RF 0, 1, 2, 3)
B3. Splenomegaly on palpation or on ultrasound echogram (> 12 cm length in diameter)	Grading of reticulin and collagen fibrosis; myelofibrosis MF grade 1, 2 and 3

World Health Organization (WHO) and European Clinical, Molecular and Pathological (ECMP) criteria criteria for early and overt polycythemia vera (PV). A0, A2, B1 and P1 establish prodromal PV (ET stage 2) PV ECMP stage 0, or masked PV; A1, A2, P1 and none of B establish so-called idiopathic erythrocytosis or polycythemic PV ECMP stage 1. A1, A2, P2 and one or more of B establish WHO defined classic and advanced PV ECMP stage 2 and 3. A1 and P3 with normal or increased values of serum EPO is consistent with congenital or secondary erythrocytosis. A3 confirms early and overt PV without the need of red cell mass measurement for clinicians who do not have access to a hematopathologist expert in myeloproliferative neoplasms. MPD: Myeloproliferative disorders; LAP: Leukocyte alkaline phosphatase; MF: Myelofibrosis; RF: Reticulin fibrosis.

Table 4 World Health Organization and European Clinical, Molecular and Pathological criteria for diagnosis and staging of primary megakaryocytic granulocytic myeloproliferation, or primary myelofibrosis

Michiels JJ	Clinical criteria (2005)	Thiele J	pathological criteria (2005/2008)
A1	Hypercellular JAK2/MPL wild type ET and no preceding or allied other subtype of myeloproliferative neoplasm: JAK2V617F or MPL515 normocellular ET, prodromal or classical PV, Ph1+ CML or MDS	B1	PMGM and relative reduction of erythroid precursors. Abnormal clustering and increase in atypical giant to medium sized dysmorphic megakaryocytes containing bulky/clumsy (cloud-like) hypolobulated nuclei and definitive maturation defects
C	Clinical stages	MF	Staging of myelofibrosis
C1	Early clinical stages
	Normal hemoglobin or slight anemia, grade I: hemoglobin > 12 g/dL	MF 0	Prefibrotic stage PMGM/PMF	RF 0/1
	No, slight or moderate splenomegaly on ultrasound scan or CT	MF 1	Early fibrotic PMGM/PMF	RF 2
	Hypercellular ET, platelets in excess of 400, 600 or even > 1000 × 109/L
	No leuko-erythroblastic blood picture and/or tear drop erythrocytes
C2	Intermediate clinical stage
	Anemia grade II: hemoglobin > 10 g/dL	MF 2	Manifest fibrotic PMGM/PMF	RF 3 = RCF
	Definitive leuko-erythroblastic blood picture and/or tear drop erythrocytes	MF 3	Advanced fibrotic PMGM/PMF	RF 4 = RCF
	Splenomegaly, increased LDH
C3	Advanced clinical stage
	Anemia grade III: hemoglobin < 10 g/L	MF 3	Osteosclerosis
	Splenomegaly and increased, normal or decreased platelet count
	Thrombocytopenia, leukocytosis,leukopenia, increased circulating CD34+ cells

ET: Essential thrombocythemia; PMGM: Primary megakaryocytic and granulocytic myeloproliferation; CT: Computed tomography; LDH: Lactodehydrogenase; PV: Polycythemia vera; CML: Chronic myeloid leukemia; PMF: Primary myelofibrosis; MF: Myelofibrosis; RF: Reticulin fibrosis; RCF: Reticulin/collagen fibrosis.

Table 5 Grading of reticulin fibrosis and myelofibrosis

Grading[78,79]	Grading of MF[80]	Description of RF and RCF in MF as a secondary event in MPN
Normal RF-0	MF 0	No reticulin fibers, occasional individual fibers or focal areas with tiny amount of reticulin fiber network
RF 1 Slight increase	MF 0	Fine reticulin fiber network throughout much of section and no course reticukin fibers
RF 2 Moderate increase	MF 1	Diffuse fine reticuline network with focal collections of thick course reticulin fibers and no collagenisation
RF 3 = RCF Marked increase	MF 2	Diffuse and dense increase in reticulin with extensive intersections, and presence of collagen fibers and no or minor osteosclerosis
RF 4 = RCF and O OS Dry tap	MF 3 Sclerotic	Diffuse and dense reticulin with with coarse bundles of collagen associated with significant O

MF: Myelofibrosis; RF: Reticulin fibrosis; RCF: Reticulin/collagen fibers; MPN: Myeloproliferative neoplasms; O: Osteosclerosis.

Table 6 World Health Organization and European Clinical, Molecular and Pathological staging of prodromal, classical and advanced polycythemia vera related to therapy

PV, ECMP stage	0		1	2	3	4	5
Michiels ECMP Clinical diagnosis	Erythrocy-themic PV	Prodromal PV mimicking ET	Polycythemic PV prefibrotic	Classic PV prefibrotic	Advanced PV PMF stage	Post-PV MF Spent phase PV	Leukemic evolution MDS AL
LAP-score	N/↑	↑	↑	↑	↑/↑↑	Variable	Variable
Red cell mass	↑	N	↑	↑	↑	Variable	N/↓
Serum EPO	N/↓	N/↓	↓	↓	↓	Variable	N/↓
Leukocytes × 109/L	< 12	< 12	< 12	N-> 12	> 15	> 20	> 20
Platelets × 109/L	< 400	> 400	< 400	> 400	< or > 1000	Variable	Variable
Hemoglobin g/dL (mmol/L)	> 16 (10)	< 16 (10)	> 16 (10)	> 16 (10)	> 16 (10)	N/> 12	< 12
Hematocrit	> 0.51	< 0.51	> 0.51	> 0.51	> 0.51	Variable	N↓
Erythrocytes × 1012/L	> 6	< 6	> 6	> 6	> 6	Variable	N/↓
ECMP bone marrow	Early PV	Pro-PV	Early PV	Trilinear PV	Trilinear PV	Myelofibrosis	AML
Bone marrow cellularity (%) Grading myelofibrosis[57]	50-80 RF 0-1	50-80 RF 0-1	60-90 RF 0-1	80-100 RF 0/1, MF 0	80-100 RCF 2/3 MF 1/2	Decreased RCF 3/4 MF 2/3	Increased No MF
Splenomegaly on palpation	No	No/+	No/+	+	++/+++	/Large	Large
Spleen size, echogram cm	< 12	< 12-15	12-15	12-18	18-> 20	> 20	> 20
Spontaneous EEC+	+	+	+	+	+	+	No
JAK2V617F in granulocytes BFU-e (exon 12)	+	+	+	+/++	+/++	++	No or +
	+(++)	+(++)	+(++)	++	++	++	No
Therapeutic implications	Low risk	Low risk	Low risk	Intermediate risk PV	High risk PV	Post-PV MF Spent phase	Acute leukemia
First line treatment option[82,83] Asp/Phleb[82,83] IFN[84-86] MPN reductive treatment Hydroxyurea[83] JAK2 inhibitor[87-90]	Aspirin phlebotomy	Aspirin phlebotomy low dose IFN?	Phlebotomy Aspirin Low dose IFN → Complete response	Phlebotomy1 Aspirin IFN→ if resistant →HU	If IFN resistant→ HU or HU first line	JAK2 inhibitor→Bone marrow transplantation Aspirin?	Chemotherapy Bone marrow transplantation? Supportive

¹↑: Increased; ↓: Decreased; N: Normal; +: Present or heterozygous; ++: Homozygous. ET: Essential thrombocythemia; PV: Polycythemia vera; MPN: Myeloproliferative neoplasms; MF: Myelofibrosis; AMM: Agnogenic myeloid metaplasia; IFN: Interferon; EEC: Endogenous erythroid colony; ECMP: European Clinical, Molecular and Pathological; LAP: Leukocyte alkaline phosphatase; RCF: Reticulin/collagen fibrosis; RF: Reticulin fibrosis.