Myeloproliferative and “Overlap” Myelodysplastic/Myeloproliferative Neoplasms

Clinical Features

Chronic myeloid leukemia is one of the most common leukemias, with an incidence of 12.8 cases per 100,000 persons per year, accounting for approximately 15% of all adult leukemias. The median age is between 46 and 53 years, with a male-to-female ratio of approximately 1.8 : 1. The median age has decreased over the years because of increased incidental diagnosis of early disease and owing to the common use of the routine complete blood cell counts in well-patient examinations. Rare cases can be seen in children.

CML most frequently presents in a chronic phase, and patients are increasingly asymptomatic at diagnosis. When symptoms are present, they include fatigue, lethargy, bleeding, weight loss, and those related to splenomegaly. Less common symptoms include night sweats, bone pain, and symptoms related to hyperviscosity owing to elevated cell counts. Physical findings include pallor, splenomegaly, and occasionally bone tenderness and stigmata of thyrotoxicosis.

Pathologic Features

Chronic Phase

The diagnosis of CML requires the evaluation of blood and bone marrow, and ancillary studies, the most important of which is cytogenetic or molecular analysis to identify the Ph chromosome or the BCR-ABL1 fusion.

Peripheral Blood

Laboratory evaluation plays a critical role in the diagnosis, and the peripheral blood findings are frequently highly suggestive, if not diagnostic in themselves. Patients usually have marked leukocytosis with white blood cell (WBC) count ranging from 20 to 500 × 10 ⁹ /L, with a mean count somewhere between 134 and 225 × 10 ⁹ /L. The peripheral blood smear also shows a characteristic myelocyte bulge, where the myelocytes are greater in percentage than metamyelocytes ( Fig. 17.2 ). This finding is in contrast to the more common reactive granulocytic or leukemoid reaction where there is a progressive decrease in the number of bands, metamyelocytes, myelocytes, promyelocytes, and blasts. Dysplasia in the maturing granulocytic elements is usually absent, and, if present, it is very minimal, and the presence of severe dysplasia should suggest a different diagnosis. Evaluation of the peripheral blood with the leukocyte alkaline phosphate (LAP) test, sometimes referred to as the neutrophil alkaline phosphate (NAP) test, yields a low score. The smear also shows an absolute basophilia in essentially 100% of cases. This finding may be difficult to appreciate because frequently the basophils are slightly hypogranular and not easily recognizable as basophils to the untrained eye ( Fig. 17.3 ). There may also be an eosinophilia, and some of the eosinophils may be immature with basophilic granules; these may resemble the abnormal eosinophils seen in AML with inv(16)/t(16;16) but are usually not as atypical (see Fig. 17.3 ). Patients frequently have an absolute monocytosis (>1000 × 10 ⁹ /L) owing to a high WBC, but the percentage of monocytes is usually low and less than 3%. Patients also frequently have a moderate normochromic, normocytic anemia, and elevated platelets with counts as high as 1000 × 10 ⁹ /L. This marked thrombocytosis can cause confusion with essential thrombocythemia. Thrombocytopenia is rare, and, if present, another entity should be considered in the differential diagnosis.

Bone Marrow

A bone marrow study is usually performed, and it is important to help exclude other entities and to obtain a specimen for cytogenetic analysis. The marrow is hypercellular, frequently approaching 100%. The marrow shows a marked proliferation of myeloid and megakaryocytic elements, with an elevated myeloid-to-erythroid ratio (≈10 : 1 to 20 : 1). Frequently the myeloid elements are expanded along the bony trabeculae, producing an expanded cuff of immature cells ( Fig. 17.4 ). In normal bone marrows, this cuff is approximately three cells thick, but in CML it can be 15 to 20 cells thick. The blast count is low, and the cellular features resemble those seen in the blood with a myelocyte bulge, basophilia, and eosinophilia. Megakaryocytes are frequently increased, although in some cases the megakaryocytic proliferation is not that prominent, whereas in others it is accentuated ( Fig. 17.5 ). The megakaryocytes are characteristically small with hypolobated nuclei, which some refer to as dwarf megakaryocytes . They are not large and atypical, nor are they tiny micro-megakaryocytes. This feature is important to recognize, because it helps to distinguish CML from the other MPNs, which have larger than normal megakaryocytes, and from the MDS or MDS/MPNs, in which true micro-megakaryocytes are seen ( Fig. 17.6 ). Numerous micro-megakaryocytes in a suspected case of CML should make one consider another diagnosis. Frequently, in approximately 20% to 40% of cases, the marrow shows histiocytes that resemble Gaucher cells; these are referred to as pseudo-Gaucher cells . These cells have the characteristic crumpled tissue paper–like cytoplasm and frequently show hemophagocytosis ( Fig. 17.7 ). The presence of these cells is not diagnostic of CML as they can be seen in any of number of hematologic disorders. However, in CML they are derived from the neoplastic clone, as they have been shown to be BCR-ABL1 ⁺ through fluorescence in situ hybridization (FISH) analysis.

Ancillary Studies

As mentioned previously, the LAP or NAP score is usually below the normal range. Although not commonly used, this screening method is helpful but not absolute, because CML in the accelerated phase can show increased scores. Indeed, the LAP score is being replaced by more specific molecular testing for CML and other MPNs and is being discontinued by many laboratories. Vitamin B ₁₂ is increased by 10- to 20-fold the normal range, and uric acid is usually elevated.

Although the diagnosis of CML usually can be made with a great degree of certainty from the features in the blood and marrow, confirmation requires the demonstration of the characteristic t(9;22) (or variant) or the associated BCR-ABL1 fusion gene. This demonstration can be accomplished by conventional cytogenetic analysis, by FISH with probes to BCR and ABL1, or by polymerase chain reaction (PCR). The t(9;22) is seen in its characteristic form in greater than 95% of cases. In a small number of cases, however, there is a variant translocation involving the 9q34, the 22q11.2, and another involved chromosome (e.g., t[9;14;22]). In slightly less than 5% of cases, there is submicroscopic translocation, which cannot be identified by conventional cytogenetics, and the karyotype appears normal. However, with FISH probes or with specific PCR primers to identify the variant fusion gene, the underlying BCR-ABL1 can be recognized. These cases are called Ph-negative CML and should probably be referred to as Ph-negative, BCR-ABL1–positive CML for clarity.

It is important to recognize that the BCR gene can be broken into three different regions, giving rise to three different BCR-ABL1 proteins of different size. Almost all cases of CML are associated with the major breakpoint region, at exon 12-16 (formerly referred to as exon b1-b5) and fusion to the ABL1 at its exon 2. This fusion is referred to as the major BCR-ABL1 or fusion associated with the p210 kilodalton (kd) BCR-ABL1 protein. Rare cases of CML can have a fusion involving the first exon of BCR , e1-e2, and this is referred to as the fusion associated with the minor breakpoint and a smaller fusion protein with 190-kd size (p190). This fusion is far more common in Ph+ ALL, but when associated with CML is associated with increased monocytes, which makes it difficult to differentiate from CMML. Last, rare cases of CML can have a fusion of BCR involving the regions around exons 17 to 20 (previously referred to as c1-c4) resulting in a µ breakpoint or a larger protein with 230-kd weight (p230). This fusion is also rare but may be associated with CML that has markedly increased platelets or CML with a predominance of mature neutrophils (CML-N). These can mimic either ET or CNL, respectively.

As will be discussed in the latter part of the chapter that CALR mutations have primarily been reported in ET and PMF and are exceedingly rare in the setting of CML with two case reports described in the literature. In both these reports of atypical myeloproliferative neoplasms, CALR mutation proceeded BCR-ABL1 fusion. However, the bone marrow morphologic features simulating primary myelofibrosis ( CALR mutation positive) became apparent only after the Philadelphia-positive clone was eradicated with dasatinib therapy.

Accelerated Phase

Chronic myeloid leukemia sometimes progresses to a blast phase first through an accelerated phase. The accelerated phase is associated with worsening overall performance, fever, night sweats, weight loss, bone pain, progressive splenomegaly, and loss of responsiveness to therapy. Although different criteria had been used to diagnose the accelerated phase, the WHO committee writing on hematologic disorders developed a list of six features, any of which would indicate an accelerated phase. These features include peripheral blood or bone marrow blasts accounting for 10% to 19% of the cells ( Fig. 17.8 ), persistent thrombocytopenia (<100 × 10 ⁹ /L) unrelated to therapy or thrombocytosis (>1000 × 10 ⁹ /L) unresponsive to therapy, persistent or increasing WBC (>10 × 10 ⁹ /L), and spleen size unresponsive to therapy, basophilia greater than 20% ( Fig. 17.9 ), and evidence of clonal cytogenetic evolution (see details under criteria for the accelerated phase). Dysplasia and increased fibrosis are frequently seen in the accelerated phase but in themselves are not considered sufficient for a diagnosis according to the WHO guidelines.

Although almost any additional chromosomal change can be seen in the accelerated phase, the most common changes include an extra Ph chromosome (+Ph or +der[22]), +8, i(17q), and +18. These abnormalities are seen singly or in combination in 81% of cases showing cytogenetic evolution. Other more common abnormalities include −7, −17, +17, +21, and −Y.

Blast Phase

Blast phase, or blast crisis, occurs in virtually all patients with untreated CML. In the era of imatinib and other TK inhibitors it occurs much less frequently, sometimes either following an accelerated phase or suddenly without warning. Blast phase resembles an acute leukemia. Because the CML clone originates in the pluripotent stem cell, blast phase can occur in the myeloid series or the lymphoid series, or it can be biphenotypic or bilineal. Blast phase is diagnosed when there are 20% or more blasts in the blood or marrow, but sometimes it is seen only focally on the biopsy as sheets of blasts (focal intramedullary blast transformation; Fig. 17.10 ). Although most of the time blast phase is diagnosed from the blood and/or marrow, in some instances it can occur at an extramedullary site. In fact, the development of a mass lesion in a patient with CML should always warrant investigation with a biopsy.

Myeloid Blast Phase

Blast phase is of a myeloid type in approximately 50% to 60% of cases. Before TK inhibitor therapy, it commonly occurred following an accelerated phase and was seen more frequently in older patients with higher blood counts, more severe anemia, and larger spleens. Although blast phase responds to TK inhibitor therapy, it still is refractory to therapy and has a poor survival. The myeloid blast crisis of CML is heterogeneous. In some cases it can resemble a de novo AML without maturation, with maturation, with a monocytic component, or it can even resemble erythroleukemia or megakaryoblastic leukemia ( Fig. 17.11 ). In rare cases, the blast phase can have a t(8;21), inv(16)/t(16;16), or t(15;17) cytogenetic abnormality usually associated with de novo AML. In these latter types, the blast phase component is morphologically and immunophenotypically identical to the de novo leukemia associated with these recurring chromosomal abnormalities ( Fig. 17.12 ). More frequently the myeloid blast phase can be of a mixed myeloid type in which the blastic elements include myeloblasts, monoblasts, megakaryoblasts, and immature basophils ( Fig. 17.13 ). This type of blast phase is distinctive morphologically and does not have a well-known de novo AML counterpart, although a mixed myeloid blast population can be seen in some AMLs arising from MDS.

Lymphoid Blast Phase

Before the use of TK inhibitor therapy, lymphoid blast phase accounted for 16% to 30% of cases of blast crisis and was more uniform morphologically and immunophenotypically than myeloid blast crisis. Clinically, lymphoid blast phase occurs in younger patients with lower counts and less splenomegaly than in patients with myeloid blast phase. Interestingly, the lymphoid blast phase occurs abruptly and is not associated with a preceding accelerated phase. Thus, there is usually no gradual increase in lymphoblasts of lymphoid blast phase. Morphologically the lymphoid blast phase shares features with ALL, although the background of CML is frequently still evident ( Fig. 17.14 ). Most cases are of B lymphoblasts, and, less commonly, the blast phase can be of a precursor T-cell type. Although lymphoid blast phase also responds to second- and third-generation TK inhibitors and other chemotherapeutic agents, the overall survival is still poor, and patients are frequently taken to stem cell transplant.

Bilineal or Biphenotypic Blast Phase

In some cases the blasts in blast phase can be a mixture of lymphoblasts and myeloblasts. These bilineal processes may be associated with two distinct cytogenetic clones that have evolved separately from the Ph ⁺ clone. The clones likely represent separate lymphoid and myeloid blast phases occurring simultaneously. In the biphenotypic blast phase, the blasts show lymphoid and myeloid markers simultaneously on the same blasts. These blasts may be precursor B/myeloid or precursor T/myeloid. This type of blast phase requires the same diagnostic criteria as de novo acute leukemia with mixed phenotype.

Extramedullary Blast Phase

In approximately 5% to 10% of cases, blast phase can manifest at extramedullary sites; therefore, the development of a mass lesion in a patient with CML should prompt appropriate evaluation. The most common sites of extramedullary blast phase include lymph node, soft tissue, and the central nervous system (CNS). Extramedullary disease is usually of the myeloid type, but not always. Bone marrow involvement can be simultaneous; if not, it usually develops in a short time after the extramedullary presentation.

Chronic Myeloid Leukemia—Pathologic Features

Microscopic Features

Blood

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  Leukocytosis with granulocytes at all stages of maturation

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  Myelocyte bulge, blasts usually <1% to 2%

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  Absolute basophilia (100% of cases), absolute eosinophilia (80% of cases)

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  Absolute monocytosis common, but monocytes <10%, usually <3% of leukocytes

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  Thrombocytosis common, can be prominent; thrombocytopenia very rare

Bone marrow

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  Hypercellular due to granulocytic and megakaryocytic proliferation

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  M:E ratio: 10–20 : 1

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  Widened paratrabecular cuff of immature granulocytes

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  Basophilia, blasts usually <10%

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  Small hypolobated (dwarf) megakaryocytes

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  Mild reticulin fibrosis

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  Pseudo-Gaucher histiocytes (in ≈ 20% to 40% of cases)

Ancillary Studies

Peripheral blood

• • –

    LAP (NAP) score low; vitamin B ₁₂ increased

Bone marrow

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    Cytogenetic analysis: t(9;22) or variant (>95% of cases)

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    Molecular: BCR-ABL1 ⁺ by FISH or PCR (100% of cases)—may be done on peripheral blood

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    Most cases: major BCR-ABL1 , (e13 or14/a2 or 3), p210 protein

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    Rare cases: minor BCR-ABL1 , (e1/a2 or 3), p190 protein

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    Rare cases: mu BCR-ABL1 , (e19/a2 or 3), p230 protein

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  Differential diagnosis

  • –

    Leukemoid reaction

  • –

    CMML

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    Atypical CML (Ph-neg, BCR-ABL1 -neg)

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    Chronic neutrophilic leukemia or ET

Accelerated Phase Criteria (WHO 2016)

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  Any 1 or more of the following hematologic or cytogenetic criteria or response-to-TKI therapy criteria

Hematologic or Cytogenetic Criteria

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  10% to 19% blasts in the PB and/or BM

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  20% or more basophils in the PB

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  Persistent thrombocytopenia (<100 × 10 ⁹ /L) unrelated to therapy

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  Additional clonal chromosomal abnormalities in Ph positive cells at diagnosis that include “major route” abnormalities (second Ph, trisomy 8, isochromosome 17q, trisomy 19), complex karyotype, or abnormalities of 3q26.2

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  Persistent or increasing WBC (>10 × 10 ⁹ /L), unresponsive to therapy

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  Persistent or increasing splenomegaly, unresponsive to therapy

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  Persistent thrombocytosis (>1000 × 10 ⁹ /L), unresponsive to therapy

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  Any new clonal chromosomal abnormality in Ph positive cells that occurs during therapy

“Provisional” Response-to-TKI Criteria

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  Hematologic resistance to the first TKI (or failure to achieve a complete hematologic response ^a to the first TKI)

  a Complete hematologic response = WBC <10 ⁹ /L, platelets <450 × 10 ⁹ /L, no immature granulocytes in the differential, no palpable splenomegaly.

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  Any hematologic, cytogenetic, or molecular indications of resistance to 2 sequential TKIs

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  Occurrence of 2 or more mutations in BCR-ABL1 during TKI therapy

Blast phase

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  20% or more blasts in blood or bone marrow

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  Myeloid blast phase: 50% to 60% of cases

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  Lymphoid blast phase: 16% to 30% of cases

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  Bilineal or biphenotypic blast phase: rare

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  Extramedullary blast phase: rare; lymph node, soft tissue, central nervous system

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  Differential diagnosis

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  De novo Ph+ acute myeloid leukemia

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  De novo Ph+ B-ALL

Differential Diagnosis

A discussion of differential diagnosis in CML needs to take into account the stage at presentation. Although most patients present in chronic phase, some initially present in blast phase or accelerated phase, and the entities considered in the differential diagnosis differ widely among these. Key to establishing the differential diagnosis is the evaluation for t(9;22) or BCR-ABL1 , or both. CML must be shown to have the t(9;22), a variant translocation, or the BCR-ABL1 by FISH or by molecular techniques.

Differential Diagnosis of Accelerated Phase and Blast Phase

Chronic myeloid leukemia does not commonly manifest initially in accelerated phase; in the rare case in which it does, the differential considerations would include entities in the MDS/MPN category as well as some of the MPNs, particularly PMF. Morphologic distinction from CML might be difficult, because the finding of dysplasia characteristic of MDS and MDS/MPN can also be seen in accelerated phase of CML. Cytogenetic and molecular studies are key to the correct diagnosis.

Patients with CML can initially present in the lymphoid blast phase. In some, the chronic phase may have gone unnoticed, but in others there may not have been a chronic phase at all. In either of these cases, the patient usually exhibits leukocytosis with lymphoblasts and the background typical of CML ( Fig. 17.15 ). In other cases in which there is no recognizable CML in the background of the blastic process, the diagnosis can be made only after treatment because many patients revert to a chronic phase after therapy. Identification of t(9;22) or BCR-ABL1 does not necessarily help in the initial evaluation because ALL can be t(9;22) and BCR-ABL1 positive. If a minor BCR-ABL1 (p190) is present, then CML would be unlikely, but if the major BCR-ABL1 (p210) is seen, either Ph+ ALL or CML presenting in lymphoid blast phase is possible. Identification of the BCR-ABL1 specifically in the granulocytic, erythroid, or megakaryocytic components might be helpful to distinguish between these, because CML is a stem cell disorder involving all cell lineages, whereas Ph+ ALL is believed to be a lymphoid-restricted process.

Rarely patients present with an AML that is shown to be t(9;22) positive or BCR-ABL1 positive. Many of the patients have an aggressive course with no reversion to chronic phase CML after therapy. Whether these are truly Ph+ AML or just CML presenting in the myeloid blasts phase is difficult if not impossible to determine. Currently, Ph+ AML is considered as a provisional category per WHO 2016. Some of the features that are suggested in the literature can help to differentiate between the two entities is highlighted in Table 17.1 . In addition, in some patients the blastic proliferation is a mixed population of myeloblasts, monoblasts, erythroblasts, and megakaryoblasts similar to the mixed-myeloid blast phase of CML. This clue can be a strong indication for CML myeloid blast phase rather de novo Ph+ AML. Other patients may have t(9:22) in addition to a common recurring cytogenetic abnormality in AML, such as inv(16) or t(8;21), whereas other patients will exhibit a mixed or bilineal acute leukemia. Some reports have noted increased incidence of the p190 BCR/ABL1 in cases presenting as AML or mixed lineage leukemia.

TABLE 17.1

Clinicopathologic Features Distinguishing Ph+ Acute Myeloid Leukemia ^a

Features	Ph+ acute myeloid leukemia a	Myeloid blast phase of chronic myeloid leukemia (CML)
Presentation	Lack of history of chronic phase of CML, acute presentation, lack of splenomegaly	Previous history of chronic phase of CML, presence of splenomegaly
Peripheral blood and BM basophilia	Absent	Present
Bone marrow findings	Usually lower cellularity; decreased myeloid-to-erythroid (M:E) ratio; presence of dwarf megakaryocytes is uncommon	Usually higher cellularity; increased M:E ratio; presence of dwarf megakaryocytes
Cytogenetic findings	Additional chromosomal abnormalities less common (25% to 59.9%) Deletion 7q/-7 abnormality is seen in a few studies	Additional chromosomal abnormalities more common (60% to 80%) including extra Ph chromosome, trisomy 8, trisomy 19, or isochromosome 17q
Molecular findings	NPM1 mutation in 25% of cases No ABL1 mutation Deletion of antigen receptor genes ( IGH, TCR ), IKZF1 and/or CDKN2A	Usually absent ABL1 mutations are common No IGH and TCR gene deletion or antigen deletions present

a Provisional category as per WHO 2016 from myeloid blast phase of chronic myeloid leukemia.

Prognosis and Therapy ( Box 17.1 )

The development of imatinib (Gleevec) in the late 1990s has revolutionized the treatment of CML and provides successful management for most CML cases. After trials of imatinib—first in patients who failed interferon therapy, then in patients with accelerated or blast phase, and finally in an up-front comparison to interferon—imatinib has become the treatment of choice because of its superiority to other therapies. A controversy that still exists is in the treatment of young patients with a suitable stem cell donor and whether first to treat with imatinib or whether to transplant immediately. Currently, three tyrosine kinase inhibitors (imatinib, nilotinib, and dasatinib) are approved by the U.S. Food and Drug Administration for initial treatment of chronic-phase CML. Box 17.1 summarizes the National Comprehensive Cancer Network (NCCN) 2015 guidelines that define complete hematologic, cytogenetic, and molecular response in CML at 3 months, 12 months, and 18 months, respectively. Results with nilotinib and dasatinib as frontline therapy in single-institution trials at MD Anderson Cancer Center have shown very high rates of complete cytogenetic response (CCyR) and major molecular response (MMR), which are superior to those in historical populations treated with standard-dose imatinib.

Imatinib inhibits the constitutive phosphorylation activity of the BCR-ABL1 TK. The drug sits in a pocket of the ABL1 portion of the BCR-ABL1 fusion protein and blocks the adenosine triphosphate from binding. Usually after approximately 3 months of therapy there is normalization of blood counts, reduction of bone marrow cellularity with correction of the myeloid-to-erythroid (M:E) ratio, and normalization of megakaryocytes ( Fig. 17.16 ). Frequently there are lymphoid aggregates composed of mostly small lymphocytes, which are a mixture of B and T cells ( Fig. 17.17 ). These lymphocytes are reactive in nature. At the 3-month mark, many patients achieve a complete cytogenetic remission, but a molecular remission by PCR analysis for BCR-ABL1 is relatively uncommon, occurring in only 10% to 15% of all patients. Resistance to imatinib can occur because of mutations, overexpression of BCR-ABL1, or reduced cellular uptake of the drug. Although newer drugs with more powerful TK inhibitor activity are being used, these still have little effect in cases in which clonal evolution resulted in a stimulation of leukemogenic pathways that are independent of the BCR-ABL1 –associated constitutive activity of TK. The presence of T315I ABL1 kinase domain mutation shows a poor response to all TKIs except the third-generation TK inhibitor ponatinib. Currently, mutational testing is not recommended at the start of treatment for patients in chronic phase, as they are present at very low allele frequencies that may not be clinically pertinent. However, the T315I ABL1 kinase domain mutation testing is recommended in patients in accelerated or blast phase or in those who fail first-line TK inhibitor therapy.

The development of clonal chromosomal abnormalities in BCR-ABL1– negative (Ph negative) cells can occur in patients undergoing TK inhibitor therapy. Studies suggest that the incidence of these abnormalities is low (3% to 9%), some are transient in nature, and there usually is no adverse clinical consequence in the majority of patients on imatinib therapy. The median interval from starting TK inhibitor therapy to emergence of clonal chromosomal abnormality is about 12 to 18 months. The most common cytogenetic abnormality includes loss of chromosome Y, trisomy 8, and monosomy 7. Balanced translocations, structural abnormalities, and a complex karyotype are also reported. The emergence of clonal karyotypic abnormalities usually does not affect patient response to TK inhibitor therapy, as confirmed by molecular and cytogenetic studies. Although the majority of these clonal karyotypic abnormalities are not associated with MDS or AML and have no clinical consequences, a small number of patients may develop MDS and AML at follow-up. The latter is often associated with a complex karyotype, inv(3) or monosomy 7. Thus, to summarize, the presence of clonal chromosomal abnormalities can occur in patients with CML on TK inhibitor therapy, but only rarely can these clonal abnormalities indicate an underlying or impending MDS/AML; thus, close follow-up of peripheral blood counts and monitoring of cytogenetic clone on follow-up bone marrow specimens may be helpful.

Successful therapy has now led to the idea that TK inhibitor therapy may at some point be discontinued, and monitoring for minimal residual disease at levels lower than that defined for a major molecular response ( BCR-ABL1 international scale ≤0.1%) will be required (levels <0.01%, so-called deep molecular response). However, further studies will be required to better define criteria for discontinuing or restarting TK inhibitor therapy.

Clinical Features

The incidence of PV is approximately 1 to 3 per 100,000 persons per year. The incidence increases with age, with the median age at diagnosis being 60 years. There is a slight male predominance and an increased incidence in Ashkenazi Jews. Rare familial cases have been described, but the genetic basis for most of these is not known. Some familial cases have been shown to be associated with mutations in the erythropoietin (EPO) receptor that results in hypersensitivity to EPO.

Most of the presenting symptoms in PV are related to the increased red cell mass that is central to the disease process. Patients may exhibit a hyperviscosity-related headache and blurry vision or arterial thrombosis and hemorrhage. There may also be symptoms related to gastrointestinal ulcers or bleeding. Many patients will have splenomegaly and pruritus, which is frequently provoked by warm water; this is referred to as aquagenic pruritus . Other common symptoms are those related to gout from hyperuricemia and erythromelalgia, which is reddening and painful swelling usually of the lower extremities.

Pathologic Features

The discovery first of an elevated red blood cell count, hemoglobin, or hematocrit through clinical signs and symptoms and then through laboratory studies is the usual starting point in the diagnosis of PV. The major difficulty in distinguishing PV from reactive or spurious polycythemia has been overcome by JAK2 mutation testing. JAK2 V617F mutation is by far the most common variant seen in 95% of PV cases, whereas the rest are due to JAK2 exon 12 mutation. Very rarely, MPL and CALR mutations have been described in PV. To establish polycythemia, the value of hemoglobin is lowered in the 2016 WHO classification from more than 18.5 g/dL to more than 16.5 g/dL in men and from more than 16.5 g/dL to more than 16 g/dL in women to capture masked PV cases. This change in a lower Hb level requirement from the 2008 WHO criteria may make a distinction from other MPNs challenging, since other MPNs can have mild to moderately elevated Hb. For that caveat, bone marrow morphology is now considered one of the major criteria to diagnose PV besides erythrocytosis and JAK2 V617F or JAK2 exon 12 mutations. Of note, endogenous erythroid colony formation in vitro is no longer one of the minor criteria, leaving subnormal EPO level as the only minor criterion in the diagnosis of PV. A bone marrow evaluation is usually obtained as a baseline for comparison to future evaluations, in addition to diagnostic purpose. It should be noted that JAK2 mutation–negative cases are extremely rare but can still be diagnosed as PV by meeting the other two major criteria and one minor criterion, highlighting the importance of bone marrow morphologic evaluation at the time of diagnosis.

Blood and Bone Marrow: Polycythemic Phase

In the polycythemic phase, the peripheral blood shows erythrocytosis, and the red blood cells are normochromic and normocytic with little poikilocytosis ( Fig. 17.18 ). Neutrophils may be elevated and there is often a basophilia. A significant left shift in the granulocytic elements is not common, although some immature cells may be seen. Platelets are elevated in at least half of the patients. The bone marrow cellularity is usually elevated, but some patients could have a normocellular marrow. When the marrow is hypercellular there is usually a panmyelosis, but the increase in erythroid precursors and megakaryocytes is most prominent. The erythropoietic cells are fairly unremarkable; however, the megakaryocytes are atypically large, but show variability in size, and are sometimes clustered around sinuses and close to the bone ( Fig. 17.19 ). They do not exhibit the bizarre features of the megakaryocytes seen in PMF, nor the prominent lobulated nature of those in ET, and they are clearly different from the small hypolobated (dwarf) megakaryocytes in CML. Fibrosis is usually not increased, and stainable iron is absent in many if not most patients (95%). Patients with JAK2 exon 12 mutations have a somewhat different appearance in their bone marrow. These patients have been reported to show more prominent erythroid proliferation without involvement of the megakaryocytic and granulocytic cell lines.

Differential Diagnosis

With the association of JAK2 mutations in such a high percentage of cases of PV, there are fewer differential diagnostic considerations than in the past when a host of apparent (spurious), secondary and congenital causes of polycythemia had to be considered. These diagnoses must still be considered in patients with polycythemia to avoid costly workup. Apparently polycythemia occurs because of hemoconcentration; secondary causes are hypoxia driven, owing to abnormal production of erythropoietin or to medications, and the congenital form is sometimes caused by mutations in the erythropoietin gene, but more commonly it is of unknown causes. Hemoglobinopathies may also result in polycythemia. The JAK2 V617F mutation occurs in significant numbers of the other MPNs and in some MDS/MPNs, but these are much less likely to manifest with elevated hemoglobin or hematocrit, which are the key features for considering PV.

Prognosis and Therapy

Most patients with PV are treated with phlebotomy with or without myelosuppressive agents. The 15-year survival for PV is approximately 65%, and an independent prognostic indicator is whether the patient has a history of thrombosis. Life expectancy is reduced, particularly when a diagnosis is made before the age of 50 years. This reduction is possibly caused by the longer disease course and more time for complications and natural evolution to the fibrotic stage or to acute leukemia. Thus the goals of therapy in PV are to improve disease-related symptoms, splenomegaly, to prevent the occurrence or recurrence of thrombosis, and to delay or prevent the progression to myelofibrosis or AML and increase survival. Cytoreductive therapies (hydroxyurea or IFN-α) have been the standard treatment for older patients with polycythemia vera or those with a history of prior thrombosis. Recently, the JAK2 inhibitor ruxolitinib, an FDA-approved drug in this setting, has demonstrated significant reduction in hematocrit and splenomegaly especially in PV patients who have failed or are intolerant to hydroxyurea therapy.

Clinical Features

Essential thrombocythemia is an uncommon disorder with an incidence of approximately 1 to 2.5 cases per 100,000 persons per year. The median age is approximately 60 years, and as in PV there may be a slight female predominance. In addition, there is an increased incidence in Ashkenazi Jews. Familial cases are extraordinarily rare, and some have been found to be caused by mutations in the TPO gene that results in increased production of thrombopoietin.

Clinically, patients are usually asymptomatic and usually come to be evaluated for ET because of an elevated platelet count found on a routine complete blood cell count performed for a well-patient check-up. Some patients exhibit symptoms believed to be related to thrombotic occlusion of the microvasculature. Such symptoms include headaches, lightheadedness, blurring vision and scotomata, palpitations, chest pain, and distal paresthesia. Erythromelalgia, characterized by erythema, warmth, and pain in the distal extremities is another symptom that is unusual, but not entirely specific for ET, as it is seen with some frequency in PV. More severe clinical features of ET include large vessel thromboses of either arterial or venous circulation. Pulmonary embolism and deep venous thromboses occur, but thrombotic events can also develop in less common sites such as hepatic or portal vein or retinal vein. Bleeding is another serious clinical manifestation and complication. Despite the high platelet count, patients are at risk for bleeding, which may be due to an acquired von Willebrand factor deficiency related to platelet absorption. Hemorrhage can occur in mucocutaneous areas and more seriously in the gastrointestinal tract. Splenomegaly is not as prominent as in the other MPNs, but it occurs in approximately 3% to 50% of cases. Hepatomegaly is less common.

Pathologic Features

The diagnosis of ET is made by identifying a sustained elevation in the platelet count (≥450 × 10 ⁹ /L); examining bone marrow and identifying marrow findings consistent with the disease; excluding other MPN, MDS, or other myeloid neoplasm associated with elevated platelets; and showing JAK2 or CALR, MPL mutations, cytogenetic clonality or in the absence of these, ruling out reactive thrombocytosis.

Blood and Bone Marrow

The peripheral smear in ET shows marked thrombocytosis with a significant size variation (anisocytosis) of the platelets. Some giant platelets may be present ( Fig. 17.20 ). The platelets are usually fairly normally granulated. White blood cells are usually normal in number, with no left shift, and there is no dysplasia. There is usually no absolute or relative basophilia. Red blood cells are normocytic and normochromic, except in patients with significant hemorrhage and iron deficiency, in which case they may be hypochromic and microcytic. Red cell morphology should be otherwise unremarkable. A rare teardrop form and a mild leukoerythroblastic picture are reasons to consider PMF rather than ET.

The bone marrow is generally moderately hypercellular. Megakaryocytes are prominent and have a particular morphology that can help to distinguish them from those seen in PV, PMF, and CML. The megakaryocytes are generally large and have abundant cytoplasm frequently with cells within the cytoplasm (emperipolesis); however, the latter finding is not diagnostic, because it is typical even in normal megakaryocytes. The nuclei are highly lobulated, a feature that has led some to refer to them as staghorn-like ( Fig. 17.21 ). Bizarre nuclear forms with hypercondensed chromatin and tight clustering (typical of PMF) and small megakaryocytes with hypolobated nuclei (typical of CML) are not seen. Granulocytic proliferation is usually absent or only minimal, and there is no left shift or increase in blasts. The mature granulocytes do not show dysplasia. Erythroid activity may be increased. Reticulin fibrosis may be minimally increased, but significant fibrosis should suggest another diagnosis. Stainable iron is usually present. Absent or decreased marrow iron is a common feature in ET and should not be used as a finding to favor an alternative diagnosis of PV over ET.

Molecular and Ancillary Studies

In approximately 50% to 60% of ET cases there is a JAK2 V617 mutation, followed by mutated CALR (calreticulin gene) in 25% to 30% of cases and mutated MPL W515 L/K in 3% to 5% of cases. These mutations are mutually exclusive and lead to constitutive activation of JAK/STAT pathways that stimulate megakaryocyte proliferation and platelet production. Molecular analyses for JAK2 V617F, CALR , and MPL M515K/L are particularly helpful in ruling out a reactive process. It is noteworthy, in 5% to 10% of ET cases, the underlying driver proliferation of ET is unknown and such cases are known as triple-negative cases. In ET, the allele burden of the JAK2 V617 mutation is lower than that of PV, and it is believed to be a more lineage-restricted mutation. Nevertheless, cases of ET with the JAK2 mutation are believed to have some similarities to PV, in that there is a more panmyelosis with increased granulopoiesis and erythropoiesis compared to the mutation-negative cases in which thrombopoiesis is most prominent. On the other hand, CALR -mutated ET shows some unique features, not only at the molecular level but also with respect to clinical presentations and outcomes. CALR-mutated ET is an MPN that affects relatively young individuals and is characterized by markedly elevated platelet count but relatively low thrombotic risk as compared to JAK2 V617F and MPL M515K/L-mutated ET. Patients with CALR -mutated ET more frequently progress to the accelerated or blast phases compared with patients with JAK2 mutations. CALR mutations lead to an alteration of the C-terminal of the protein that results in the loss of an endoplasmic reticulum retention motif and activates STAT signaling. There are two types of mutations, including a 52-bp deletion in exon 9 (type 1) and a 5-bp insertion (type 2 mutation), with type 1 mutation being more common than type 2 mutations. Mutation subtypes in CALR contribute to a patient's clinical phenotype and outcomes. Type 1- CALR mutations are mainly associated with a myelofibrosis phenotype and a significantly higher risk of myelofibrotic transformation in ET. Type 2- CALR mutations are preferentially associated with a low risk of thrombosis despite very high platelet counts and an indolent clinical course. These mutations result in a novel epitope that can be detected by immunohistochemistry in fixed tissues using a mutation-specific monoclonal antibody (CAL2). Thus, presence of CALR mutations can now be easily tested in routine biopsy material.

Besides these driver mutations, mutations in epigenetic regulatory genes, such as TET2, ASXL1, IDH1/2 , or DNMT3A , are also found in ET. The exact molecular pathology in ET is not well understood, but it is believed that there is loss of control of the proliferative activity in the megakaryocytes, leading to autonomous platelet production. The megakaryocytes are believed to be hypersensitive to stimulation by one of a number of growth factors, such as interleukin 3, or the megakaryocytic growth factor, thrombopoietin (TPO). However, it should be noted that TPO levels are not sufficiently different among ET, other myeloproliferative disorders, and reactive conditions; therefore, they cannot be used for diagnostic purposes. Mutations in TPO have not been found except in rare familial cases.

Cytogenetic analysis does not add much in resolving a differential diagnosis. Cytogenetic clones are rare in ET, as they are found in only 5% to 10% of cases. The cytogenetic findings can be used to support the diagnosis of a clonal MPN over a reactive thrombocytosis, but the genetic changes are nonspecific and do not help with cases that are difficult to distinguish from PV or early PMF. The abnormalities seen in ET include +8, +9, and del(13q).

Differential Diagnosis

As mentioned previously, the differential diagnosis includes reactive conditions leading to elevated platelet counts, other MPNs with markedly elevated platelet counts, and rarely AML, MDS, or MDS/MPNs, associated with increased platelets (most notably AML with t[3;3] or inv[3], the 5q- syndrome, and MDS/MPN-RS-T).

Reactive conditions causing thrombocytosis include infection, inflammatory diseases, blood loss and chronic iron deficiency, malignancy, trauma and surgery (especially splenectomy), and rebound following chemotherapy or replacement therapy for vitamin B ₁₂ or folate deficiency. Reactive conditions are more frequently associated with elevated acute phase reactants like C-reactive protein. The reactive conditions should not be persistent or associated with splenomegaly, and they are not likely associated with a history of thrombotic episodes.

The myeloid disorders with thrombocytosis that are JAK2 mutation negative are common, but they usually have some features that allow them to be recognized when considering ET in the differential diagnosis. For example, CML can frequently develop with thrombocytosis, but it shows the full spectrum of myeloid proliferation, with a myelocyte bulge and basophilia in the peripheral blood that is usually quite distinctive and not to be expected in ET. In addition, the small, dwarf megakaryocytes in the bone marrow are also distinguished easily from the larger staghorn-like megakaryocytes in ET. Even in cases with prominent thrombocytosis and less easily distinguished blood or marrow morphology, the t(9;22) or BCR-ABL1 will clarify any dilemma. Some patients with CML with markedly elevated platelets can have the p230 BCR-ABL1 protein.

MDS 5q- syndrome can present with increased platelets, but typically the megakaryocytes are small, hypolobated, and distinctive from the large, hyperlobulated megakaryocytes seen in ET. In addition, the del(5q) would unlikely be seen in ET.

AML can sometimes present with elevated platelets; this is a feature of AML with inv(3)(q21;q26.2) or t(3;3)(q21;q26.2) or in some megakaryoblastic leukemias. In addition to the abnormal cytogenetic finding, these patients will have elevated blasts and usually highly dysplastic megakaryocytes, including the classic micro-megakaryocytes that make them easily distinguishable from ET.

The other MPNs or MDS/MPNs with JAK2 mutations that must be considered in the differential diagnosis include PV, PMF, and MDS/MPN-RS-T; however, even these have features that make the distinction possible. The high hemoglobin–hematocrit of PV, the dysplasia and ring-sideroblasts of MDS/MPN-RS-T, and the fibrosis of the fibrotic phase of PMF all provide ample clues for a differential diagnosis. The most difficult differential is between ET and the prefibrotic phase of PMF, and features distinguishing the two entities are highlighted in Table 17.2 .

Prognosis and Therapy

The prognosis of ET is good. In the first decade the disease does not lower life expectancy. Overall median survival from the time of diagnosis is approximately 20 years. Risk factors that predict a worse overall survival in ET include age more than 60 years, WBC count more than 11 × 10 ⁹ /L, and a history of previous thrombosis. Treatment is aimed at lowering both platelet counts and the risk for thromboses. Acute leukemic transformation in ET is rare, occurring in approximately 1% at 10 years and 2% at 15 years. A post-ET myelofibrotic transformation has been described, occurring in approximately 5% at 10 years and 10% at 15 years.

Clinical Features

The incidence of PMF is approximately 0.5 to 1 per 100,000 persons. The average age is between 54 and 62 years, with an equal sex distribution. Occurrence is rare in children, and there is an increased incidence in Ashkenazi Jews.

Clinically, many patients are asymptomatic (30%). For those with symptoms, complaints are related to anemia, splenomegaly, or constitutional symptoms. Other symptoms, such as weight loss, gouty arthritis, or renal stones from hyperuricemia may also be present.