Updated on: 09.06.2025

Introduction:

• Chronic eosinophilic leukaemia is a myeloproliferative neoplasm in which there is autonomous clonal proliferation of eosinophilic precursors resulting in persistently increased numbers of eosinophils in blood (>1500/cmm), bone marrow and peripheral tissues.
• It is diagnosed after possibility of other myeloid or lymphoid neoplasms (such as Myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions) is ruled out.
• They have evidence of clonality (Cytogenetics or molecular) or abnormal bone marrow morphology (>5% blasts)
• For approach to diagnosis of eosinophilia: Click here 

Epidemiology:

• M: F – 1: 1
• Usually seen at around 40 years

Clinical Features:

• 10% are asymptomatic
• Constitutional symptoms- Fever, fatigue, cough
• End organ damage (HES): Refer to eosinophilia section

Investigations:

• Hemogram
  • WBC count- > 30 x 10⁹ /L
  • Marked eosinophilia – 30 – 70%
  • Mainly mature eosinophils with only few eosinophilic myelocytes or promyelocytes
  • Eosinophils show sparse granulation with clear areas of cytoplasm, cytoplasmic vacuolization, nuclear hypersegmentation /  hyposegmentation (Eosinophil dysplasia)
  • Associated neutrophilia and blasts > 2% prompt the diagnosis of CEL
  • Normo or macrocytic anemia
  • Thrombocytopenia in some cases
• Bone marrow
  • Hypercellular due to eosinophilic proliferation
  • Orderly maturation of eosinophils seen
  • Charcot – Leyden crystals often present
  • Erythropoiesis – Normal/ dysplastic
  • Myelopoiesis- Dysplasia may be seen.
  • Megakaryopoiesis – Dysplasia is commonly seen (hypolobated/non-lobated nuclei or separated nuclear lobes)
  • Fibrosis may be seen.
  • Marrow should be carefully searched for any process which might explain eosinophilia as a secondary reaction – vasculitis, lymphoma, ALL, granulomatous reaction etc
• Cytochemistry 
  • Cyanide resistant myeloperoxidase positive
  • Naphthol – ASD – chloroacetate esterase – positive (Normally absent in eosinophils)
• Immunophenotyping: No specific immunophenotypic abnormalities
• Cytogenetics
  • Chromosome 5 band 31-35 contains several genes relevant to eosinopoiesis- such as IL5, IL3, GM-CSF, PDGFR beta
  • t (1:5), t (2:5), t (5:12), t (6:11)
  • Other abnormalities include- +8,   i(17q)
  • t (5:12) – chronic myelomonocytic leukaemia with eosinophilia
  • t (8:13) -(p11; q22) & other 8p11 translocations- Related to FGFR1 gene
  • 8p11 syndrome includes – eosinophilic leukemia, AML, precursor T lymphoblastic leukaemia / lymphoma, Precursor B lymphoblastic leukaemia
  • Microdeletion on chromosome 4 leads to fusion of FIP1L1 & PDGFR alfa genes. This results in generation of constitutively active tyrosine kinase. FIP1L1-PDGFRA fusion tyrosine kinase is seen in 50% of HES. Discussed below.
  • Age-related loss of the Y chromosome should not be considered as evidence of clonality
• NGS panel for myeloid mutations:
  • Mutations other than DNMT3A, TET2, and ASXL1 are considered  indicators of a neoplastic process
  • Mutations with variant allele fraction (VAF) <2% should not be used to support clonality
• Serum cobalamin, uric acid and muramidase- Increased
• LAP score- Normal
• Annual echocardiography- To assess cardiac damage

Criteria for Diagnosis CEL-NOS:

Essential criteria

• AEC >1500/cmm on at least 2 occasions over an interval of at least 4 weeks
• Evidence of clonality (Cytogenetics or molecular) or abnormal bone marrow morphology (>5% blasts)
• WHO criteria for other myeloid or lymphoid neoplasms not met (negative for BCR-ABL, PDGFRA/B, FGFR1 or PCM1-JAK2)

Desirable

• None

Prognosis:

• Median survival: 2 years
• Causes of death include infection, bleeding, and disease-related organ failure
• Poor prognostic markers:
  • Marked splenomegaly
  • High blast count in blood/ bone marrow
  • Cytogenetic abnormalities
  • Dysplastic changes in other myeloid lineages
  • Lack of response to steroids
  • Markedly elevated eosinophil count
  • Normal IgE levels
  • Male sex

Pretreatment Work-up: 

• History
• Examination:   Spleen:     Skin:
• BMA and Bx
• Haemoglobin
• TLC, DLC, AEC
• Platelet count
• Peripheral smear: For monocytosis, dysplasia or circulating blasts
• LFT: Bili- T/D       SGPT:       SGOT: Albumin:     Globulin:
• Creatinine
• Uric acid
• S. IgE levels
• Vitamin B12 level
• S. Tryptase level
• LDH
• HIV
• Cytogenetics
• FISH/ PCR- BCR-ABL1
• JAK 2
• CAL-R
• MPL
• NGS/ FISH for PDGFRA
• NGS/ FISH for PDGFRB
• NGS/ FISH for FGFR1
• Inv (16)
• t (16:16)
• Cardiac Troponin, proBNP, ECHO, ECG, Chest x ray, PFT, Electromyography- For end organ damage
• Treatment consent after informing about disease, prognosis, cost of therapy, side effects, hygiene, food and contraception
• Tumor board meeting and decision
• Inform primary care physician

Treatment:

• Aim: Decrease eosinophil count and prevent further end organ damage.
• Treatment of neoplasms with tyrosine kinase gene fusions is discussed separately below.
• For all other patients (CEL-NOS): A trial of Imatinib can be given. If there is no response, steroids (Prednisolone- 1mg/Kg) can be used. If this also fails, other options include hydroxyurea, IFN-alpha, Vincristine, Cyclophosphamide, Etoposide and Cladribine.
• Other options (in trials and not approved): Anti- IL-5 antibodies (mepolizumab), Cyclosporine, Azathioprine, alemtuzumab, Allo-HSCT.

Myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions

They include:

• Myeloid/ Lymphoid neoplasm with PDGFRA rearrangement
• Myeloid/Lymphoid Neoplasm with PDGFRB Rearrangement
• Myeloid/Lymphoid Neoplasm with FGFR1 Rearrangement
• Myeloid/Lymphoid Neoplasm with JAK2 Rearrangement
• Myeloid/Lymphoid Neoplasm with FLT3 Rearrangement
• Myeloid/Lymphoid Neoplasm with ETV6::ABL1fusion
• Myeloid/Lymphoid Neoplasms with other tyrosine kinase gene fusions

Myeloid/ Lymphoid neoplasm with PDGFRA rearrangement

• Most of them are associated with eosinophilia
• M:F= 7:1
• Median age- 5^th decade
• Pathogenesis: Cryptic deletion at 4q12 which leads to formation of FIP1L1-PDGFRA fusion gene
• Other partners genes include: KIF5B, CDK5RAP2, STRN, ETV6, BCR, and TNKS2
• t (1:4) and t (4:10) can also lead to this fusion gene formation.
• Can present with
  • Chronic eosinophilic leukemia
  • MPN in blast crisis
  • AML with eosinophilia
  • T- ALL
  • Myeloid sarcoma
• Associated with
  • Elevated vitamin B12 levels
  • Elevated S. Tryptase
  • Splenomegaly
• Peripheral smear: Eosinophils are large with abnormal distribution of cytoplasmic granules, cytoplasmic vacuolation, abnormal granule size or color, and/or abnormal nuclear lobation
• BMA: Hypercellular with increased eosinophilic precursors. Dense clusters of mast cells are present 
• Serum tryptase level:  Elevated
• RT-PCR/ FISH/ RNA sequencing for PDGFRA fusion
• Cytogenetics: Additional chromosomal abnormalities such as trisomy 8 are associated with increased risk of disease progression.
• Immunophenotyping
  • Eosinophil activation markers: CD23, CD25 and CD69
  • Mast cells: KIT (CD117), tryptase and aberrant expression of CD25 (However CD2 is negative, compared to mast cells of systemic mastocytosis)
• Diagnostic criteria:
  • Essential:
    • A myeloid (more frequent) or lymphoid neoplasm, usually with prominent peripheral and/or tissue eosinophilia
    • Presence of a PDGFRA fusion gene, usually with FIP1L1.
  • Desirable: In the absence of molecular demonstration of the fusion gene, the diagnosis should be suspected if
    • There is a BCR::ABL1-negative myeloproliferative neoplasm with prominent eosinophilia associated with splenomegaly
    • Marked elevation of serum vitamin B12
    • Increased serum tryptase
    • Increased bone marrow mast cells
• Prognosis: Good
• Poor prognostic markers include:
  • Blast phase of disease
  • Complex karyotype
  • Cardiac involvement
• Treatment:
  • Imatinib- Start with 100mg OD, then titrate the dose up to 400mg-OD, depending on AEC.
  • Concurrent corticosteroids for 7–10 days if there is cardiac involvement
  • It is useful both in chronic phase and blast crisis
  • Hence, even if patient has manifestation of acute leukaemia- Imatinib has to be given. As these patients can enter into remission with Imatinib alone. 
  • If there is development of resistance (Ex. T674I mutation)- Sorafinib or midostaurin can be used.

Myeloid/Lymphoid Neoplasm with PDGFRB Rearrangement

• PDGFRB mutations occur due to translocations involving chromosome 5q32. Most common counterpart is ETV6 which results from t (5:12).
• Male predominance
• Usually seen in 5^th decade
• Presentations:
  • CMML with eosinophilia
  • Atypical CML with eosinophilia
  • JMML
• Splenomegaly is common
• BM shows features of dysplasia along with eosinophilia
• PDGFRB mutation testing can be done by RT-PCR or FISH or RNA sequencing
• Criteria for diagnosis:
  • Essential:
    • A myeloid or lymphoid neoplasm, often with prominent eosinophilia with varying degrees of neutrophilia or monocytosis associated with the formation of a PDGFRB fusion gene.
    • Cases of BCR::ABL1-like B-ALL without evidence of an associated myeloid neoplasm are excluded from this category.
  • Desirable:
    • Cytogenetic and molecular identification of the partner gene, e.g., t(5;12)(q32;p13.2) with ETV6::PDGFRB or other partner genes.
• Prognosis: Good. 10 year overall survival when treated with Imatinib is 90%.
• Treatment- 
  • Standard dose Imatinib
  • Concurrent corticosteroids for 7–10 days if there is cardiac involvement
  • At 3 months, PB or BM standard cytogenetics and/or FISH
  • In subset of patients with blast crisis Imatinib alone is useful. If there is no response, HSCT needs to be done.

Myeloid/Lymphoid Neoplasms with FGFR1 Rearrangement

• Gene is located on chromosome 8p11.2. So far 16 partner genes have been identified (Most common- ZMYM2)
• Presentations include (all have associated eosinophilia):
  • MPN 
  • AML
  • B-ALL
  • T-ALL
  • Mixed phenotype acute leukemia
• Mild male predominance
• Mostly seen in 4^th decade
• Testing for FGFR1 can be done by RT-PCR/ FISH
• Prognosis: Poor. Has aggressive clinical course.
• Criteria for diagnosis:
  • Essential:
    • Myeloproliferative or myelodysplastic/myeloproliferative neoplasm with prominent eosinophilia, +/- neutrophilia or monocytosis or with increased blasts of myeloid, T-cell or B-cell lineage, or mixed phenotype
    • Demonstration of t(8;13)(p11.2;q12.1) or a different translocation leading to formation of an FGFR1 fusion gene
  • Desirable:
    • Molecular identification of the partner gene of FGFR1
• Treatment: 
  • Chronic phase: Pemigatinib/ midostaurin/ Ponatinib- Followed by HSCT (If fit)
  • Blast crisis: (Pemigatinib/ midostaurin/ Ponatinib) + ALL/AML type induction chemotherapy- Followed by HSCT (If fit)

Myeloid/Lymphoid Neoplasm JAK2 Rearrangement

• Most common fusion partner is PCM1
• Can present with
  • MPN with eosinophilia
  • MDS/MPN with eosinophilia
  • ALL
  • AML
• Male predominance
• Aggressive clinical course
• Cytogenetics: Alterations involving 9p24.1 are seen
• Diagnostic criteria
  • Essential:
    • A myeloid or lymphoid neoplasm, often with prominent eosinophilia and the presence of a JAK2 fusion gene
    • Cases of BCR::ABL1-like B-ALL without evidence of an associated myeloid neoplasm are excluded
  • Desirable:
    • Cytogenetic identification of the translocation. 
    • Molecular identification of the fusion gene, e.g., PCM1::JAK2.
• Treatment- 
  • Ruxolitinib/ Fedratinib (Doses adopted to platelet count). HSCT should be considered irrespective of response to Ruxolitinib.
  • If there is blast crisis, ALL/AML type induction chemotherapy must be given along with Ruxolitinib/ Fedratinib. This must be followed by Allo-HSCT (if fit)

Myeloid/Lymphoid Neoplasms with FLT3 Rearrangement

• The most common partner gene is ETV6/12p131
• Others include: BCR, ZMYM2, TRIP11, SPTBN1, GOLGB1, CCDC88C, ZBTB44 and MYO18A
• May resents as
  • CEL
  • MDS
  • MDS/MPN
  • T- Lymphoblastic lymphoma
  • B- ALL
• Cytogenetics: Chromosomal rearrangements involving 13q12
• Diagnostic criteria: A myeloid or lymphoid neoplasm, with or without associated eosinophilia with chromosomal rearrangements leading to the formation of a FLT3 fusion gene.
• Aggressive clinical course
• Treatment- Consider sunitinib or sorafenib or midostaurin. Allo HSCT is to be considered at the earliest.

Myeloid/Lymphoid Neoplasm with ETV6::ABL1fusion

• Occurs due to t(9;12)(q34;p13)
• BM: Similar to CML but has marked eosinophilia
• Others may present as
  • MDS/MPN
  • CEL
• Diagnostic criteria:
  • Essential: A haematopoietic (myeloid or lymphoid) neoplasm in chronic phase associated with ETV6::ABL1.
  • Desirable: Cytogenetics: t(9;12)(q34;p13) or complex aberrations involving other chromosomes
• Prognosis: Poor
• Treatment: Dasatinib/ Nilotinib/ Imatinib/ bosutinib

Myeloid/Lymphoid Neoplasms with other tyrosine kinase gene fusions

• Present with MDS/MPN with eosinophilia
• Fusions include: ETV6::FGFR2; ETV6::LYN; ETV6::NTRK3; ANBP2::ALK; BCR::RET and FGFR1OP::RET
• Criteria for diagnosis:
  • Essential
    •  A myeloid and/or lymphoid neoplasm
    • Detection of a tyrosine kinase fusion gene, other than those specifically defined as distinct entities (i.e., PDGFRA, PDGFRB, FGFR1, JAK2, FLT3, ETV6::ABL1 etc.).
  • Desirable
    • Eosinophilia
    • Cytogenetic identification of a translocation, suggesting the involvement of a tyrosine kinase gene and prompting the selection of appropriate break apart FISH probes or other molecular investigation
• Prognosis and treatment: Not well defined