What is the recommended approach for assessing Chronic Myeloid Leukemia (CML)?

Comprehensive Assessment of Chronic Myeloid Leukemia (CML)

The recommended approach for assessing Chronic Myeloid Leukemia (CML) requires a mandatory combination of cytogenetic analysis and qualitative RT-PCR, with additional testing based on disease phase and clinical context. 1

Initial Diagnostic Testing

Mandatory Tests

• Cytogenetic Analysis: Essential for detecting the Philadelphia chromosome and additional cytogenetic abnormalities (ACAs) which have prognostic significance 1

  • Analyze minimum 20 metaphases (ideally 25) to ensure detection of ACAs 1
  • Cannot be used in isolation as 5% of CML patients have normal karyotype 1
  • Performed on bone marrow aspirate (preferred) or peripheral blood 1

• Qualitative RT-PCR: Required to determine the exact BCR::ABL1 transcript type 1

  • Essential for identifying typical and atypical BCR::ABL1 variants 1
  • Usually performed on peripheral blood 1
  • Sequence confirmation may be required for atypical variants 1

Recommended Additional Tests

• FISH (Fluorescence In Situ Hybridization): Alternative approach for initial screening 1

  • Dual fusion probes recommended over ES (extra signal) probes 1
  • Mandatory for cases with insufficient metaphases or normal karyotype to exclude cryptic abnormalities 1
  • Cannot detect ACAs or identify transcript type, so positive cases need follow-up with cytogenetics and RT-PCR 1

• Quantitative RT-qPCR: Strongly recommended at diagnosis 1

  • Provides baseline for determining early response kinetics 1
  • Useful for identifying cases requiring investigation for atypical BCR::ABL1 variants 1

Disease Phase-Specific Testing

• Chronic Phase (CP):

  • NGS panel for myeloid and lymphoid genes is not recommended for routine clinical management 1
  • BCR::ABL1 tyrosine kinase domain (TKD) mutation analysis not recommended at diagnosis as mutations are unlikely to be detected prior to TKI therapy 1

• Blast Phase (BP) or de novo BP:

  • NGS panel analysis is strongly recommended to identify potential therapeutic targets in addition to BCR::ABL1 1
  • Can provide additional prognostic information 1

Monitoring During Treatment

• Molecular Monitoring: Sequential monitoring by RT-qPCR is essential 1

  • Peripheral blood BCR::ABL1 mRNA levels serve as surrogate for disease burden 1
  • Results should be reported on the International Scale (IS) 1
  • RT-dPCR (digital PCR) is a valid alternative to RT-qPCR 1

• Cytogenetic Monitoring: Follow-up analysis at 3,6, and 12 months post-treatment until complete cytogenetic response (CCyR) is achieved 1

  • Analyze minimum 20 cells for disease monitoring 1
  • More frequent monitoring required for patients with additional abnormalities at diagnosis 1
  • Once CCyR achieved, can be replaced by FISH or standardized RQ-PCR 1

Response Assessment Criteria

• Complete Hematologic Response (CHR): White blood cells <10×10^9/L, no immature granulocytes, <5% basophils, platelets <450×10^9/L, non-palpable spleen 1

• Cytogenetic Response:

  • Partial (PCyR): 1-35% Ph+ metaphases 1
  • Complete (CCyR): No Ph+ metaphases 1

• Molecular Response:

  • Major Molecular Response (MMR): BCR::ABL1:ABL ratio <0.10% by International Scale 1

Common Pitfalls and Caveats

• Diagnostic Pitfalls:

  • Up to 5% of CML cases have a normal karyotype, requiring FISH or molecular confirmation 1
  • 5-10% of cases have variant translocations involving chromosomes beyond 9 and 22 1
  • 1-5% have cryptic BCR::ABL1 fusion not visible by conventional cytogenetics 1

• Monitoring Pitfalls:

  • Accurate interpretation of FISH follow-up requires knowledge of signal pattern at diagnosis 1
  • Cases with single fusion signal cannot be reliably monitored by FISH 1
  • Care needed when interpreting variants in samples with reduced BCR::ABL1 levels, as they could represent clonal hematopoiesis rather than CML 1

• Exclusion Criteria:

  • Absence of Philadelphia chromosome or BCR::ABL1 fusion excludes CML diagnosis 2
  • Cases previously considered as BCR::ABL1-negative CML should be classified as another myeloid neoplasm 1

By following this comprehensive assessment approach, clinicians can accurately diagnose CML, determine disease phase, monitor treatment response, and make appropriate therapeutic decisions to improve patient outcomes.