Can a male patient develop an extramedullary blast crisis (myeloid sarcoma/granulocytic sarcoma) of chronic myeloid leukemia while having a normal complete blood count and a low BCR‑ABL transcript level of 0.04%?

Yes, Extramedullary Blast Crisis Can Occur with Normal CBC and Low BCR-ABL1

A male patient can absolutely develop extramedullary blast crisis (myeloid sarcoma/granulocytic sarcoma) of CML while maintaining a normal complete blood count and a BCR-ABL1 level of 0.04%, representing a rare but well-documented phenomenon that requires high clinical suspicion and tissue biopsy for diagnosis.

Key Diagnostic Principle

The critical concept here is that myeloid sarcoma represents a pathologic diagnosis based on extramedullary tissue, not peripheral blood or bone marrow findings 1. According to WHO classification, myeloid sarcoma can precede the appearance of blood or bone marrow disease entirely, and when it does, the diagnosis should be considered synonymous with AML requiring full evaluation 1.

Evidence from Clinical Cases

Multiple case reports document this exact scenario:

• A 26-year-old man achieved deep molecular remission (BCR-ABL1 0.0058% International Scale) on flumatinib yet developed a painful sacrococcygeal mass that proved to be granulocytic sarcoma-type CML extramedullary blast crisis 2. This demonstrates that profound molecular response does not preclude extramedullary disease.

• A 49-year-old man presented with cervical mass and only slight platelet elevation without leukemoid reaction, confirmed as extramedullary blast crisis 3. The aleukemic presentation was the initial manifestation of CML.

• A patient with CML in complete hematological, molecular, and cytogenetic remission for 42 months developed spinal granulocytic sarcoma 4. This case emphasizes that complete remission status in blood/marrow does not eliminate extramedullary risk.

Pathophysiologic Mechanism

Extramedullary blast crisis represents blast transformation occurring in tissue sanctuaries where systemic BCR-ABL1 monitoring may not reflect local disease burden. The International Bone Marrow Transplant Registry specifically defines blast crisis as including extramedullary disease regardless of blast percentage in blood or marrow 5, 6.

Diagnostic Algorithm When Suspecting Extramedullary Disease

1. Obtain tissue biopsy of any suspicious mass - this is mandatory and non-negotiable

   • Morphology alone may be misleading (can mimic lymphoma or sarcoma)
   • Immunohistochemistry is essential for myeloid lineage confirmation
   • FISH analysis for BCR-ABL1 on tissue confirms clonal relationship to CML 2, 3

2. Perform targeted RNA sequencing on tissue when morphology is ambiguous

   • Can detect BCR-ABL1 fusion in tissue even when peripheral levels are undetectable
   • May reveal rare transcript variants (e.g., e1a3, e6a2) that standard RT-PCR misses 3, 7
   • Can identify concurrent molecular alterations (e.g., NPM1 fusions) 2

3. Complete staging evaluation:

   • Bone marrow aspirate and biopsy with cytogenetics
   • PET-CT to identify additional extramedullary sites
   • CSF examination if neurological symptoms present 8

Critical Pitfalls to Avoid

• Never rely solely on peripheral BCR-ABL1 levels to exclude blast crisis - extramedullary disease can exist independently of marrow/blood involvement 4
• Do not dismiss tissue masses as unrelated to CML simply because blood counts are normal 9
• Recognize that standard RT-qPCR may miss rare BCR-ABL1 transcripts - if clinical suspicion is high but standard testing is negative, pursue RNA sequencing 3
• Understand that deep molecular remission (BCR-ABL1 <0.01%) does not eliminate extramedullary blast crisis risk 2

Treatment Implications

Extramedullary blast crisis requires systemic AML-intensity chemotherapy plus TKI therapy, not TKI monotherapy 10. The specific approach:

• Intermediate-to-high dose cytarabine-based chemotherapy
• Continue or escalate TKI (consider second/third-generation agents)
• Bridge to allogeneic hematopoietic stem cell transplantation when feasible 10, 11
• Local radiation may be considered for symptomatic masses but is not curative 10

Why This Occurs

The BCR-ABL1 level of 0.04% (MR3.4 range) indicates excellent systemic disease control but does not reflect clonal evolution or sanctuary site disease. Additional chromosomal aberrations and secondary mutations can drive extramedullary transformation independent of systemic disease burden 11, 2. The presence of concurrent genetic alterations (like NPM1::CCDC28A fusion) may facilitate extramedullary dissemination through HOX gene upregulation 2.

Bottom line: Normal CBC and low BCR-ABL1 do not exclude extramedullary blast crisis. Any unexplained mass in a CML patient—regardless of remission status—mandates tissue diagnosis with comprehensive molecular characterization.