## Cytogenetic Risk Stratification in Childhood ALL ### Favorable Prognostic Features **Key Point:** ETV6-RUNX1 (formerly TEL-AML1) and hyperdiploidy are the two most important favorable cytogenetic findings in childhood ALL, each present in ~20–25% of cases. | Feature | Prognosis | Frequency | Mechanism | |---------|-----------|-----------|----------| | t(12;21) ETV6-RUNX1 | Favorable | ~20–25% | Transcription factor dysregulation; chemotherapy-sensitive | | Hyperdiploidy (>50 chr) | Favorable | ~25–30% | Increased DNA content; enhanced drug uptake | | t(9;22) BCR-ABL | **Unfavorable** | ~3–5% | Constitutive tyrosine kinase; high relapse risk | | Low WBC (<50K) | Favorable | — | Reflects lower tumor burden | ### Why t(9;22) BCR-ABL is Unfavorable 1. **Constitutive kinase activity:** BCR-ABL produces a 210 kDa protein with unregulated tyrosine kinase activity, driving aggressive proliferation. 2. **High relapse rate:** Even with intensive chemotherapy, BCR-ABL ALL has historically had 5-year event-free survival (EFS) <50% without targeted therapy (TKI). 3. **Requires TKI escalation:** Modern management mandates tyrosine kinase inhibitors (imatinib, dasatinib, nilotinib) in addition to chemotherapy. 4. **Philadelphia chromosome ALL:** This is the hallmark of Philadelphia chromosome-positive ALL, classified as a distinct, high-risk entity. **High-Yield:** BCR-ABL (Philadelphia chromosome) is the **only** listed option that represents an **unfavorable** prognostic marker — making it the correct answer to an "EXCEPT" question. ### Clinical Pearl In modern pediatric ALL protocols (COG, UKALL), BCR-ABL ALL is now treated with chemotherapy + TKI, which has improved outcomes significantly, but it remains a high-risk subtype requiring intensified therapy and close monitoring for TKI resistance. [cite:Harrison 21e Ch 110]
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