## Cytogenetic Prognostic Markers in Childhood ALL **Key Point:** Hyperdiploidy (>50 chromosomes) is the single most favorable prognostic cytogenetic finding in childhood ALL, associated with 5-year event-free survival (EFS) >90%. ### Prognostic Cytogenetic Abnormalities in ALL | Abnormality | Prognosis | 5-Year EFS | Mechanism | |---|---|---|---| | Hyperdiploidy (>50) | Excellent | >90% | Increased chromosomal instability; better chemotherapy response | | t(12;21) ETV6-RUNX1 | Favorable | 85–90% | Fusion transcription factor; common in standard-risk ALL | | t(9;22) Philadelphia | Adverse | 50–60% | BCR-ABL tyrosine kinase; requires TKI therapy | | MLL rearrangement | Adverse | 40–50% | Aggressive biology; high-risk disease | | Hypodiploidy (<45) | Adverse | 50–60% | Chromosomal loss; poor response | **High-Yield:** Hyperdiploidy is found in ~25% of childhood ALL cases and is a hallmark of standard-risk disease when combined with low initial WBC and age 1–9 years. **Clinical Pearl:** The ETV6-RUNX1 fusion (formerly TEL-AML1) is the most common favorable translocation in childhood ALL (~20% of cases) but is NOT the single best prognostic marker — hyperdiploidy trumps it. **Mnemonic:** **HARM** = Hyperdiploidy is Awesome, Philadelphia is Rough, MLL is Miserable. ### Why Hyperdiploidy Is Favorable 1. Increased genomic instability paradoxically improves chemotherapy sensitivity. 2. Associated with lower-risk clinical features (younger age, lower WBC). 3. Trisomies of chromosomes 4, 10, and 17 confer additional benefit. 
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