A 7-year-old boy from Delhi presents with a 2-month history of progressive pallor, bone pain, and recurrent infections. On examination, he has hepatosplenomegaly and generalized lymphadenopathy. Complete blood count shows: Hemoglobin 6.8 g/dL, WBC 120,000/μL with 85% blasts, platelets 25,000/μL. Bone marrow aspirate reveals >90% blasts with fine chromatin, prominent nucleoli, and abundant cytoplasm with vacuoles. Flow cytometry shows blasts positive for CD10, CD19, CD20 (partial), HLA-DR, and TdT, negative for MPO and CD33. Cytogenetics reveals t(12;21)(p13;q22). What is the most likely diagnosis?

Explanation

## Diagnosis: Acute Lymphoblastic Leukemia (ALL) with t(12;21) — Favorable Prognosis ### Clinical Presentation The child presents with classic ALL features: anemia (pallor), thrombocytopenia (easy bruising), leukocytosis with blasts, and extramedullary involvement (hepatosplenomegaly, lymphadenopathy). Bone pain is common in pediatric ALL due to leukemic infiltration of the bone marrow. ### Morphologic Features **Key Point:** The blasts in this case show fine chromatin, prominent nucleoli, and abundant cytoplasm with vacuoles—morphology consistent with L2 (common) or L3 (Burkitt-like) ALL by FAB classification. The vacuoles suggest cytoplasmic lipid droplets, which are characteristic of ALL blasts. **High-Yield:** Auer rods are **absent** in ALL. Their absence, combined with the immunophenotype, is crucial for distinguishing ALL from AML. ### Immunophenotype Analysis | Marker | ALL | AML | Significance | | --- | --- | --- | --- | | CD10 | Positive (common ALL) | Negative | B-cell precursor marker | | CD19 | Positive | Negative | B-lineage marker | | CD20 | Partial/variable | Negative | B-lineage marker | | HLA-DR | Positive | Positive | Activation antigen | | TdT | Positive | Negative | Terminal deoxynucleotidyl transferase—lymphoid marker | | MPO | Negative | Positive | Myeloid marker | | CD33 | Negative | Positive | Myeloid marker | **Key Point:** The combination of CD10+, CD19+, HLA-DR+, and TdT+ with MPO− and CD33− is diagnostic of B-cell precursor (BCP) ALL, also called common ALL (cALL). This is the most frequent ALL subtype in children (~80%). ### Cytogenetics: t(12;21) — The Most Favorable Prognostic Marker **High-Yield:** The translocation t(12;21)(p13;q22) results in fusion of the ETV6 gene (chromosome 12) with the RUNX1 gene (chromosome 21), creating the ETV6-RUNX1 fusion gene. This is the most common recurrent translocation in pediatric ALL (~25% of cases). **Clinical Pearl:** t(12;21) ALL is associated with **excellent prognosis**: - 5-year event-free survival (EFS) >90% with modern chemotherapy - Favorable response to standard-risk chemotherapy protocols - Lower risk of CNS involvement - Better overall survival compared to other ALL subtypes ### Why This Is Favorable-Risk ALL ```mermaid flowchart TD A[Pediatric ALL]:::outcome --> B{Cytogenetics?}:::decision B -->|t(12;21)| C[ETV6-RUNX1 fusion]:::outcome B -->|t(9;22)| D[BCR-ABL fusion]:::urgent B -->|t(4;11)| E[KMT2A rearrangement]:::urgent B -->|Hyperdiploid| F[>50 chromosomes]:::action C --> G[Excellent prognosis]:::action D --> H[Poor prognosis]:::urgent E --> I[Very poor prognosis]:::urgent F --> J[Good prognosis]:::action ``` **Mnemonic:** **CALLA** = **C**ommon **ALL**a (CD10+, CD19+) — this is the classic immunophenotype. When combined with t(12;21), it defines the most favorable-risk ALL subtype. ### Treatment Implications **Key Point:** Patients with t(12;21) ALL are typically treated with standard-risk chemotherapy protocols (e.g., BFM 95, COG protocols) rather than high-risk protocols. The presence of this translocation is one of the strongest favorable prognostic factors in pediatric ALL.