A 4-year-old boy presents with a 2-week history of fever, pallor, and petechiae. Complete blood count shows hemoglobin 7.2 g/dL, WBC 45,000/µL with 85% blasts, and platelets 20,000/µL. Bone marrow aspiration confirms acute leukemia. What is the most common type of acute leukemia in this age group?

Explanation

## Epidemiology of Childhood Leukemias **Key Point:** Acute lymphoblastic leukemia (ALL) accounts for approximately 75–80% of all childhood acute leukemias, making it the most common malignancy in children. ### Incidence and Age Distribution | Leukemia Type | Frequency in Children | Peak Age | Key Features | |---|---|---|---| | ALL | 75–80% | 2–5 years | B-cell precursor (80%), good prognosis with modern therapy | | AML | 15–20% | Bimodal: <2 years and adolescence | Higher mortality, requires intensive chemotherapy | | CML | 2–5% | Adolescence > younger children | BCR-ABL fusion, tyrosine kinase inhibitor responsive | | Burkitt lymphoma | <1% with leukemic phase | 8–14 years | Translocation t(8;14), highly aggressive | **High-Yield:** The peak incidence of ALL is between ages 2–5 years, which matches the clinical presentation in this case. ### Why ALL Dominates in Childhood 1. **Developmental origin:** ALL arises from lymphoid progenitors in the bone marrow during the period of rapid lymphoid development in early childhood. 2. **Cytogenetics:** Favorable cytogenetics (e.g., hyperdiploidy, t(12;21)) are common in childhood ALL and associated with better prognosis. 3. **Treatment response:** Modern multiagent chemotherapy protocols achieve 5-year event-free survival rates of >90% in standard-risk ALL. **Clinical Pearl:** The presence of hepatosplenomegaly, lymphadenopathy, and mediastinal mass (if present) would further support ALL diagnosis, as these are more common in ALL than AML in children. ### Differential Considerations - **AML:** Second most common; typically presents with higher platelet counts and more myeloid maturation; associated with t(15;17) (APL) in some cases. - **CML:** Rare in young children; usually presents in adolescence; characterized by BCR-ABL fusion and basophilia. - **Burkitt lymphoma:** Typically presents as a lymphoid mass with leukemic phase being uncommon; associated with t(8;14) and c-MYC amplification. [cite:Robbins 10e Ch 7]