11 General introduction 1 ACUTE LYMPHOBLASTIC LEUKEMIA IN CHILDREN Pediatric acute lymphoblastic leukemia (ALL) is the most common childhood cancer type, with approximately 120 new patients each year in The Netherlands.1 This hematologic malignancy originates from the bone marrow, where under normal circumstances hematopoietic stem cells produce all lineages of blood and immune cells (Figure 1). In ALL, normal hematopoiesis is interrupted by maturation arrest of one of the lymphatic cell lines, followed by uncontrolled growth of malignant immature monoclonal lymphoid cells. This expansion of leukemic cells leads to a decreased production of erythrocytes, platelets and functional leukocytes.2 Both precursor B-cell and T-cell leukemia can occur in children, with precursor B-cell ALL being the most common variant (85%). The peak incidence of ALL in children is between the age of two and five years and boys are slightly more often affected than girls.1 Figure 1. Normal hematopoiesis
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