Introduction

Acute lymphoblastic leukemia (ALL), also known as lymphoblastic leukemia/lymphoma in the WHO classification, is a malignant expansion of immature lymphoid cells that results from multi-step genetic changes in a single lymphoid progenitor cell. Its incidence peaks between the ages of 2 and 4 years; rates are lower during later childhood, adolescence and young adulthood but the incidence rises in the sixth decade, reaching a second, smaller peak in the elderly. ALL is the most common malignancy diagnosed in patients younger than 15 years. Childhood ALL appears to have a prenatal origin in many cases. In the case of identical twins, when leukemia occurs in one twin, there are a 20% probability that it will also occur in the other twin due to ALL transfer through the placental circulation. In identical twins with the t(4;11); (q21;q23) MLL-AFF1, the chances of ALL becoming clinically overt in the other twin in a short period of time are nearly 100%. The concordance rate in twins is lower in cases of ALL with the ETV6-RUNX1 fusion or T-cell phenotype, probably because of the requirement for additional genetic events for leukemic transformation.

A small proportion of patients (< 5%) have hereditary genetic abnormalities that predispose to the disease, including Down syndrome, ataxia telangiectasia and Bloom's syndrome; children with Down syndrome have a 10- to 30-fold higher risk of developing ALL.