EPIDEMIOLOGY AND ETIOLOGY
Acute lymphoblastic leukemia (ALL) is characterized by the proliferation and accumulation of lymphoid progenitor cells in the blood, bone marrow, and other tissues. It has a bimodal distribution. The overall age-adjusted incidence is 1.7 per 100,000 persons, but ALL affects 4 to 5 per 100,000 persons during age 4 to 5 years and half that number around the fifth decade of life. Approximately 60% of cases are diagnosed in patients ≤20 years old, with a median age at diagnosis of 14 years. In 2014, the American Cancer Society estimated that approximately 6,000 individuals would be diagnosed with ALL that year (1,2). Acute lymphoblastic leukemia represents 20% of adult leukemias but is the most common childhood acute leukemia, representing approximately 80% of cases (1,2).
The etiology of ALL is unknown in most cases (3,4,5,6,7). Chromosomal translocations occurring in utero during fetal hematopoiesis have suggested genetic factors as the primary cause for pediatric ALL and postnatal genetic events as secondary contributors. Monozygotic and dizygotic twins of patients with ALL and individuals with genetic disorders, such as Klinefelter (XXY and variants) and Down (trisomy 21) syndromes, or inherited diseases with excessive chromosomal fragility, such as Bloom syndrome, Fanconi anemia, and ataxia telangiectasia, have all been found to have higher incidence of ALL, implicating a possible genetic predisposition. Additional studies have postulated infectious etiologies (4). Human T-cell lymphotropic virus type-1 is known to cause adult T-cell leukemia/lymphoma (5); Epstein-Barr virus has been associated with lymphoproliferative disorders, including Burkitt lymphoma and mature B-cell ALL (6); and varicella has been linked to childhood ALL (7).
CLINICAL PRESENTATION AND LABORATORY ABNORMALITIES
Presenting symptoms can be nonspecific, particularly in children. They largely reflect bone marrow failure and include malaise, fatigue, bleeding or bruising, and secondary infections. The B symptoms, such as fever, night sweats, and weight loss, are frequent. White blood cell (WBC) count at presentation varies widely, and circulating blasts are generally noted. Symptoms related to hyperleukocytosis are rare in ALL, given the lymphoblast morphology, even when WBC counts are high.
Leukemic involvement of the central nervous system (CNS) ranging from cranial neuropathies to meningeal infiltration occurs in <10% of patients at presentation. It is more common in mature B-cell ALL (Burkitt leukemia) (8). A history or findings of abdominal masses, significant spontaneous tumor lysis syndrome, and chin numbness (mental nerve) indicating cranial nerve involvement are also more common in this subtype of ALL (9). Lymphadenopathy and hepatosplenomegaly, although rarely symptomatic, are noted in approximately 20% of patients (9).
The diagnosis of ALL is largely based on flow cytometric immunophenotyping, although identification of cytogenetic-molecular abnormalities plays a significant role ...