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INTRODUCTION

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Acute lymphoblastic leukemia (ALL) is a highly aggressive neoplasm of hematopoietic cells of lymphoid lineage. Collections of abnormal T- or B lymphoblasts may be found in the bone marrow, peripheral blood, and other extramedullary sites. ALL is predominantly a childhood cancer, with two-thirds of new cases diagnosed in children younger than 15 years of age. ALL was uniformly fatal until the 1960 but, due to advances in chemotherapy and supportive care, is now cured in over 80% of children. Adults diagnosed with ALL, in contrast, have a poor overall prognosis. Important factors in assessing prognosis are the age of the patient, type of lymphoid cell involved (T cell vs B cell), and the presence of high-risk cytogenetic markers, such as the t(9;22) (BCR–ABL) translocation. Burkitt's lymphoma, a malignancy of mature B cells, has been historically classified as a B-ALL due to its high-grade leukemia-like features but is both diagnostically and prognostically a separate entity from precursor B-ALL. Burkitt's lymphoma is addressed in the chapter on non-Hodgkin lymphomas.

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  • ALL is the most common malignancy of childhood.

  • Childhood ALL has a much better prognosis than adult ALL.

  • Highly aggressive lymphoid malignancy with B-cell (80%) and T-cell (20%) immunophenotypes.

  • Certain cytogenetic alterations, such as t(9;22), are associated with inferior prognosis while others, such as t(12;21), are associated with favorable outcomes.

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EPIDEMIOLOGY AND ETIOLOGY

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Leukemia comprises 32% of malignancies in children younger than 15 years. Of these, the majority are ALL. Each year approximately 2400 children in the United States are diagnosed with ALL. The peak incidence in children is between ages 2 and 5. Leukemia rates are significantly higher in Caucasian children, with a nearly threefold higher incidence over African-American children. ALL is almost 30% more common in males than females. Overall, the incidence of childhood ALL has increased in the past 20 years at a rate of 0.9% per year. Adult ALL is less common, with approximately 1000 new cases diagnosed per year. The incidence of ALL decreases from age 15 until 50; then a second, minor increase in new cases appears. A third peak appears at age 80. The lifetime risk of developing ALL is 0.13%, or approximately 1 in 789 men and women (1).

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Several reports have suggested that inadvertent exposure to radiation in utero and postnatal radiation treatment for such conditions as tinea capitis and thymic enlargement increase the risk of ALL (2). A common cytogenetic translocation involving ETV-6 was retrospectively detected in neonatal blood spots of children who were diagnosed with ALL between ages 2 and 5, suggesting that ALL can be initiated by somatic translocation in utero but requires additional molecular events to fully develop (3). Limited and/or inconsistent evidence links ALL to parental smoking, infection, diet, electromagnetic fields, hydrocarbons and, possibly, radiation delivered during the course of diagnostic studies such as CT scans (4).

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