Recent improvements in the treatment results of patients with childhood leukemia have resulted from a careful assessment of the failure risk based on many variables linking to the prognosis, such as initial leukocyte count, age at diagnosis, and cytogenetic abnormalities (risk factors), and also from the development of risk-directed therapy. In acute lymphoblastic leukemia (ALL), however, the stratification of patients by conventional risk factors is inadequate ; relapses occur in about 15% of low-risk cases, and some cases with t (9 ; 22), which is extreme high-risk characteristics, can be cured with chemotherapy alone. Recent reports suggest that poor early response to therapy and the presence of minimal residual disease (MRD) during the post-remission-induction period are reliable indicators of a poor prognosis. Patients with>1, 000 leukemic blasts/μl on day 8 of induction show a significantly poor outcome. The MRD at a level of 10
-2 or more at the end of induction is a strong adverse risk factor of relapse. These factors can greatly improve the accuracy of risk assessment and thus contribute to the decision making of optimal treatment intensity. In acute myelogenous leukemia (AML), the cytogenetic features have been proved to be the most important prognostic factor. Patients with t (8 ; 21) or inv (16) are cured with chemotherapy alone, which can reduce long-term complications associated with highly intensive therapy including stem cell transplantation. Further studies on leukemic cells of individual patients, such as drug sensitivity, genetic polymorphism and pharmacokinetics, and distinct gene expression detected by DNA microarray, will lead to more accurate patient stratification, which prevents both over-and undertreatment.
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