To evaluate the outcome of hematopoietic stem cell transplantation using targeted busulfan, fludarabine, etoposide conditioning regimen in childhood and adolescent ALL.

Busulfan is a highly toxic drug with narrow therapeutic window used for the conditioning of hematopoietic stem cell transplantation. High exposure is associated with systemic toxicity such as veno-occlusive disease (VOD) and underexposure is associated with graft failure or relapse. In this study the investigators plan to improve the outcome of hematopoietic stem cell transplantation by using optimal busulfan dose through pharmacokinetic study.

Inclusion Criteria:

1. Patients who are diagnosed as ALL.
2. Patients who need hematopoietic stem cell transplantation
3. Age: up to 21 years
4. Performance status: ECOG 0-2.
5. Patients must be free of significant functional deficits in major organs, but the following eligibility criteria may be modified in individual cases. 1. Heart: a shortening fraction > 30% and ejection fraction > 45%. 2. Liver: total bilirubin < 2 × upper limit of normal; ALT < 3 × upper limit of normal. 3. Kidney: creatinine <2 × normal or a creatinine clearance (GFR) > 60 ml/min/1.73m2.
6. Patients must lack any active viral infections or active fungal infection.
7. Appropriate donor is available: Matched in 6/6 of A, B, DR loci.
8. Patients (or one of parents if patients age < 19) should sign informed consent.

Exclusion Criteria:

1. Pregnant or nursing women.
2. Malignant or nonmalignant illness that is uncontrolled or whose control may be jeopardized by complications of study therapy.
3. Psychiatric disorder that would preclude compliance.