PURPOSE: Minimal residual disease (MRD) before allogeneic stem-cell transplantation was shown to predict outcome in children with relapsed acute lymphoblastic leukemia (ALL) in retrospective analysis. To verify this, the Acute Lymphoblastic Leukemia Relapse Berlin-Frankfurt-Münster (ALL-REZ BFM) Study Group conducted a prospective trial. PATIENTS AND METHODS: Between March 1999 and July 2005, 91 children with relapsed ALL treated according to the ALL-REZ BFM 96 or 2002 protocols and receiving stem-cell transplantation in >or= second remission were enrolled. MRD quantification was performed by real-time polymerase chain reaction using T-cell receptor and immunoglobulin gene rearrangements. RESULTS: Probability of event-free survival (pEFS) and cumulative incidence of relapse (CIR) in 45 patients with MRD >or= 10(-4) leukemic cells was 0.27 and 0.57 compared with 0.60 and 0.13 in 46 patients with MRD less than 10(-4) leukemic cells (EFS, P = .004; CIR, P <.001). Intermediate-risk patients (strategic group S1) with MRD >or= 10(-4) leukemic cells (n = 14) had a pEFS of 0.20 and CIR of 0.73, whereas patients with MRD less than 10(-4) leukemic cells (n = 21) had a pEFS of 0.68 and CIR of 0.09 (EFS, P = .020; CIR, P <.001). High-risk patients (S3/4, third complete remission) who received transplantation with an MRD load of less than 10(-4) leukemic cells (n = 25) showed a pEFS and CRI of 0.53 and 0.18, respectively. In contrast, pEFS and CRI were 0.30 and 0.50 in patients who received transplantation with an MRD load of >or= 10(-4) leukemic cells. Multivariate Cox regression analysis revealed MRD as the only independent parameter predictive for EFS (P = .006). CONCLUSION: MRD is an important predictor for post-transplantation outcome. As a result, new strategies with modified stem-cell transplantation procedures will be evaluated in ALL-BFM trials.
