The population-based Nordic/Baltic acute lymphoblastic leukaemia (ALL) Nordic Society for Paediatric Haematology and Oncology (NOPHO) ALL2008 protocol combined minimal residual disease (MRD)-driven treatment stratification with very intense first line chemotherapy for patients with high risk ALL. Patients with MRD ≥5% at end of induction or ≥10(-3) at end of consolidation or following two high risk blocks were eligible for haematopoietic
cell transplantation (HCT) in first remission. After at least three high risk blocks a total of 71 children received HCT, of which 46 had MRD ≥5% at end of induction. Ten patients stratified to HCT were not transplanted; 12 received HCT without protocol indication. Among 69 patients with evaluable pre-HCT MRD results, 22 were MRD-positive, one with MRD ≥10(-3) . After a median follow-up of 5.5 years, the cumulative incidence of relapse was 23.5% (95% confidence interval [CI]: 10.5-47.7) for MRD-positive versus 5.1% (95% CI: 1.3-19.2), P = 0.02) for MRD-negative patients. MRD was the only variable significantly associated with relapse (hazard ratio 9.1, 95% CI: 1.6-51.0, P = 0.012). Non-relapse mortality did not differ between the two groups, resulting in disease-free survival of 85.6% (95% CI: 75.4-97.2) and 67.4% (95% CI: 50.2-90.5), respectively. In conclusion, NOPHO block treatment efficiently reduced residual leukaemia which, combined with modern transplant procedures, provided high survival rates, also among pre-HCT MRD-positive patients.