- Minimal residual disease (MRD) is highly prognostic in pediatric B-precursor acute lymphoblastic leukemia (ALL). In COG High Risk B-ALL study AALL0232 we investigated MRD in subjects randomized in a 2X2 factorial design to receive either High-Dose (HD-MTX) or Capizzi Methotrexate (C-MTX) during interim maintenance (IM), or Prednisone or Dexamethasone during induction. Subjects with end induction MRD>=0.1% or those with morphologic slow early response were non-randomly assigned to receive a second IM and delayed intensification phase. MRD was measured by 6-color flow cytometry in one of two reference labs, with excellent agreement between the two. Subjects with end induction MRD<.01% had a 5y EFS of 87±1% vs 74±4% for those with MRD 0.01%-0.1%; increasing MRD amounts was associated with progressively worse outcome. Subjects converting from MRD positive to negative by end consolidation had a relatively favorable 79±5% 5 y DFS vs 39±7% for those with MRD>=0.01%. Although HD-MTX was superior to C-MTX, MRD retained prognostic significance in both groups (86±2% vs 58±4% for MRD negative vs positive C-MTX subjects; 88±2% vs 68±4% for HD-MTX subjects). Intensified therapy given to subjects with MRD>0.1% did not improve either 5y EFS or OS. However, these subjects showed an early relapse rate similar to that seen in MRD negative ones, with EFS/OS curves for patients with 0.1%-1% MRD crossing those with 0.01%-0.1% MRD at 3 and 4 years, thus suggesting that the intensified therapy altered the disease course of MRD positive subjects. Additional interventions targeted at the MRD positive group may further improve outcome. NCT00075725 at www.clinicaltrials.gov.Copyright © 2015 American Society of Hematology.