Impact of minimal residual disease on the outcome of hematopoietic stem cell transplantation for childhood acute lymphoblastic leukemia within the FORUM Trial

Abstract

In the randomized cohort of the international phase-III FORUM trial, which showed the superiority of total-body irradiation (TBI) over chemotherapy-based conditioning prior to hematopoietic stem cell transplantation (HSCT) for pediatric acute lymphoblastic leukemia (ALL), type of conditioning and remission phase, but not pre-HSCT minimal residual disease (MRD), were associated with outcome. We report the impact of MRD within the extended FORUM cohort. Patients (n=1014), 4-21 years old, transplanted from a matched donor who had ≥1 MRD measurement prior to and/or 100 days and/or 1 year after HSCT were eligible. A threshold of 0.01% defined MRD positivity versus negativity. Prior to HSCT, 21% of patients were MRDpos. Three-year event-free survival (EFS) was 0.73 and 0.59 (p<0.001), and 3-year cumulative incidence of relapse (CIR) was 0.20 and 0.33 (p<0.001) in MRDneg and MRDpos patients, respectively. The level of MRD positivity pre-HSCT (<0.1% versus ≥0.1%), did not significantly affect outcome. Pre-HSCT MRDneg and TBI/etoposide conditioning were associated with a 2-fold lower risk of relapse, whereas MRDpos had a 2-fold higher risk of any failure and/or death. No detrimental effect of MRDpos pre-HSCT could be demonstrated in patients with T-cell ALL. MRDpos versus MRDneg patients at day 100 had an EFS of 0.47 versus 0.77 (p<0.001) and a CIR of 0.51 versus 0.17 (p<0.001), respectively, but post-HSCT MRDpos did not necessarily imply relapse. In conclusion, the MRD status pre-HSCT and at day 100 post-HSCT was a strong prognostic factor for children transplanted for ALL in the extended FORUM cohort.