In our series, more than half of the patients achieved long term OS with AHSCT. Less toxic conditioning regimens or more intensive GVHD prophylaxis could lead to better results in some children.

Introduction: Despite the advances observed in the outcome of pediatric acute lymphoblastic leukemia (ALL) treatment during the last 20 years, relapse remains the most common cause of treatment failure in childhood ALL. Several factors have been associated to the prognosis of these patients; however, minimal residual disease (MRD) emerges as a relevant predictor of outcome. Objectives: The aims of this study were to assess MRD by flow-cytometry in relapsed ALL and to evaluate its prognostic impact as a predictor factor of outcome at the end of the induction therapy and prior to hematopoietic stem cell transplantation (HSCT). Patients and Methods: From Aug'10 to Jun'15, 123 ALL patients were treated at our center. MRD determination at least at two time-points during relapse treatment was a requirement for considering a patient eligible for the present study. Sixty-six cases were excluded due to the following causes: 10 patients died during induction, 2 died early in complete remission (CR), 29 did not respond to chemotherapy, in 13 patients MRD determination was not performed: 4 did not have clinical data available, 4 patients were Down Syndrome and 4 children received treatment for relapse in other centers. Thus, fifty-seven patients achieved CR and were evaluated for MRD at two time points. Of them, 56 patients belonged to S4 and S3 and 1 patient to S1 group as defined by the Berlin-Frankfurt-Münster stratification for relapsed ALL. MRD was analyzed by multiparametric flow-cytometry following ALL-IC 2009 guidelines. Negative MRD was defined as disclosing less than 0.1% of blasts. For this analysis, patients were stratified based on MRD levels at two different time points: after end of induction, before HSCT or at any other time point during the follow-up for patients who did not undergo HSCT. Three groups were defined: Group-1: negative at both time points (n= 23), Group-2: positive at 1 time point (n= 13) and Group-3: positive at both time points (n= 21). Patients who relapsed before receiving HSCT were considered Group-3. Twenty-five patients underwent HSCT: 13 of them from Group-1, 9 from Group-2 (2 had positive MRD previous to receive HSCT) and 3 patients from Group-3. HSCT was performed with matched familiar donor in 16 cases and matched unrelated donor in 9 cases. Results: The distribution of events according to receiving or not HSCT was: 5 died due to transplant related mortality (TRM), 9 relapsed after receiving HSCT and 16 during treatment with chemotherapy. With a median follow-up of 16 (range: 6-67) months, overall 3-year EFS probability (EFSp) (SE) was 32 (8)%. The 3-year EFSp was 75 (11)% for Group-1, 24 (14)% for Group-2 and 0% for Group-3 (p-value <0.00001). Comparing patients who did not receive HSCT vs. patients who did, EFSp (SE) was 32 (12)% and 29 (11)% respectively (p-value: non-significant). The EFSp (SE) according to MRD groups in patients who underwent HSCT was: Group-1: 53 (19)%, Group-2: 14 (13)% and 0% for Group-3 (p-value: 0.06). Conclusions: MRD quantification by flow-cytometry demonstrated to be a significant prognostic factor for relapsed ALL. Both, TRM and death in CR rates, were high and should be decreased by improving supportive measures. MRD determination by flow-cytometry in patients who underwent HSCT showed a trend to achieve a better EFSp, thus representing a relevant tool for stratifying relapsed ALL patients in order to achieve a better selection of patients to receive HSCT. Disclosures No relevant conflicts of interest to declare.