Minimal Residual Disease (MRD) in Childhood Acute Lymphoblastic Leukemia (ALL). Correlation with Prognostic Factors and Outcome
Authors: Paisiou Anna , Paterakis Georgios , Tsagarakis Nikolaos , Kentrou Nektaria , Papadakis Vassilios , Papadhimitriou Stefanos I , Parcharidou Agapi , Basdekis Vasillios , Papadimitriou Konstantinos , Skoumi Dimitra , Tsitsikas Konstantinos , Polychronopoulou Sophia 
Published: April 2020

Abstract
Aim: The aim of this study was the prospective evaluation of MRD during childhood ALL therapy and its correlation with specific prognostic criteria of ALL-BFM 95 protocol and with patient outcome.
Patients/Methods: 127 children (49 girls) with ALL were studied during the period 1999–2008. The median age at diagnosis was 9,32 years (range, 0,6–16,48). All patients were diagnosed in the same center and treated uniformly with the ALL-BFM 95 protocol, modified in two therapeutic branches, medium and high risk, as we have published previously. We used three or five colours’ flow cytometric panels for MRD quantification at sequential standardized treatment time-points:
at day 15 of induction (T1), at day 33 (T2) of induction, before consolidation (T3), before re-induction (T4), before maintenance (T5), at maintenance completion (T6).
Additionally for the high risk patients, 6 more determinations before each consolidation treatment cycle were performed. The median follow-up time was 48,4 months (range, 1,7–110,3). For statistical analysis, descriptive statistics and Kaplan-Meier were used.
Results: Immunophenotypical analysis resulted in 119 patients with ALL of B-origin and 8 of T-origin. Median WBC at diagnosis was 10×109/lt, while extra-BM infiltration was found in 9 children. According to ALL-BFM 95 protocol’s criteria: 40 patients were fulfilling the criteria of the standard risk (SR), 61 of medium (MR) and 26 of high risk (HR), respectively, and therapeutically were divided into two groups: A (101 patients, SR+MR) and B (26 patients, HR). MRD was detected in: 59/123 patients at treatment time-point (T1) (39/59 from group A, of which 26/39 with high MRD levels, and 20/59 from group B, all with high MRD levels). In time-point (T2), disease was detected in 19/124: 5/19 from group A (3/5 high MRD levels), 14/19 from group B (11/14 high MRD levels). At treatment-point (T3), 3/127 had detectable disease (all from group B). None of the patients of group A had minimal residual disease at the following time-points, while only 2 patients of group B had persistent presence of MRD. In total, 14/127 children relapsed (4/SR, 2/MR, 8/HR), with significant levels of MRD in 7 (6/7 HR) and 4 (all HR) patients, at time-point (T1) and (T2), respectively. Among all, 114 children survived (CR1: 110, CR2: 4), while 13 children died (9/disease, 4/therapy-related toxicity).
Conclusions: Our results suggest that MRD detection in continuous standardized treatment time-points of childhood ALL correlates with shorter disease free (DFS) and overall survival (OS), however in our cohort there was no sufficient evidence of MRD independency as prognostic factor (cox-regression analysis) compared to the classical prognostic criteria of the ALL-BFM 95. The enlargement of the group of patients and the expansion of the follow-up period will lead to more reliable conclusions.