Determine minimal residual disease in children with acute lymphoblastic leukemia in treatment at the Children's Hospital of Brasilia according to the BFM 2009 adapted protocol; Evaluate risk stratification and molecular alterations as prognostic factors.

Patients with ALL in treatment at the Hospital with the BFM 2009-adapted protocol from June 2018 to December 2022 were included in this study. Exclusion criteria were: patients with less than 1-year old; patients with Down Syndrome; history of chemotherapy for other cancers; and relapse at admission. Samples were collected at the Hospital's facilities and processed at the Laboratory of Translational Research. Immunophenotyping analysis were made using Infinicyt software and statistical analysis were made in SPSS software. Approval by the Ethical Committee: 85960218.5.0000.5553.

Of all 117 patients included, 51 were female and 66 were male, and 110 (94%) were diagnosed with B ALL, while only 7 (6%) with T-ALL. Risk stratification were made according to the following criteria: patients with age between 1 and 6 years old, initial leucometry < 20.000 mm3, MRD < 0.1% at D15 and < 0.01% at D33 and D78 and no risk factors were classified as Low Risk (LR); patients with MRD D15 > 5%, D78 > 0.01% or genetic alterations hypodiploidy, t(9;22) or t(4;11) were classified as High Risk (HR); patients who did not meet the criteria above were classified as Intermediate Risk. Twenty (17.%) patients were stratified as LR, 77 (65.8%) as IR and 20 (17.1%) as HR. Molecular biology tests showed that the majority (53%) of the patients did not have alterations; the most common alteration found was the t(12;21) translocation (23%), followed by t(1;19) (5%), t(9;22) (3%), del(1p32) (2%), alteration on gene IKZF1 (2%) and t(4;11) (1%); 6 patients (5%) had their test pending.

Of all 116 samples analysed at D15, 95 (81.9%) were positive and 21 (18.1%) were negative; IR and HR patients represented approximately 73.2% of all positive samples. At D33, number of negative MRDs was greater, representing 76.9% of all 108 samples processed. At D78, 90.6% of the 85 samples were negative. These results reflect the success of the treatment protocol; the decrease of the number of samples collected, however, is due to deaths and samples pending. Overall survival was 83.8%, a result close to that found in literature and excellent to a public health system hospital. As expected, high risk patients had a significantly lower survival when compared to the other risk groups or molecular alterations. Patients with bad prognostic molecular alterations (IKZ mutations, t(4;11) and t(9;22)) had the lowest overall survival , while patients with good prognostic alterations (t(1;19) and t(12;21)) had the greatest. Even though molecular biology was negative for more than half of the patients, OS was lower than 90%, which indicates the need for more sensitive techniques, like Next-Generation Sequencing (NGS).

MRD analysis, although added to the ALL treatment protocol at the Children's Hospital of Brasilia only in 2018, was confirmed to be an effective and sensitive approach to disease monitoring. Furthermore, risk stratification as well as molecular alterations showed to have a high prognostic value for those patients.