This retrospective-prospective observational cohort study aims to evaluate peripheral blood plasma circulating tumor DNA (ctDNA) dynamics in pediatric acute leukemia and compare ctDNA results with concurrent bone marrow multiparameter flow cytometry minimal residual disease (MFC-MRD), droplet digital PCR (ddPCR), and RNA sequencing findings. The study includes a retrospective cohort with available clinical and molecular data and a prospective cohort with serially collected peripheral blood and bone marrow samples at predefined treatment time points. The study will assess consistence between plasma ctDNA and conventional bone marrow-based assays, characterize longitudinal ctDNA dynamics, and explore the association between ctDNA patterns and relapse or survival outcomes.