Circulating Tumor DNA and microRNA-Based Liquid Biopsy for Longitudinal Treatment Monitoring and Recurrence...

Background: Pediatric sarcomas are aggressive malignancies characterized by marked biological heterogeneity and a high risk of relapse. Standard surveillance relies on imaging and invasive biopsies, which may fail to detect early molecular disease. Liquid biopsy using circulating tumor DNA (ctDNA) and microRNAs offers a minimally invasive strategy for longitudinal monitoring. This study aimed to evaluate dynamic changes in ctDNA and circulating microRNAs during treatment and examined their associations with treatment response, disease recurrence, and survival outcomes. Methods: This prospective cohort study included 100 pediatric patients with histologically confirmed sarcomas. Serial peripheral blood samples were collected at diagnosis, during treatment, at treatment completion, and during follow-up. Plasma-derived ctDNA and miRNAs were quantified using droplet digital Polymerase Chain Reaction (PCR) and quantitative Reverse Transcriptase-PCR (qRT-PCR). Associations between biomarker levels, clinical and radiologic response, recurrence, and survival outcomes were analysed using correlation analyses. Results: The mean age at diagnosis was 10.5 ± 4.9 years, with a male predominance (59.0%). The most common tumor subtype was Ewing sarcoma (49.0%), followed by rhabdomyosarcoma (27.0%) and osteosarcoma (24.0%). Both ctDNA and circulating miRNA concentration declined significantly from baseline to the end of treatment (p < 0.001 for both). End-of-treatment biomarker levels showed weak and insignificant correlations with clinical response, radiologic response, and survival outcomes (all p > 0.05). In multivariable analysis, disease recurrence status was strongly associated with survival, as expected; however, circulating biomarker levels did not independently predict outcome (Odds ratio (OR) = 0.04, 95% Confidence Interval (CI): 0.01-0.15; p < 0.001). Conclusions: ctDNA and microRNA levels showed dynamic, treatment-related changes in pediatric sarcomas, reflecting therapeutic response at the population level. However, single end-of-treatment measurements were not predictive of outcomes. These findings support liquid biopsy for longitudinal monitoring rather than static prognostication and emphasize the need for prospective validation of serial biomarker approaches.