Researchers tracked endothelial injury markers in 48 children with leukemia and lymphoma receiving anthracycline chemotherapy, measuring circulating endothelial cells (CECs) and endothelial progenitor cells (EPCs) across four timepoints over one year. While acute endothelial damage (elevated CECs) resolved after treatment completion, the capacity for vascular repair remained severely compromised. EPCs dropped below healthy control levels during remission and became predominantly senescent, persisting up to one year post-treatment. This finding reframes our understanding of anthracycline cardiotoxicity in pediatric patients. Rather than ongoing tissue damage driving long-term cardiovascular risk, the culprit appears to be defective regenerative capacity. The endothelial progenitor pool becomes corrupted with senescent cells that cannot effectively repair vascular damage. This mechanism could explain why childhood cancer survivors face elevated cardiovascular disease risk decades later, even when acute cardiac markers normalize. The research suggests therapeutic strategies should focus on restoring EPC function rather than simply preventing initial damage. However, this preprint awaits peer review, and the retrospective single-center design limits broader applicability. The findings represent an important mechanistic insight that could reshape cardio-oncology approaches for pediatric patients.