Circulating Cell-free DNA as a biomarker for radiation-induced injury: From mechanistic insights to clinical translation.

Circulating cell-free DNA (cfDNA) has attracted increasing attention as a minimally invasive biomarker for assessing radiation-induced tissue injury. Radiation exposure can alter cfDNA concentration, fragmentation profiles, and methylation patterns. These changes may reflect cellular damage, tissue origin, and temporal responses to irradiation. During radiotherapy, however, cfDNA can arise from tumor cells, irradiated normal tissues, endothelial cells, immune cells, and hematopoietic cells. This mixed origin complicates the interpretation of radiation-related signals. Current evidence is derived mainly from preclinical studies and small, heterogeneous clinical cohorts. Sampling schedules, analytical workflows, and clinical thresholds also vary across studies. Recent advances in fragmentomics, tissue-specific methylation analysis, mitochondrial cfDNA, multi-omics integration, and artificial intelligence have expanded the information obtained from cfDNA. In particular, tissue-specific methylation and fragmentation features may help identify the cellular or organ origin of injury. Nevertheless, prospective multicenter validation and methodological standardization are required before routine clinical use. This review summarizes the mechanisms linking radiation injury to cfDNA release, current detection technologies, and evidence for organ-specific applications. It also discusses clinical confounders, translational barriers, and future directions for integrating cfDNA into precision radiation oncology.