Circulating Exosomal miRNA as a Novel Predictive Marker of Radiation Toxicity in Breast Cancer Patients

Radiotherapy is an essential component of breast cancer management; however, interpatient variability in radiation-induced normal tissue toxicity remains a significant clinical challenge. The identification of minimally invasive biomarkers capable of predicting susceptibility to radiotoxicity could facilitate personalized treatment strategies. Exosomal microRNAs (miRNAs) are stable in circulation and play key roles in cellular stress and inflammatory responses, making them attractive biomarker candidates. In this study, plasma-derived exosomal miRNA profiles were analyzed in breast cancer patients receiving adjuvant radiotherapy and stratified according to the severity and timing of normal tissue toxicity. Exosomes were isolated using a combined size-exclusion chromatography and precipitation approach and characterized by nanoparticle tracking analysis and protein marker expression. Differential miRNA expression was assessed using quantitative PCR following normalization with stably expressed endogenous controls. Two exosomal miRNAs were associated with increased susceptibility to radiotherapy-induced toxicity. MiR-222-3p was significantly upregulated in patients who developed acute high-grade toxicity, while miR-144-5p was increased in both acute and late high-toxicity groups. Additionally, miR-200a-3p and miR-335-3p were enriched in patients exhibiting minimal or no toxicity. These findings identify exosomal miRNAs with potential utility as biomarkers of radiation-induced normal tissue toxicity and support further investigation into their mechanistic roles and clinical applicability in personalized radiotherapy.