Conclusions: Our method is a valuable tool that enables us to characterize exosomes and microparticles, and, unlike other available nanoparticle analysis methods, this method permits subset identification and sorting for functional studies of single particles. At 3 months after SABR in this clinical cohort, we observed an increase in the plasma microparticle population. Whether this increase is due to tumor responses to radiation, tissue responses to radiation, tumor progression, or tissue repair and immune reconstitution following chemotherapy requires further study. Future studies will use this method to identify and sort subsets of treatmentassociated exosomes and microparticles, with the goal of identifying new and minimally invasive biomarkers to monitor tumor and normal tissue responses to radiation therapy, chemotherapy, and/or immunotherapy. Author Disclosure: J.C. Jones: None. H. Cao: None. M. Limaye: None. A. Koong: None. S. Knox: None.