Background: Currently, pancreatic cancer (PanC) is one of the serious gastrointestinal diseases, and more than 90% of the patients die within five years of diagnosis. Therefore, to improve PanC-related mortality, new therapeutic and diagnostic/prognostic measures are urgently needed. In this substudy, we examined the usefulness of plasma small extracellular vesicles (sEV) to discover molecular biomarkers associated with treatment response and overall survival from the plasma samples collected on a prospective clinical trial assessing the efficacy of SBRT following chemotherapy in pancreatic cancers (NCT03600623). Methods: PanCpatients (n=22) with locally advanced and borderline inoperable disease were recruited at the University of Alabama Comprehensive Cancer Center. They were administered either FOLFIRINOX (5-FU, folinic acid, oxaliplatin, and irinotecan) or paclitaxel (gemcitabine-nab-paclitaxel) for 2 months followed by SBRT (33 Gray in 5 fractions). The primary objective of this single-center pilot study was to evaluate the safety and tolerability of this treatment regimen. Further, blood was collected at baseline and, at the end of chemotherapy and radiotherapy. sEV were isolated from archived plasma samples by an established ultracentrifugation method and characterized for size and concentration by nanoparticle tracking analyses (NTA), shape and size by transmission electron microscope (TEM), and surface expression of exosomal tetraspanin markers (CD63, CD9, and CD81) and a PanC marker (CA19-9) by nano-flow cytometry. Lastly, sEV in longitudinal plasma samples were characterized for the expression of specific PanC-related miRNAs (miR196a-5p, miR155-5, miR194-5p, miR301-3p, miR21-5p, miR1246, and miR34a-5p) by real time-PCR (RT-PCR). Results: Neoadjuvant FOLFIRINOX and gemcitabine-nab-paclitaxel followed by SBRT were safe and well tolerated by most patients. NTA data showed that the ultracentrifugation method yielded highly pure sEV (with average diameter of <200 nm) from archived baseline and longitudinal plasma samples. TEM analysis further confirmed the shape and size of the isolated sEV. Nano-flow cytometry showed the expression of exosomal markers, CD63, CD9, and CD81, as well as PanC marker CA19-9 on the surfaces of sEV. The expression of various PanC-related miRNAs in sEV was heterogenous and correlated with corresponding clinical parameters, including treatment response and overall survival. Conclusions: Neoadjuvant chemotherapy in combination with SBRT is safe and tolerable regimen to treat patients with locally advanced and borderline PanC. Further, sEV in the plasma of PanC patients could serve as useful prognostic and predictive markers.