8540 Background: The standard of care for patients with inoperable early stage or locally advanced NSCLC is definitive stereotactic body radiotherapy (SBRT) or conventional radiation therapy (RT) with systemic therapy. Circulating tumor DNA (ctDNA) testing can be used for the assessment of MRD and predict risk of recurrence. Few studies have prospectively evaluated MRD detection and ctDNA dynamics specifically among patients with early or locally advanced NSCLC receiving definitive RT. Methods: In a prospective clinical cohort of patients with stage I-III NSCLC (n = 17), serial plasma samples (n = 70) were collected before and after SBRT as well as before, during, and after conventional RT with or without concurrent systemic therapy and adjuvant durvalumab. Patients were followed-up for a median of 29 months (range: 4 to 54 months) with the last serial plasma collected at a median of 5 months from completion of RT (range: 1 – 26 months). A personalized, tumor-informed multiplex PCR assay (Signatera™ bespoke mPCR NGS assay) was used for the detection and quantification of ctDNA and tracked 16 tumor variants among 16 patients and 15 tumor variants in one patient. This study evaluated the prognostic value of ctDNA, correlating MRD status with clinical outcomes, in addition to ctDNA clearance kinetics during RT. Results: Among 17 patients with early-stage and locally advanced NSCLC, baseline ctDNA was detected in 82% of patients (14/17). Clinical progression was confirmed radiographically for 53% (9/17). All events of clinical progression were detectable by ctDNA (sensitivity 100%, 0.63 – 1.0), with a median lead-time of 5.5 months for MRD detection compared to radiographic disease progression. Durable ctDNA clearance was observed in 29% (5/17) of patients, all of whom then remained recurrence-free until the end of follow-up (median 12 months; specificity 100%, 95% CI 0.6 – 1.0). Transient ctDNA clearance was observed in 3 patients, and recurrent ctDNA was detected before or at the time of disease progression in all 3. ctDNA status after treatment at a single time point and longitudinally were highly predictive of disease recurrence (p < 0.0001). Conclusions: ctDNA detection is feasible for patients with stage I-III NSCLC undergoing definitive chemoradiation. and can serve as a powerful predictive biomarker for disease recurrence. High baseline detection rate is essential for feasibility of a ctDNA-based MRD assay. Residual detectable ctDNA represents a powerful predictive tool to identify patients who might benefit from intensification of adjuvant therapy following definitive RT.