e21068 Background: Surgical resection is recommended for the stage I NSCLC patients. However it is lack of standard therapies for synchronous mGGNs. This study aims to explore the feasibility of a PD-1 inhibitor (sintilimab) in high-risk residual mGGNs in post-surgery synchronous stage I NSCLC patients. Methods: This is a single arm phase I trial intending to recruit 20 subjects. Eligible patients contained at least one high-risk residual mGGN (confirmed by MDT discussion) after surgical resection of stage I NSCLC. Patients received sintilimab (200mg i.v., Q3W) for 10 cycles since 6 weeks post-surgery. Radiological evaluations were performed at every 3 cycles in treatment period and every 3 months during follow-up. Whether re-operation is needed depends on MDT evaluation. The primary endpoints were safety and ORR. The secondary endpoints included DOR, MPR of re-operated GGNs, and the one-year PFS. Results: As data cutoff (23 Jan 2020), 20 patients (9 males and 11 females) were enrolled. The pathological results revealed that all were lung adenocarcinoma. The median treatment cycles was 6 (range 3 to 10). All lesions were evaluated as stable disease by RECIST 1.1. Six patients underwent re-operation safely and12 GGNs were resected. Pathology showed that 1 had no tumor cells residue, 4 had 20-35%, and 7 had over 80% residues. Whole-exome sequencing of 12 GGNs showed 6 harboring EGFR and 2 harboring KRAS mutations. Interestingly, 3 out of 6 with EGFR-mut showed less tumor cells residues (23.3%, 30%, and 80%). The highest TMB (15.7 mut/Mb) was observed in the complete response GGN. 65% patients experienced treatment-related 1-2 grade AEs. Conclusions: The PD-1 inhibitor showed well tolerability and a certain degree anti-tumor activity was seen from the pathologic results of re-operated GGNs. Radiological evaluation using RECIST 1.1 might be unsuitable for mGGNs. Further studies are needed to demonstrate the clinical benefits. ChiCTR: 1900022159 Clinical trial information: 1900022159 .