Background: Molecular profiling is limited by access to sufficient tumor tissue for comprehensive analysis and due to tumor heterogeneity, the complete range of tumor DNA abnormalities may not be represented nor accurately reflect the clinical evolution of disease. Circulating tumor DNA (ctDNA) can be used as a minimally-invasive liquid biopsy for the detection, quantification and monitoring of molecular abnormalities for personalized treatment strategies. Methods: In a prospectively designed program, to date, we have recruited 227 advanced NSCLC patients having received prior therapy, with unknown molecular profile at time of blood collection. Blood collections (10ml K2-EDTA) were performed to assess molecular profile prior to or at time of relapse. Repeat samples were performed on patients initiated on treatment and followed for up to 18months. Patient samples were analyzed with InVision (enhanced tagged-amplicon sequencing) using a 34 gene panel. Interim analysis performed with full descriptive summary statistical analyses to be presented at conference. Results: ctDNA profiling detected somatic mutations in 182pts (80.2%), predominantly located in TP53 (46%), EGFR (28%), KRAS (11%) and STK11 (7%, half of which had concurrent KRAS). Of note, clinically actionable mutations were detected: T790M (25pts, median 1.4% AF), ERBB2 (8 pts), MET (8pts) and BRAF (4pts) providing eligibility for new therapy options. 20pts including 12 EGFR/T790M+ve were evaluated for ctDNA monitoring up to 18 months (median 10m); correlation between dynamic change in mutation allele fraction and clinical response was observed, especially predictive of relapse to treatment. 10 patients demonstrated SD/PR response to osimertinib treatment with T790M detection at low allele fraction (7/10 < 1% AF with 1pt at 0.08% AF). Conclusions: ctDNA can be used as a non-invasive ‘liquid biopsy’ for molecular profiling of NSCLC patients to detect clinically relevant and actionable mutations when tissue biopsy is unavailable. Liquid biopsies can be repeated as needed where tissue is not feasible, providing real-time information to support personalised treatment.