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Background Precision-medicine initiatives are driven by the molecular analysis of tumor samples (fresh or FFPE material). Such an approach is limited by the availability of the tumor material and the challenges related to on-purpose tumor biopsies. A very appealing alternative to advance precision-medicine initiatives is the development of liquid biopsies using cfDNA. Methods We investigated the use of NGS on tumor biopsy and plasma, and evaluated the consistency between the tissue biopsy (tDNA) and cfDNA analysis on a prospective cohort of patients with metastatic or locally advanced solid tumors enrolled in the MOSCATO 01 trial (NCT01566019). Blood samples were collected at inclusion before tumor biopsy and cfDNA was extracted from 500μl plasma. Hot-spot mutations from 50 genes were screened with Ampliseq CHP2 panel (IonTorrent, Life Technologies, Dramstadt). Only variants reported by the Torrent Suite Variant Caller v4.2 were retained for the analysis. Paired results in tumor and plasma were described using Cohen's Kappa agreement coefficient (κ). Results From November 2011 to May 2014, among the 516 patients enrolled in the MOSCATO 01 trial, 190 patients (37%) were analyzed for tDNA and cfDNA. In addition, cfDNA was evaluated in 43 patients for whom no tumor analysis was performed because of low cellularity (< 10% of tumor cells). Patient characteristics were as follows: median age at biopsy: 57 years (range, 18-78); main tumor types: lung (19%), ENT (14%), colorectal (10%), breast (10%); median of 3 previous lines of treatment. Overall, 325 mutations were identified in the tDNA of 184 patients: 146 mutations were identified both in tumor and plasma and 179 in tumor, but not in plasma. 15 mutations were only found in cfDNA, thereby providing additional information. The κ.was 59% (95%CI, 0.54-0.64). The sensibility of using NGS for 50 targeted hot-spot genes analysis in cfDNA compared to tDNA was 44.9% and the specificity was 99.8%, with a positive predictive value of 90.7% and negative predictive value of 98.1%. The ten most frequent pathogenic mutations found in the tDNA or cfDNA (>5 cases each) included KRAS (33 cases), PIK3CA (22 cases) and TP53 genes (22 cases). When considering only these ten most frequent mutations, 77 mutations were identified in 71 patients: 39 in tumor and plasma, 36 in tumor, but not in plasma and 2 mutations only in plasma, with a κ of 66% (95%CI, 56-76%). The p.H1047R PIK3CA mutation, was found only in tumor (5 cases); for the other nine mutations, the κ coefficient varied from 56% to 89%, with a median of 75%. The cfDNA analysis of the 43 patients without tDNA analysis revealed at least one mutation in 24 patients (56%), including 11 pathogenic variants of therapeutic interest. Conclusion The analysis of cfDNA using NGS represents an attractive and noninvasive alternative to tumor biopsies, and can be used as a surrogate method to screen for mutations. Further prospective validation is warranted. Citation Format: Ecaterina Ileana, Cécile Jovelet, Marie-Cécile Le Deley, Christophe Massard, Nelly Motté, Antoine Hollebecque, Amélie Boichard, Charles Ferté, Sophie Postel-Vinay, Silvia Rosellini, Maud Ngo-Camus, Thierry De Baere, Philippe Vielh, Catherine Richon, Mélanie Laporte, Siham Gouissem, Yohann Loriot, Rastilav Bahleda, Anas Gazzah, Andrea Varga, Gilles Vassal, Alexander Eggermont, Fabrice André, Jean-Charles Soria, Ludovic Lacroix. Circulating cell-free tumor DNA (cfDNA) analysis of 50-genes by next-generation sequencing (NGS) in the prospective MOSCATO trial. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2401. doi:10.1158/1538-7445.AM2015-2401