Abstract Background: Genomic profiling with tissue sequencing is still considered as the gold standard despite several limitations including screening failures due to limited tissue availability, and inability to capture intratumor spatial and temporal heterogeneity, which may impair accurate treatment selection. Several studies have demonstrated the potential of circulating tumor DNA (ctDNA) to detect genomic alterations at high accuracy compared with tissue analysis. However, no studies have comprehensively evaluated differences between tissue and ctDNA by using a large panel in the same cohort. Methods: Genomic analysis was performed for each patient by using the Foundation One Liquid CDx Assay and the Foundation One CDx Assay (324 genes, tumor mutational burden [TMB], microsatellite instability). Each individual genomic report was reviewed and discussed weekly by a multidisciplinary tumor board (MTB). Actionable targets were defined by the MTB according to the existing level of evidence (ESCAT tiers) and molecular-based treatment suggestions were proposed where possible. Results: Between Dec 2020 and Nov 2021, 1021 patients (median age: 62 years) with advanced cancer underwent both tissue and ctDNA NGS. Five most frequent tumor types were colorectal (N=137,13%), NSCLC (N=130,13%), breast (N=120, 12%), prostate (N=82, 8%) and pancreas (N=65, 6%). Median time elapsed between request and assay results was 12 days for ctDNA and 46 days for tissue. Testing failure was 15% for tissue and 3.9% for ctDNA. Overall, 824 (81%) patients had evaluable results for both tissue and liquid. Total number of cancer-related alterations and variants of unknown significance were 4704 and 11673 vs 4645 and 7481 for ctDNA and tissue, respectively. Proportion of patients with a higher number of cancer alterations identified in ctDNA compared with tissue increased in parallel with the time elapsed between the tissue and ctDNA sampling (45% vs 33% for a delay > 26 months or < 8 months). MSI and TMB status were concordant for 71% and 64% of patients, respectively. MSI status was evaluable for 97% of patients through ctDNA vs 90% through tissue. Number of actionable alterations was similar in 346 (42%) of cases, whereas it was higher in tissue for 289 (35%) and in liquid for 189 (23%) patients. ctDNA profiling allowed the identification of an ESCAT I/II or III or IV alteration not present in tissue for 74 (9%), 113 (14%) and 52 (6%) patients, respectively. Overall, MTB recommended a matched therapy for 430 patients (52%). Such a recommendation would not have been made without the results of ctDNA for 120 patients (15%). Conclusion: This systematic comparison of ctDNA vs tissue sequencing demonstrates the capacity of ctDNA for capturing clinically relevant alterations to guide therapy in cancer patients with high accuracy and rapid turnover results. Citation Format: Arnaud Bayle, Florent Peyraud, Laila Belcaid, Maxime Brunet, Miha Aldea, Rebecca Clodion, Paul Dubos, Damien Vasseur, Claudio Nicottra, Santiago Ponce, Isabelle Soubeyran, Emmanuel Khalifa, Yohann Loriot, Benjamin Besse, Ludovic Lacroix, Etienne Rouleau, Geoffrey Oxnard, Fabrice Barlesi, Fabrice Andre, Antoine Italiano. Systematic comparison of ctDNA vs tissue sequencing with a large panel to guide therapy in patients with advanced cancer: A study from the French National Center for Precision Medicine (PRISM) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3414.