PT - JOURNAL ARTICLE AU - ELISA DE CARLO AU - MONICA SCHIAPPACASSI AU - GIACOMO PELIZZARI AU - TANIA BARESIC AU - ALESSANDRO DEL CONTE AU - BRIGIDA STANZIONE AU - VALENTINA DA ROS AU - ROBERTO DOLIANA AU - GUSTAVO BALDASSARRE AU - ALESSANDRA BEARZ TI - Acquired EGFR C797G Mutation Detected by Liquid Biopsy as Resistance Mechanism After Treatment With Osimertinib: A Case Report AID - 10.21873/invivo.12586 DP - 2021 Sep 01 TA - In Vivo PG - 2941--2945 VI - 35 IP - 5 4099 - http://iv.iiarjournals.org/content/35/5/2941.short 4100 - http://iv.iiarjournals.org/content/35/5/2941.full SO - In Vivo2021 Sep 01; 35 AB - Background: Osimertinib is a third-generation EGFR-tyrosine kinase inhibitor approved for the treatment of T790M-positive non-small-cell lung cancer. More recently, osimertinib demonstrated improved disease control compared to other EGFR-TKIs. Multiple mechanisms of resistance have been described in T790M-positive patients who experienced treatment failure with osimertinib. Case Report: We report the case of a 78-year-old non-smoker woman with stage IV EGFR L858R-positive lung adenocarcinoma presented with T790M mutation after five years of treatment with gefitinib. The patient was started on osimertinib, but after two and a half years of treatment experienced disease progression. The analyses of circulating tumor DNA using next-generation sequencing showed, together with the pre-existing T790M and exon 21 L858R, the presence of the EGFR C797G resistance mutation. Conclusion: Our case report revealed a rare EGFR-dependent acquired resistance mutation to osimertinib in circulating tumor DNA. Liquid biopsy appears to be a promising resource to understand the biology of osimertinib resistance by clonal evolution monitoring and the identification of novel resistance mechanisms.