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PURPOSE: Fibroblast growth factor receptor (FGFR) 2 alterations, present in 5%-15% of intrahepatic cholangiocarcinomas (IHC), are targets of FGFR-directed therapies. Acquired resistance is common among patients who respond. Biopsies at the time of acquired resistance to targeted agents may not always be feasible and may not capture the genetic heterogeneity that could exist within a patient. We studied circulating tumor DNA (ctDNA) as a less invasive means of potentially identifying genomic mechanisms of resistance to FGFR-targeted therapies. MATERIALS AND METHODS: Serial blood samples were collected from eight patients with FGFR-altered cholangiocarcinoma for ctDNA isolation and next-generation sequencing (NGS) throughout treatment and at resistance to anti-FGFR–targeted therapy. ctDNA was sequenced using a custom ultra-deep coverage NGS panel, incorporating dual index primers and unique molecular barcodes to enable high-sensitivity mutation detection. RESULTS: Thirty-one acquired mutations in FGFR2 , 30/31 located in the kinase domain, were identified at resistance in six of eight patients with detectable ctDNA. Up to 13 independent FGFR2 mutations were detected per patient, indicative of striking genomic concordance among resistant subclones. CONCLUSION: ctDNA could be an effective means to longitudinally monitor for acquired resistance in FGFR2-altered IHC. The numerous acquired genetic alterations in FGFR2 suggest frequent polyclonal mechanisms of resistance that cannot be detected from single-site tissue biopsies. [ABSTRACT FROM AUTHOR]