INTRODUCTION: Biliary tract cancers (BTCs) are molecularly heterogeneous, and early genomic profiling is becoming increasingly important for treatment decisions. Because tissue sampling is often limited or inadequate, there is a clear need for minimally invasive biomarkers that can support treatment selection and longitudinal disease assessment. AREAS COVERED: This narrative review summarizes current and emerging liquid-biopsy (LB) applications in BTC, with a primary focus on plasma circulating tumor DNA (ctDNA). The evidence base was assembled through targeted searches of PubMed/MEDLINE and Embase up to 1 February 2026, supported by selective ClinicalTrials.gov searches for ongoing biomarker-driven studies. We discuss ctDNA-based molecular profiling for actionable alterations, its prognostic role including minimal residual disease assessment, and its use in serial monitoring of treatment response and acquired resistance. We also consider how LB may support clinical-trial enrichment and biomarker-guided endpoints, and briefly review complementary approaches using bile and other analytes, while highlighting current evidence gaps. EXPERT OPINION: In BTC, ctDNA is best viewed as a complement to tissue-based profiling, especially when tissue is inadequate or when rapid genotyping is needed. Its broader clinical impact will depend on assay standardization, clearer interpretation frameworks for low-shedding disease, and prospective studies showing that ctDNA-guided decisions improve patient outcomes.
Erul et al. (2026) studied this question.