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PurposeWhile tumor markers (carbohydrate antigen 19-9 [CA 19-9] and carcinoembryonic antigen [CEA]) can aid in the diagnosis of biliary tract cancer, their prognostic role has not been clearly elucidated. Therefore, this study was conducted to evaluate the prognostic role of tumor markers and tumor marker change in patients with advanced biliary tract cancer.Materials and MethodsPatients with pathologically proven metastatic or relapsed biliary tract cancer who were treated in a phase II trial of first-line S-1 and cisplatin chemotherapy were enrolled. Serum tumor markers were measured at baseline and after the first cycle of chemotherapy.ResultsAmong a total of 104 patients, 80 (77%) had elevated baseline tumor markers (69 with CA 19-9 elevation and 40 with CEA). A decline ≥ 30% of the elevated tumor marker level after the first cycle of chemotherapy conferred an improved time to progression (TTP), overall survival (OS), and better chemotherapy response. Multivariate analysis revealed tumor marker decline as an independent positive prognostic factor of TTP (adjusted hazard ratio [HR], 0.44; p=0.003) and OS (adjusted HR, 0.37; p < 0.001). Subgroup analysis revealed similar results in each group of patients with CA 19-9 elevation and CEA elevation. In addition, elevated baseline CEA was associated with poor survival in both univariate and multivariate analysis.ConclusionTumor marker decline was associated with improved survival in biliary tract cancer. Measuring tumor marker after the first cycle of chemotherapy can be used as an early assessment of treatment outcome.