According to current clinical guidelines, first-line treatment for advanced biliary tract cancer (BTC) is the gemcitabine plus cisplatin (GC) regimen combined with immunotherapy, which has been shown to improve patient outcomes. Additionally, the combination of radiotherapy and immunotherapy may synergistically enhance antitumor efficacy. Therefore, this study aims to assess the efficacy and safety of first-line treatment with radiotherapy in combination with sintilimab and gemcitabine/cisplatin versus sintilimab in combination with gemcitabine/cisplatin in patients with previously untreated, unresectable locally advanced or metastatic BTC.