A Study to Evaluate the Association of Biliary Complications and Treatment Duration in Unresectable BTC Patients

Inclusion Criteria: 1\. Patients diagnosed with "unresectable BTC" who meet any of the following: 1. Patients who were deemed unamenable to curative resection at the initial diagnosis. 2. Patients who relapsed after curative resection of BTC more than 6 months after the end of adjuvant therapy or after the date of surgery for patients who did not receive adjuvant therapy. 2\. Patients who received first-line systemic therapy for unresectable BTC with the date of the first dose between May 1, 2022, and December 31, 2023. However, patients who received postoperative adjuvant therapy for BTC with R2 resection is not included. 3\. Patients with ECOG performance status 0 or 1 on the most recent assessment date prior to first dose of first-line systemic therapy for unresectable BTC 4. Patients who provided written informed consent form (ICF), in cases where obtaining informed consent is not difficult, or for whom an appropriate opt-out approach was taken at the study site if obtaining informed consent is difficult. 5\. Patients aged 18 years or older at the date of first dose of first-line systemic therapy for unresectable BTC. 6\. Patients who started first-line systemic therapy with physician's expectation of 3 months or longer prognosis at the time of starting systemic therapy. Exclusion Criteria: 1. Maintenance dialysis at the date of the first dose of first-line systemic therapy for unresectable BTC. 2. Combined any locoregional therapy with first-line systemic therapy for unresectable BTC (e.g., radiation therapy, radiofrequency ablation, arterial injection chemotherapy) 3. Patient who participated in clinical trials (interventional studies) of first-line systemic therapy for BTC 4. Active cancers other than unresectable BTC within the past 2 years from the date of the first dose of first-line systemic therapy. The following are exceptions to this criterion: * Carcinoma in situ or lesions equivalent to carcinoma in situ. * Intramucosal cancers that are deemed cured by local treatment. 5. Patients who were followed up for less than 3 months after starting first-line systemic therapy for reasons other than TTF events (disease progression, death, or treatment discontinuation).