[Study objectives] To evaluate the efficacy and safety of the combined treatment of radiotherapy and endoscopic intraductal radiofrequency ablation in patients with locoregional extrahepatic cholangiocarcinoma.

○ Study flow The clinical study will be conducted in the following order. Of patients with extrahepatic cholangiocarcinoma requiring biliary drainage, those who meet the inclusion criteria and consent to the study will be enrolled in the study and hospitalized to undergo endoscopic biliary drainage. The histologic or cytologic examination will be performed at the time of the first biliary drainage, and the extent of the cholangiocarcinoma lesion will be assessed during the procedure. After the diagnosis of extrahepatic cholangiocarcinoma, ID-RFA will be performed, and a biliary stent will be inserted for biliary drainage. Radiotherapy will be performed within 1 month of ID-RFA.

Systemic chemotherapy is also one of the main treatments for patients with inoperable cholangiocarcinoma. Administration of subsequent chemotherapy is recommended after radiotherapy. The choice of chemotherapy regimen is at the discretion of the investigator based on on the patient's age, performance status, and other factors. For older patients who are inoperable, systemic chemotherapy may be difficult to administer.

Tumor response assessment will be performed using CT or MR every 2 months ± 2 weeks based on based on RECIST 1.1.

• Endoscopic biliary drainage The ERCP procedure is performed by an experienced pancreatobiliary endoscopist. A standardized endoscopic sphincterotomy (EST) is usually performed prior to stenting. If EST is difficult, a precut sphincterotomy may be performed first. After selective intubation into the bile duct according to the usual process of biliary drainage, a guidewire is placed upstream of the stricture and a biliary stent is inserted, and the type and length of the stent are selected at the discretion of the investigator. Systemic anti-cancer therapy after radiotherapy will be conducted at the discretion of the investigator, considering the patient's condition.
• ID-RFA ID-RFA requires a catheterized electrode that can be used through the channel of the ERCP. Currently, there are two commercially available RFA catheters: the Habib EndoHBP® (Boston Scientific, London, UK) and the ELRA RFA catheter® (Starmed, Goyang, Korea). This study uses the Habib catheter, which is an 8Fr bipolar catheter with two 8 mm electrodes that can be inserted into the bile duct by ERCP along a guidewire. The standard RFA is 7-10 W for 90 sec, and the energy and treatment time can be adjusted according to the operator's discretion. Repeated RFA can be performed by adjusting the catheter from proximal to distal depending on the length of the stricture during the same endoscopic session. After ID-RFA, a biliary stent is inserted to ensure bile duct patency and prevent stricture.
• Radiotherapy Radiotherapy is aimed to start within 1 month after ID-RFA. The radiotherapy dose will be 30-50 Gy in 10 fractions, considering the planning target volume based on the dose-volume standard.

Inclusion Criteria:

1. Patients 20 years of age or older, who voluntarily agree to participate in the study and sign the informed consent.
2. Patients with cytopathologically diagnosed inoperable extrahepatic cholangiocarcinoma

Exclusion Criteria:

1. Patients who refuse to sign the informed consent
2. Patients with metastatic cancer (stage IV) or malignant ascites
3. Patients with intrahepatic cholangiocarcinoma
4. Patients who are unable to undergo endoscopic procedures as determined by the investigator.
5. Patients with a life expectancy of 3 months or less
6. Patients with ID-RFA contraindications: pacemaker, pregnancy, uncorrected bleeding tendency (PT INR > 1.5, platelets < 50,000/mm3)