PULSAR Radiotherapy Combined With Immunotherapy for High-Risk Advanced Liver Cancer

The goal of this clinical trial is to learn if adding PULSAR, a personalized adaptive stereotactic radiotherapy, to standard immunotherapy can control cancer longer in adults with high-risk advanced hepatocellular carcinoma (liver cancer). It will also learn about the safety of this combination. The main questions it aims to answer are: * What percentage of participants are alive without cancer progression 12 months after starting PULSAR? * What medical problems do participants have after PULSAR with immunotherapy? There is no comparison group. Results will be compared with published results of immunotherapy alone. Participants will: * Continue their usual immunotherapy * Receive 2 to 3 PULSAR sessions, 3 to 4 weeks apart, with the plan adjusted before each session * Give blood samples and fill out symptom questionnaires * Have checkups and scans for up to 3 years

Inclusion Criteria: * Age 19 years or older * Hepatocellular carcinoma diagnosed histologically or clinically according to the Korean Liver Cancer Association-National Cancer Center guidelines * Unresectable and/or locally advanced or metastatic disease, currently receiving or scheduled to receive immune checkpoint inhibitor-based systemic therapy (e.g., atezolizumab plus bevacizumab or durvalumab plus tremelimumab) * At least one of the following high-risk features: * Tumor larger than 5 cm in maximum diameter * Portal vein tumor thrombosis (Vp2-4) * Infiltrative lesion * Bile duct invasion * Inferior vena cava invasion * Extrahepatic metastasis * At least one measurable target lesion per RECIST version 1.1, with PULSAR already planned by the treating physician * ECOG performance status 0-1 * Child-Pugh score 7 or lower * Known hepatitis B and C virus status, with antiviral therapy per institutional policy if indicated * Voluntary written informed consent Exclusion Criteria: * Pregnant or breastfeeding women * Active brain or leptomeningeal metastases * More than 5 extrahepatic metastatic lesions (multiple intrahepatic lesions are allowed) * Prior radiotherapy to the treatment site * Moderate or refractory ascites, or hepatic encephalopathy * Contraindications to immune checkpoint inhibitors, such as active autoimmune disease requiring systemic treatment (hypothyroidism stable on hormone replacement and type 1 diabetes controlled with insulin are allowed) * Uncontrolled serious comorbidities (e.g., uncontrolled infection, NYHA class II or higher heart disease, myocardial infarction or stroke within 3 months, unstable angina or arrhythmia) * Unable to comply with the study protocol * Considered unsuitable for the study by the investigator