PURPOSE: To evaluate the efficacy, safety, and clinical course following ablative radioembolization with glass microspheres for hepatocellular carcinoma (HCC) with localized portal vein tumor thrombosis (PVTT) in patients with well-preserved liver function. MATERIALS AND METHODS: This is a prospective, open-label, multi-center, single-arm, phase II trial. Key inclusion criteria are unilobar HCC, PVTT confined to the ipsilateral lobe (Vp1-3), no extrahepatic spread, Child-Pugh class A, and a performance status of ≤ 1. Main exclusion criteria are hepatic venous tumor thrombus, bile duct invasion, and massive arterioportal shunting. Depending on the extent of the tumor and PVTT, patients will undergo radiation segmentectomy, modified radiation lobectomy, or ablative lobar treatment, while adhering to dose limits for the non-tumorous liver and lung. The primary outcome measure is overall survival, with the overall survival rate at two years provided as the summary measurement. STATISTICS: This study will enroll 30 patients to explore the efficacy and safety of ablative radioembolization for HCC with localized PVTT. Efficacy will be evaluated by intention-to-treat and per-protocol populations. Safety will be assessed for all patients who received radioembolization at any dose. EXPECTED GAIN OF KNOWLEDGE: This study aims to address the potential of ablative radioembolization as a definitive or effective downstaging treatment for HCC with localized PVTT, where locoregional treatments may be more beneficial than systemic treatments. The results will help establish treatment outcomes that can serve as a standard for future comparative studies and contribute to the standardization of radioembolization approaches for HCC with localized PVTT. TRIAL REGISTRATION: ClinicalTrials.gov ( https://classic. CLINICALTRIALS: gov/ct2/show/NCT06166576 ). Identifier: NCT06166576.