Supplementary Material for: Low Tumor Burden Predicts Benefit from Transarterial Treatment in Hepatocellular Carcinoma with Extrahepatic Metastasis: A Nationwide Cohort Study

Background/Aims: In patients with hepatocellular carcinoma (HCC) and extrahepatic metastasis (EHM), intrahepatic disease control may remain clinically relevant in selected cases. This study aimed to compare survival outcomes between transarterial treatment (TAT) and systemic therapy (ST) as initial treatment. Methods: Data from the Korean Central Cancer Registry (2008–2020) were analyzed. TAT included chemoembolization, radioembolization, and hepatic arterial infusion chemotherapy. Systemic therapy consisted predominantly of tyrosine kinase inhibitors (>98%). Overall survival (OS) was the primary endpoint. Propensity score matching (PSM) and inverse probability of treatment weighting (IPTW) adjusted for confounding. Results: Among 19,753 treatment-naïve HCC patients, 2,517 had EHM. Of these, 663 received TAT and 595 received ST as initial therapy. In the overall cohort, TAT was associated with significantly longer OS than ST (median OS: 6.7 vs. 3.7 months, P < 0.001), and this benefit remained in both PSM and IPTW analyses (P < 0.001 for both). The advantage of TAT persisted across subgroups defined by metastatic site, number of metastatic organs, portal vein tumor thrombosis (PVTT), and tumor size, although the difference narrowed in tumors >10 cm. Multivariable analysis identified TAT, single-organ metastasis, AFP