Tumor serosal invasion (si) significantly worsens outcomes in perihilar cholangiocarcinoma (pcca).

• 37.7% of 725 patients had si, linked to 34.1% postoperative peritoneal dissemination versus 12.8% in non-si.
• Overall recurrence rates were 81.0% for si vs. 58.6% for non-si (p<0.001).
• Five-year survival was only 25.2% in si patients compared to 53.5% in non-si (p<0.001).

Surgeons should prioritize evaluating serosal invasion in pcca to better identify patients at high risk for poor outcomes.