Surgical Anatomy for Liver Cancer: Revisiting Segmental Liver Anatomy Beyond Couinaud in the Era of 3D Simulation and Minimally Invasive Liver Resection

Couinaud’s eight-segment model remains the universal language of liver surgery; however, contemporary multidetector computed tomography (CT), three-dimensional (3D) reconstruction, and minimally invasive surgery have exposed substantial variability in third-order or more peripheral portal branching. This variability becomes clinically important when hepatocellular carcinoma and other liver malignancies are treated with small, parenchymal-sparing anatomical resections, where third-order or more peripheral portal territories define the oncologic unit at risk. This review examines the current evidence of surgical anatomy relevant to liver surgery, focusing on third-order portal branches and comparing classical Couinaud segmentation with CT/3D reconstruction-based portal territories. Clinically relevant variations in the right anterior (S5/S8) and right posterior (S6/S7) sections are described, while the more limited and heterogeneous evidence for the left hemiliver is explicitly acknowledged. In the right anterior section, craniocaudal, ventrodorsal, trifurcation, and other branching patterns may alter the practical interpretation of S5/S8 anatomy. In the right posterior section, bifurcation and loop-type portal configurations have distinct implications for S6/S7-oriented resection and individual anatomy-based resection. Comparable variability in the left hemiliver, particularly around the umbilical portion and segment 4 branches, further supports individualized preoperative territory mapping. A two-level anatomical planning framework is proposed as a conceptual strategy: Couinaud-based anatomy for first- and second-order resection and individualized CT-based anatomy for third-order or more peripheral resection. This framework is supported by anatomical observations and technical feasibility, but it has not been shown to improve perioperative or oncological outcomes. Evidence supporting anatomical liver resection in general should not be regarded as direct validation of cone-unit (small portal territory) resection, which requires prospective and independent clinical evaluation. The current status of preoperative simulation and intraoperative implementation using ultrasound, indocyanine green fluorescence, and navigation/fusion technologies is discussed. Finally, emerging directions, including automated vessel segmentation and AI-assisted territory analysis, are considered.