Oncology Recapitulates Surgical Anatomy: Re-Defining Negative Oncologic Margins with R1 Vascular Hepatectomy in Colorectal Liver Metastases.

Complete resection with microscopically negative (R0, or no tumor cells within 1 mm of resection surface) margins has long been considered the cornerstone of curative-intent hepatectomy for colorectal liver metastases (CRLM). R1 resection is defined as the presence of tumor cells at or within 1 mm of the inked margin. A growing body of contemporary evidence, however, shows that the traditional binary R0/R1 framework obscures biologically meaningful subcategories of positive margin. An R1 parenchymal (R1p) margin on the transected liver surface carries a distinctly adverse prognosis, whereas an R1 vascular (R1v) margin (created by intentional detachment of tumor from the adventitia of a major intrahepatic vessel along Laennec's capsule) yields oncologic outcomes that are, in essence, equivalent to R0: modern series report five-year overall survival rates of 34-45% after R1v, comparable to matched R0 cohorts. This insight has radical implication, enabling parenchyma-preserving hepatectomy (PPH) for patients who would otherwise require major hepatectomy (i.e., trisectionectomy) for multifocal and centrally located tumors, as well as allowing patients who would otherwise be considered unresectable to undergo curative intent resection. Careful patient selection, taking into consideration anatomic variation, use of intraoperative ultrasound guidance, response to systemic therapy, and molecular tumor profiling is essential for the safe application of R1v PPH. This review synthesizes the evidence supporting the R1p/R1v distinction, places it within the broader evolution of margin paradigms in CRLM, and outlines the clinical, anatomical, and biological criteria that should guide its judicious use in the current era of precision hepatobiliary surgery.