Reconsidering the 1 mm rule: Contextualising R1 margin status in rectal cancer

BACKGROUND: The 1 mm rule for circumferential resection margin (CRM) involvement in rectal cancer is deeply embedded in international practice, defining R1 resection as tumour at or within 1 mm of the resection margin. While this threshold has strong evidence in major resections for primary rectal cancer, its universal application is increasingly questioned. Advances in imaging, surgical technique and pathological understanding suggest that R1 status may require context-specific interpretation across three distinct clinical settings: encapsulated nodal involvement in locally advanced rectal cancer (LARC), locally recurrent rectal cancer (LRRC) and locally excised early rectal cancer (LERC). METHODS: This opinion article reviews current literature, international datasets and emerging evidence to challenge the uniformity of the 1 mm definition. It draws upon The International Collaboration on Cancer Reporting (ICCR) dataset, Royal College of Pathologists (RCPath) guidance and recent large cohort and registry analyses to explore the biological and clinical relevance of close margins in these scenarios. RESULTS: Evidence indicates that the prognostic value of the 1 mm rule varies by anatomical and pathological context. In LARC, a lymph node metastasis abutting the CRM without extracapsular extension behaves biologically as R0 and should not be upstaged. For LRRC, narrow but clear margins (>0 mm) confer equivalent outcomes to wider margins, supporting the use of a 0 mm R1 definition. In LERC, a ≤1 mm margin may be oncologically acceptable in the absence of high-risk histological features. CONCLUSION: The current evidence supports a tailored approach to R1 definition, preserving rigour while aligning classification with modern oncological, anatomical and pathological realities.