J. D. Mason1 | K. Naidu2 | J. Tiernan2 | N. P. West3 | C. Cunningham1
Abstract
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.