The Association of Mismatch Repair Status with Microscopically Positive (R1) Margins in Stage III Colorectal Cancer: A Retrospective Cohort Study

BACKGROUND: There is mounting evidence that microscopically positive (R1) margins in patients with colorectal cancer (CRC) may represent a surrogate for aggressive cancer biology rather than technical failure during surgery. However, whether detectable biological differences exist between CRC with R0 and R1 margins is unknown. We sought to investigate whether mismatch repair (MMR) status differs between Stage III CRC with R0 or R1 margins.

METHODS: Patients treated for Stage III CRC from January 1, 2016 to December 31, 2019 were identified by using the Danish Colorectal Cancer Group database. Patients were stratified according to MMR status (proficient [pMMR] vs. deficient [dMMR]) and margin status. Outcomes of interest included the R1 rate according to MMR and overall survival.

RESULTS: A total of 3636 patients were included, of whom 473 (13.0%) had dMMR colorectal cancers. Patients with dMMR cancers were more likely to be elderly, female, and have right-sided cancers. R1 margins were significantly more common in patients with dMMR cancers (20.5% vs. 15.2%, p < 0.001), with the greatest difference seen in the rate of R1 margins related to the primary tumour (8.9% vs. 4.7%) rather than to lymph node metastases (11.6% vs. 10.5%). This association was seen in both right- and left-sided cancers. On multivariable analyses, R1 margins, but not MMR status, were associated with poorer survival, alongside age, pN stage, perineural invasion, and extramural venous invasion.

CONCLUSIONS: In patients with Stage III CRC, dMMR status is associated with increased risks of R1 margins following potentially curative surgery, supporting the use of neoadjuvant immunotherapy in this patient group.