The impact of mismatch repair status on the preoperative staging of colon cancer

Purpose: CT scan is standard in preoperative local staging of colon cancer. Tumours with a deficient mismatch repair (dMMR) system are characterised by unique clinical and pathophysiologic aspects that may impact the accuracy of the preoperative CT staging. Material and methods: Data from the Danish Colorectal Cancer Group national clinical database addressing a cohort of patients operated for stage I-III colon cancer during 2010-15 were analysed. The analyses of MMR status had been conducted consecutively through means of immunohistochemistry. All CT scans were blindly assessed by a certified radiologist. Results: Data from 590 patients, operated at a specialised cancer centre, were available for analyses. 135 (22.9%) of the patients had tumours demonstrating dMMR. The overall correlation of the clinical and pathological T-category was significant for both groups. There was an inferior correlation between clinical (cN) and pathological (pN) N-category (p>0.05) in pMMR cancers with a higher degree of over-staging assessed by CT scan compared to a significant correlation between cN and pN stages in pMMR cancers (p<0.01). Of the 91 dMMR tumours assessed node-positive by the preoperative CT scan, 59 (64.8%) showed no sign of metastatic involvement at the postoperative assessment. Conclusion: Preoperative CT staging of dMMR colon cancer seems to overestimate nodal involvement and two-thirds of the patients preoperatively judged node-positive are false positive. The overall clinical value of a routine assessment of MMR status in the diagnostic setup is evident and the current study argues for implications to treatment decisions as well.