Background and Aims: Post-Colonoscopy Colorectal Cancer (PCCRC) refers to CRC diagnosed after a colonoscopy in which no cancer was found. The rectum is the second most common location for PCCRC after the right colon. Rectal cancer has different biologic behavior than CRC located elsewhere in the colon, but whether a rectal location for PCCRC affects all-cause mortality (ACM) and colorectal cancer-specific mortality (CSM) differently than other locations is not known.
Methods: We performed a secondary analysis of a VA–Medicare administrative database that included all Veterans aged 50 to 85 years with newly diagnosed CRC between 1/1/2003 and 12/31/2013, examining prior exposure to colonoscopy. Patients whose colonoscopy occurred ≤ 6 months prior to CRC diagnosis with no other prior colonoscopy within the previous 36 months were categorized as Detected CRC (DCRC). Those whose colonoscopy occurred 6-36 months prior to CRC diagnosis were categorized as PCCRC-3y. Using Cox proportional hazards (with censoring, last follow-up 12/31/2018), we assessed ACM and CSM after a CRC diagnosis, stratified by location (rectum, right colon) and whether a PCCRC or DCRC.
Results: The analyses are summarized in the Table. There was no significant difference in CSM between rectal PCCRC-3y and rectal DCRC regardless of adjustment for stage (hazard ratios [HR] 1.11, 95% CI: 0.91-1.34 and 1.03 (0.85, 1.25), respectively). Compared with rectal DCRC, patients with rectal PCCRC-3y had comparable ACM, though this became marginally significantly higher after adjustment for stage (HR 1.15 (1.00-1.33), p=0.047). Among PCCRC-3y cases, rectal location was associated with significantly higher ACM (HR 1.35) and CSM (HRs 1.49-1.56) compared with right-sided location irrespective of adjustment for stage. Among DCRC cases, rectal location was also associated with significantly higher ACM (HRs 1.33-1.37) and CSM (HRs 1.56-1.67) than right-sided location irrespective of adjustment for stage.
Conclusions: Rectal cancer is associated with worse ACM and CSM than right-sided colon cancer, and does not appear to be influenced by stage or whether the cancer was classified as DCRC or PCCRC-3y. Additional studies in more contemporary cohorts are needed to confirm our findings, and explore underlying mechanisms.

Patrick Monahan

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