A study published in the Annals of Internal Medicine showed that screening colonoscopy was associated with significant reductions in colorectal cancer (CRC) mortality among veterans in the Veterans Affairs (VA) Health Care System.
Using national VA-Medicare data, colonoscopy was associated with a 61% reduction in CRC mortality among veterans. This reduction was observed for both left- and right-sided CRC, but the association was weaker for right-sided cancer (46% vs 72% mortality reduction).
Although screening colonoscopy is the standard of care in the US, it is not supported by randomized controlled trials. Some studies have questioned its benefit over sigmoidoscopy and whether the effect varies based on the anatomical location of the cancer.
The investigators set out to determine if colonoscopy is associated with decreased CRC mortality in veterans, and if its effect differs by anatomical location of CRC. The case-controlled study was led by Charles J. Kahi, MD, from the Richard L. Roudebush VA Medical Center and Indiana University School of Medicine in Indianapolis, Indiana.
Veterans with inpatient and/or outpatient visits at any VA facility from 1997 to 2010 who received care primarily in the Veterans Health Administration (VHA) system were identified using data from the national VA-Centers for Medicare and Medicaid Services (CMS).
Case patients were veterans who received a diagnosis of CRC at age 50 years or older and died of CRC at age 52 years or older. Those with Crohn’s disease, ulcerative colitis, or familial polyposis were excluded. Cancer was categorized as right-sided (cecum, ascending colon, hepatic flexure, or transverse colon) or left-sided (splenic flexure, descending colon, sigmoid colon, or rectum).
Control patients were veterans who were not diagnosed with CRC before an index date and did not die of the disease before December 31, 2010. For each case patient, four control patients were matched according to age (±1 year), gender, and VA medical center. As with the case patients, control patients with Crohn’s disease, ulcerative colitis, or familial polyposis were excluded.
The exposure in case patients was defined as receipt of colonoscopy between January 1997 and six months before CRC diagnosis, or the corresponding index date in control patients.
Cases were identified with the VA Central Cancer Registry (VACCR), which contains demographic information, date of diagnosis, pathologic confirmation, cancer location within the colon and rectum, previous cancer, and stage of cancer.
Other sources of data included the VA Medical SAS Data Sets, which contains data on all VA inpatient stays and outpatient visits, the Linked VA-CMS Data, which identifies veterans who had colonoscopies in non-VA facilities, and the Department of Defense Suicide Data Registry, which contains information on date and cause of death for all VA health system users.
A total of 4,964 case patients and 19,856 controls were identified; 99.3% were male, they were predominantly white, and the mean age was 70.7 years.
Case patients had higher comorbidity burden and a significantly smaller proportion were exposed to colonoscopy than controls (13.5% vs 26.4%; P < 0.001).
A total of 668 case patients and 5,250 control patients underwent colonoscopy; the indication was diagnostic in 68.7% and 60.9%, screening in 15.3% and 21.3%, and surveillance in 16.0% and 17.8%, respectively (P < 0.001). The median time between colonoscopy and CRC diagnosis was 43.5 months.
The proportions of patients who underwent colonoscopy for a screening indication increased dramatically during the study from 8.0% during 1997–2001 to 32.6% in 2007 or later.
Case patients were less likely than controls to have undergone any colonoscopy (adjusted odds ratio [OR]: 0.39).
Colonoscopy was associated with reduced odds for left-sided cancer (adjusted OR: 0.28) and right-sided cancer (adjusted OR: 0.54). Among patients exposed to screening colonoscopy, the adjusted ORs were 0.30 overall, 0.20 for left-sided cancer, and 0.48 for right-sided cancer.
The results were similar at various intervals between CRC diagnosis and exposure to colonoscopy (3, 12, or 24 months). Moreover, the odds of exposure to colonoscopy did not significantly change based on time period.
Explanations offered for reduced protection in the proximal colon include bowel preparation quality, patient factors such as diet or cigarette smoking, operator-dependent factors, or system-related factors, as well as factors such as altered tumor biology with accelerated progression to invasive cancer.
The investigators reported that the study had limitations. Some confounding factors such as body mass index or physical exercise could not be measured, and misclassification according to indication may have occurred. In addition, nonprescription use of aspirin and other NSAIDs could not be determined, and information on procedure quality (bowel preparation adequacy, adenoma detection, and cecal intubation rates) was not available.
“Our study showed that colonoscopy was associated with a reduction in CRC mortality of approximately 60% in the VHA system, although the reduction was less pronounced in the right colon,” concluded the authors. They believe that reducing variability in colonoscopy effectiveness, particularly against right-sided CRC, is critical for prevention of CRC.
In addition, they acknowledge that ongoing studies comparing colonoscopy to fecal immunochemical tests will provide information to determine if colonoscopy is the best CRC screening test based on factors such as cost-effectiveness, availability, and patient compliance.
To read more about this study, click here.