Evidence of Benefit
Table 3. Randomized Controlled Screening Trials: Fecal Occult Blood Testing continued...
Flat or difficult-to-detect lesions include "serrated polyps," which may be more common in the right colon than they are in the left. The term serrated polyp is currently used to include hyperplastic polyps, sessile serrated adenomas, traditional serrated adenomas, and mixed serrated polyps.[65,66] The clinical significance of these lesions is uncertain, because the natural history is so difficult to learn for any polypoid lesion. However, the histologic and molecular characteristics of some serrated lesions suggest possibly important malignant potential (mutations in the BRAF gene may be an early step toward carcinogenesis in serrated polyps). This potential, along with the challenges of detecting flat lesions, may partially account for recent reports of a colonoscopy's lesser protective effect in the right colon compared to the left.
In 2011, authors of one study reported variability of detection rates for proximal serrated polyps. They studied 15 colonoscopists on faculty at one university and showed, during the years 2000 to 2009, a wide variation in detection rate for proximal serrated polyps, ranging (per colonoscopy) from 0.01 to 0.26, suggesting that many proximal serrated lesions may be missed on routine exam. The overall proportion of polyps that are "serrated" is unknown, in part because these lesions have been unappreciated and/or difficult to identify.
Evidence about colorectal cancer mortality reduction
Although there is no RCT to assess reduction of CRC incidence or mortality by colonoscopy, some case-control evidence is available. Based on case-control data about sigmoidoscopy, noted above, it has been speculated in the past that protection for the right colon might be similar to that found for the left colon. However, a recent case-control study of colonoscopy raises questions about whether the impact of colonoscopy on right-sided lesions might be different than the impact on left-sided lesions. Using a province-wide administrative data base in Ontario, investigators compared cases of persons who had received a diagnosis of CRC from 1996 to 2001 and had died by 2003. Controls were selected from persons who did not die of CRC. Billing claims were used to assess exposure to previous colonoscopy. The OR for the association between complete colonoscopy and left-sided lesions was 0.33, suggesting a substantial mortality reduction. For right-sided lesions, however, the OR of 0.99 indicated virtually no mortality reduction.