Colorectal Cancer Screening (PDQ®): Screening - Health Professional Information [NCI] - Evidence of Benefit
Table 3. Randomized Controlled Screening Trials: Fecal Occult Blood Testing continued...
This difference, which was striking and unexpected, might be explained in several possible ways and has been discussed extensively. It is possible that exams were incomplete and did not reach the cecum even though they were coded as complete. It is possible that poor prep or incomplete mucosal inspection caused important lesions to be missed. It is also possible that examination was complete but some right-sided lesions simply grow rapidly and are not detected and treated by periodic colonoscopy. In other words, it is impossible to determine the reason for the dramatically different results on the right side compared to the left side, which are either the result of physician-related and patient-related factors or are the result of the biology of cancer.
Even if it is not possible to determine which reasons may be responsible for the right side versus the left side difference, the findings beg the question, "What is the degree of mortality reduction from colonoscopy?" While a figure of 90% is sometimes cited as the degree of mortality reduction, the question will not be properly answered until the European study is completed. In the meantime, the results of the study mentioned above  question what is known about the degree of CRC mortality reduction provided by colonoscopy. Until there are more reliable results from RCTs, it may be prudent to expect an approximately 60% to 70% CRC mortality reduction, based on consideration of these studies and other data.
Evidence about colorectal cancer reduction
A case-control study assessed CRC reduction (not CRC mortality reduction) in the right side versus the left side. In a population-based study from Germany, data were obtained from administrative records and medical records; 1,688 case patients (with CRC) were compared with 1,932 participants (without CRC), aged 50 years or older. Data were collected about demographics, risk factors, and previous screening examinations. According to colonoscopy records, the cecum was reached 91% of the time. Colonoscopy in the previous 10 years was associated with an OR for any CRC of 0.23, for right-sided CRC of 0.44, and for left-sided CRC of 0.16. While this study does not assess CRC mortality, the results suggest that the magnitude of the right side versus the left side difference may be smaller than previously found. It would be extremely useful to assess right side versus left side differences in a RCT.