Colorectal Cancer Screening (PDQ®): Screening - Health Professional Information [NCI] - Evidence of Benefit
Table 3. Randomized Controlled Screening Trials: Fecal Occult Blood Testing continued...
Detection rates in colonoscopy screening vary with the rate at which the endoscopist examines the colon while withdrawing the scope. Detection rates among gastroenterologists (mean number of lesions per patient screened, 0.10–1.05; range of the percentage of patients with adenomas, 9.4%–23.7%) and the times to withdraw (3.1–16.8 minutes for procedures not including polyp removal). Examiners whose mean withdrawal time was 6 minutes or more had higher detection rates than those with mean withdrawal times of less than 6 minutes (28.3% vs. 11.8%; P < .001 for any neoplasia) and (6.4% vs. 2.6%; P < .005 for advanced neoplasia). Overall detection rate of adenomas and cancer may be affected by how thoroughly endoscopists search for flat adenomas and flat cancer. While the phenomenon of flat neoplasms has been appreciated for years in Japan, it has more recently been described in the United States. In a study in which endoscopists used high-resolution white-light endoscopes, flat or nonpolypoid lesions were found to account for only 11% of all superficial colon lesions, but they were about 9.8 times as likely to contain cancer (in situ neoplasia or invasive cancer) compared with polypoid lesions. However, because the definition of flat or nonpolypoid was height less than one-half of the diameter, it is likely that many lesions classified as nonpolypoid in this study would be routinely found and described by U.S. endoscopists as sessile. At the same time, the existence of very flat or depressed lesions (depressed lesions are very uncommon but are highly likely to contain cancer) means that endoscopists will want to pay increasing attention to this problem. Flat lesions may play a role in the phenomenon of missed cancers.
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.