Screening for Colorectal Cancer in 2006 -Advantages and Limitations of the Different Screening Options
John H Bond, MD Chief, Gastroenterology Section, Minneapolis Veterans Affairs Medical Center and
Professor of Medicine, University of Minnesota
The Combination of Annual FOBT and FS Every Five Years
Although this option has not been directly studied, indirect evidence suggests it is a highly effective screening approach. FOBT screening is insensitive for detecting small polyps and distal cancers, while FS is highly accurate for diagnosing all neoplasia located in the high-risk left colon. FS when done alone, however, misses about 30% of neoplasia located in the right colon beyond its reach. Such lesions, as they advance, will likely be detected by a program of annual FOBTs before they become incurable. Although this approach is complicated and frequent screening is required, it largely corrects the limitations of performing either FOBT or FS screening alone.
Double-contrast Barium Enema Every Five Years
Barium enema is not used much for population-based CRC screening in the US, and there are no direct studies demonstrating efficacy. In addition, barium enema examinations are substantially less sensitive and specific than colonoscopy for detecting neoplasia. The US National Polyp Study performed a single-blinded comparison of double-contrast barium enema and colonoscopy performed back-to-back in the same 580 patients that showed that the sensitivity of the barium enema for detecting polyps >1cm was only 48%. A large retrospective study of patients with CRC in Indiana indicated that the sensitivity of barium enema for detecting CRCs was only 83% (versus 95% for colonoscopy).
Direct Colonoscopy Screening Every 10 Years
Most gastroenterologists and many patients now prefer the option of direct screening with colonoscopy because it is clearly the most accurate way to accomplish with a single test both of the major objectives of screening. Unfortunately, there are no randomized, controlled trials of direct screening colonoscopy proving efficacy. All of the scientific evidence that supports this option, although compelling, are indirect. The National Polyp Study showed that colonoscopy reduced the incidence of metachronous CRC in adenoma-bearing patients by 76–90%. Case-control studies suggest that screening colonoscopy or FS reduces mortality from cancers located in the examined colon by 50–85%. Lastly, CRC mortality and incidence reductions in the large FOBT trials was largely due to colonoscopy performed in subjects with a positive screening test.
Advantages of screening colonoscopy, in addition to its accuracy and efficacy, are that infrequent (every 10 years) screening is recommended, and the examination is both diagnostic and therapeutic at a single sitting with a single bowel-cleansing preparation. When performed by experienced, well-trained endoscopists, screening colonoscopy is feasible and has an acceptable safety record. The large VA Multicenter Colonoscopy Screening Study performed screening colonoscopy in 3,196 asymptomatic volunteers. Colonoscopy was complete to the cecum in 97.7% of cases and the incidence of major complications (mainly bleeding after polypectomy) was only 0.3%. In patients who did not have polyps, there was only one major complication (a cardiovascular event) in 1,492 cases.
Disadvantages of direct colonoscopy screening that still need attention include questions of patient acceptance and colonoscopy capacity. Many healthy people are reluctant to endure the direct and indirect costs of colonoscopy. While the risk is small, complications (especially perforation) may be very serious for a test that detects an advanced neoplasm in only 6–10% of patients. The bowel preparation, sedation, procedure, and recovery time require up to two days lost from normal activity. Lastly, capacity to perform screening colonoscopy on all average-risk people over the age of 50 likely is insufficient, and many endoscopy clinics are experiencing long waiting times for patients to undergo recommended screening. A study by the Centers for Disease Control and Prevention (CDC) reported that the capacity to conduct direct colonoscopy screening of the 41 million eligible Americans who have not yet been screened may be severely lacking. If half the currently available US colonoscopy capacity were dedicated completely just to screening, it would take 10 years to complete the task and reach a steady state.
