Advantages and Limitations of the Five Screening Options

FOBT

The FOBT is the most intensively studied of the different screening options and is the only method that has been shown to be efficacious in randomized, controlled trials. The Minnesota FOBT Trial demonstrated a reduction in col-orectal cancer mortality of 33.4% and 21%, respectively, for annual and biennial FOBT screening followed by colonoscopy for anyone with a positive screening test (Bond, 2002). When the data were analyzed just for those who complied with all recommended screening, annual FOBT screening resulted in a 45% colorectal cancer mortality reduction. This is an important number because it is the benefit that clinicians can inform their patients to expect if they comply with recommended screening. Further follow-up in the Minnesota Trial also demonstrated a significant reduction in colorectal cancer incidence in those screened annually, presumably as the result of detection and resection of advanced adenomatous polyps. Although FOBT screening has been disparaged by many proponents of alternative methods, it does have a number of proven advantages. A program of annual screening, using a reasonably sensitive FOBT (ie, HemoccultSensa guaiac cards [Beckman-Coulter, Palo Alto, CA] or one of the newer immunochemical FOBTs) followed by colonoscopy for a positive result, detects most colorectal cancers and many advanced adenomas. Screening reduces both colorectal cancer mortality and incidence and is feasible, widely available, and generally acceptable to patients. Furthermore, this option of screening has a very low upfront cost. Disadvantages of FOBT screening include low sensitivity for polyps, especially smaller ones, and a relatively high false-positivity rate for advanced neoplasia. In addition, to be effective, relatively frequent screening is required.

Flexible Sigmoidoscopy

Flexible sigmoidoscopy screening also has a number of important advantages. It detects most colorectal cancers and many advanced adenomas. An analysis from the Veterans Affairs Multicenter Colonoscopy Screening Study indicated that a single screening flexible sigmoidoscopy would detect about 70 to 80% of all advanced colorectal neoplasia, provided that those who have a left-sided neoplasm detected undergo subsequent full colonoscopy (Lieberman and Weiss, 2001). Flexible sigmoidoscopy can be performed by trained, experienced examiners accurately, safely, and quickly following a simple bowel preparation. The procedure is generally well tolerated by patients, and has been shown in cohort and case-control studies to reduce mortality from colorectal cancer within its reach by 60 to 80%. These studies also indicate that the protective effect of a single examination lasts for 5 to 9 years; therefore, infrequent screening is possible.

COMBINATION FOBT PLUS FLEXIBLE SIGMOIDOSCOPY

The combination of annual FOBT screening plus flexible sigmoidoscopy every 5 years largely corrects the limitations of doing either method of screening alone. The FOBT misses many polyps and has been shown to be relatively insensitive for distal rectosigmoid cancers. When performed annually, however, it will detect most colorectal cancers before they become incurable. The flexible sigmoidoscopy is highly accurate in the high risk left colon, but will miss up to 30% of proximal advanced neoplasia in patients who do not have a synchronous distal polyp or cancer.

BARIUM ENEMA

Screening DCBE, although included in the menu of guideline options, is not used much for screening in the United States and has not been directly studied for this purpose. Furthermore, DCBE recently has been shown to be relatively insensitive for detecting advanced neoplasia. A retrospective study by Rex and colleagues (1997) showed that about 15% of colorectal cancers are missed by barium enema examination. The National Polyp Study performed back-to-back DCBE and colonoscopy on 580 patients undergoing postpolypectomy surveillance and showed that the sensitivity of this method for detecting large polyps (> 1 cm) was only 48% (Winawer et al, 2000). For these reasons, when this method is used for screening, the guidelines recommend a screening interval of 5 years.

Three-Dimensional Virtual Colonscopy

A recent New England Journal of Medicine editorial suggested that three-dimensional computed tomography (CT) scanning and reconstruction may be a consideration for screening in the near future. The article by Pickhardt and colleagues (2003) described 1233 asymptomatic adults who underwent a new sophisticated 3-dimensional virtual colonoscopy and same-day conventional colonoscopy. More than 97% were at average risk for colorectal neoplasmia. The sensitivity and specificity of virtual colonoscopy for adenomatous polyps was comparable to standard colonoscopy, 94% and 96% respectively for adenomatous polyps > 10 mm on virtual colonoscopy. The sensitivity for polyps at least 6 mm was 88.7%. Only two cancers were found, both on virtual colonoscopy, and only one was found on standard colonoscopy until results of the virtual colonoscopy were revealed. Although this study should be repeated to verify the results, its findings appear to be a breakthrough in the use of virtual three-dimensional colonoscopy. As the editorial asks: Is it ready for prime time?

Colonscopy Screening

Increasingly in the United States, direct colonoscopy screening has become the overwhelming preference of gastroen-terologists and many others. In the broad area of preventive screening, this option is somewhat of a perturbation ofthe classic definition of a screening test. Instead of performing a simple, acceptable, inexpensive and indirect test to identify those in the healthy at-risk population who might benefit from further examination, we are substituting upfront a highly definitive, complex, expensive and somewhat invasive, diagnostic and therapeutic method. Direct screening colonoscopy, however, is now being increasingly championed by physician and patient groups because it detects almost all cancers and advanced adenomas, and it allows for resection of most polyps during a single sitting. Thus it is the most effective way of achieving both the major goals of colorectal cancer screening—cancer prevention through polypectomy and reduced mortality through the detection of early cancers. Because of colonoscopy's great accuracy and the relatively long natural history of the adenoma-carcinoma sequence, infrequent screening (every 10 years) is possible. The VA Multicenter Colonoscopy Screening Study demonstrated that, when performed by well-trained experienced colonoscopists, colonoscopy screening is feasible and very safe (Nelson et al, 2002).

Although there are no randomized controlled trials of screening colonoscopy, compelling indirect evidence suggests that this approach is very effective at reducing both the incidence and mortality of colorectal cancer. For example, colonoscopy and polypectomy in the National Polyp Study cohort reduced colorectal cancer incidence by up to 90%; there are a number of supportive case-control studies of both flexible sigmoidoscopy and colonoscopy, and the FOBT trials effected their demonstrated reduction of cancer incidence and mortality by doing colonoscopy on those with a positive screen. Limitations of direct screening colonoscopy that have not yet been satisfactorily addressed include questions of risk, cost, patient accept ability, and capacity. Conscious sedation usually is required with its attendant risk, cost, and inconvenience. A screen requires the better part of 2 days to complete the bowel purging preparation, the examination, and recovery. Although screening colonoscopy has been shown to be safe when performed by experienced physicians, I still have concerns about both the accuracy and risk of this option when it is carried out in increasing numbers by less experienced examiners. Last, the great demand for screening colonoscopy already shows signs of overwhelming the capacity to perform these additional examinations. In some parts of the country, long waiting times to have a screening colonoscopy may be diminishing the attractiveness or practicality of this option.

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Constipation Prescription

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