Traditional Methods of Colorectal Screening: Advantages, Problems, and Comparisons
Screening for colon cancer decreases both the incidence and the death rate of the disease by detecting and leading to the removal of precancerous adenomatous polyps (growths that develop on the membrane of the large intestine) before they become invasive cancers (1-8). Health care providers and policy makers agree that screening for colorectal cancer is worthwhile (1). The current options available for colorectal cancer screening include digital rectal examination, fecal occult blood testing, barium enema, sigmoidoscopy, fiberoptic colonoscopy, or any combination of these procedures (8). Following is a brief description of current colorectal screening methods.
Digital rectal examination: The physician inserts a finger into the rectum to feel for polyps or other abnormalities.
Fecal occult blood testing (FOBT): A sample of feces is tested for microscopic amounts of blood. This could be an indication of a bleeding tumor, although there are many other causes of blood in the stool as well.
Barium enema: A suspension of barium sulfate is injected through the anus, which coats the rectum and colon and makes these areas appear opaque on an x-ray. When an x-ray is subsequently taken, abnormalities can be seen. In a double contrast barium enema, air is pumped into the rectum after the initial x-ray and the x-ray is repeated.
Sigmoidoscopy: In this procedure a sigmoidoscope, which is a thin, lighted tube with a camera at the end, is inserted into the rectum and guided into the sigmoid colon (the lower portion of the colon). This test reveals polyps, tumors, and other abnormalities in the rectum and sigmoid colon.
Fiberoptic colonoscopy: In this test, which requires sedation, the lining of the entire colon and rectum is examined. Like a sigmoidoscopy, the test is performed with a thin, lighted tube with a camera attached, but in a colonoscopy, the tube is guided further into the colon to visualize the entire colon. The colonoscope pumps air into the colon, while the video camera records the images on a screen for the doctor to see during the procedure. If polyps or other abnormal tissue are discovered, they can be removed and biopsied.
The following strategies have been recommended by The United States Agency for Health Care Policy and Research for colon screening in the average risk patient (3). Fecal occult blood testing (FOBT) is recommended every year beginning at age 50. If this test is positive at any year, a diagnostic exam should be performed including either colonoscopy or double contrast barium enema (possibly in combination with sigmoidoscopy) (9). For individuals with no symptoms, either flexible sigmoidoscopy or double contrast barium enema should be performed every five years or colonoscopy every 10 years for the detection of non-bleeding polyps and tumors (3, 4).
Despite consensus on the need for and efficacy of screening, there consistently are approximately 60,000 deaths from colon cancer every year in the United States (2). Since most colorectal cancers grow slowly from precancerous adenomas, and screening can detect the precancerous adenomas, the continued high prevalence of colon cancer is alarming. There are many potential reasons for the high rate of colon cancer, including limitations of current screening options, confusion about when and how to perform current screening options, and patient reluctance to undergo screening.
Regarding patient reluctance to comply with current screening options, a survey found that only 17.3 percent of patients over age 50 had undergone fecal occult blood testing within the last year and only 9.4 percent had undergone sigmoidoscopy within the last three years (9). Studies have also demonstrated that even health care professionals are reluctant to undergo colon screening (10). The Centers for Disease Control has set goals and committed funding to decrease the number of deaths from colorectal cancer. Among the strategic goals is to increase the number of patients accessing colorectal cancer screening from the current levels of approximately 37 percent to 50 percent by the year 2010 (http://www.cdc.gov/cancer/screenforlife/index.htm).
Each current colon screening option has important limitations. While the performance of yearly FOBT has demonstrated a mortality reduction from colorectal cancer, FOBT does not directly evaluate mucous membrane of the colon. (11). Many large adenomatous polyps do not bleed and occasionally cancers will not bleed. In addition, there are many false positive fecal occult blood tests for colon cancer, which can lead to further testing and expense. A study demonstrated that in more than 50 percent of stool examinations in which blood is found, the source was from the upper gastrointestinal tract (12).
Screening sigmoidoscopy has been shown to decrease the death rate of colorectal cancer (13). However, sigmoidoscopy fails to evaluate the entire colon and therefore complete colon screening is not obtained (14, 15). Recent studies evaluating sigmoidoscopy and colonoscopy found that if only sigmoidoscopy were performed for colon screening in people with no symptoms, many advanced cancers in the upper colon would be missed (14, 15). This is true even taking into account the fact that if a significant lesion were detected in the part of the colon examined during sigmoidoscopy, it would prompt a complete colon examination with colonoscopy. In fact, half of the cases of upper-colon cancers did not have a lower-colon polyp which would have prompted colonoscopy. Moreover, it appears that the combination of FOBT and sigmoidoscopy do not result in a significant improvement in the efficacy of screening (16).
There are currently two options for a full colonic evaluation: colonoscopy and double contrast barium enema (DCBE). The sensitivity of the double contrast barium enema for polyp detection is incompletely documented. Norfleet et al determined that the sensitivity of DCBE was 26 percent in detecting polyps larger than 5 mm as compared to 13 percent for single contrast barium enema (17). Importantly, a recent study comparing double contrast barium enema with colonoscopy in detecting polyps in patients who previously had a polyp removed demonstrated poor sensitivity of the double contrast barium enema. In this study the barium enema missed over 50 percent of polyps greater than 1 cm in diameter (18).
Of the numerous options for colon cancer screening, complete fiberoptic colonoscopy allows the most thorough evaluation of the colon with the added benefit that suspicious lesions can be excised during the exam and then biopsied. Colonoscopy is considered the "gold standard" for colonic evaluation (1,19). Yet there are many limitations to the widespread use of colonoscopy for screening, including examination time, need for sedation, potential risk of perforation and bleeding, costs of the procedure including the need for sedation, failure to complete the examination in 5-10 percent of patients, and an insufficient workforce of trained endoscopists (physicians trained to perform internal visualization procedures such as colonoscopies) to meet the increased demand (20, 21).
A final limitation of conventional fiberoptic colonoscopy is that in order to perform the examination, the colon must be thoroughly cleansed of residual fecal material. This is typically performed with polyethylene-glycol solutions or phospho-soda preparations. Many patients find bowel cleansing the most difficult aspect of screening, whether sigmoidoscopy, DCBE, or traditional colonoscopy, or virtual colonoscopy is used (22, 23, 24).
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