Colon and Rectal Cancers
DescriptionAn in-depth report on the causes, diagnosis, treatment, and prevention of colorectal cancer.
Alternative NamesRectal Cancer
Colon and rectal cancers are diagnosed using the screening tests discussed below. These tests can detect precancerous polyps and colorectal cancers at stages early enough for complete removal and cure.
Unfortunately, only 30% to 40% of adults over 50 years old (mostly in the upper socioeconomic group) have regular screening tests that could detect a cancer early enough for curative treatment. A survey reported that many people are not screened because they are too embarrassed and revealed that they would rather lose months off their life than face these tests. Those who had already had the tests were willing to have them again if they saved one additional day of their lives. There is some debate about what is the best screening modality. However almost all experts agree that not enough people are screened and that if these tests were adopted with the same regularity as other screening tests, such as Pap smears, they would save many lives. It is especially important that anyone at increased risk or with symptoms, such as rectal bleeding, undergo testing.
Digital Rectal Examination (DRE)
The digital rectal examination is used to detect tumors in the rectum, lower intestine, and prostate gland. The doctor inserts a lubricated-gloved finger into the patients rectum and feels for lumps or other abnormalities. The exam is quick and painless but embarrassing for some. Fewer than 10% of colon cancers develop within the region that can be evaluated by a DRE, so it is not useful as a sole screening test.
Fecal Occult Blood Test (FOBT)
Blood in bowel movements is not always visible, in which case it is called occult blood. Fecal occult blood tests (FOBTs) are used to detect this hidden blood. The most common FOBT method is called the guaiac-based test. The patient is asked to supply up to six stool specimens in a specially prepared package. A small quantity of feces is smeared on specially treated paper, which reacts to hydrogen peroxide. If blood is present, the paper turns blue.
Accuracy. FOBTs can miss more than 75% of advanced cancers. Nevertheless, large studies have indicated that this simple test, performed annually, saves lives and may reduce the risk of dying from colon cancer by 15% to 33%. The following may affect its accuracy:
Even if none of these conditions is present, a test that shows hidden (occult) blood does not necessarily mean that cancer is present. About 20% to 30% of people with occult blood have noncancerous polyps or other conditions, such as gastritis, and only 5% to 10% actually have cancer. Any abnormal result, however, requires further testing, such as colonoscopy.
Lack of Compliance. Compliance is a major problem. Patients are asked to perform the tests at home and send the test cards to the laboratory; only 35% to 50% of patients actually follow through. Occult-blood tests that give results at home are available but are extremely inaccurate. In one large study, these tests failed to detect advanced cancer in about 62% of cases, although they may detect some early cancers.
Visualizing the Colon: Colonoscopy, Sigmoidoscopy, and Barium Enema
If a digital rectal exam (DRE) or fecal occult blood test (FOBT) shows signs of trouble, several methods to visualize the colon are available. They include colonoscopy, sigmoidoscopy, and double-contrast barium enema. They have the following similarities and differences:
Sigmoidoscopy. Sigmoidoscopy examines the rectum and the lower two feet of the colon. It cannot, however, detect the roughly half of cancers that occur in the right colon. Right-sided cancers are more common in older people.
This procedure has been found to reduce the risk of fatal cancers in the rectal and sigmoid area by 60%. If polyps are detected, a colonoscopy is then used.
Colonoscopy. Colonoscopy is the most accurate testing method and can reduce cancer incidence by up to 90%. It is clearly indicated for anyone with an increased risk for colorectal cancer, including those with a personal or family history of the disease. As with sigmoidoscopy, a colonoscopy uses a flexible tube but it is snaked through the entire large intestine.
Complications are rare, but include the following:
Barium Enema. The double-contrast barium enema, which uses an x-ray image, is the less expensive alternative for viewing the entire colon. It is not as accurate as colonoscopy, and if any polyps or abnormalities are revealed on x-ray, a colonoscopy is then required to remove suspicious tissue, so it is now recommended much less often than in the past.
Screening for FAP. Genetic screening for familial adenomatous polyposis (FAP) and hereditary nonpolyposis colon cancer (HNPCC) is now available and may be recommended for high-risk patients. The test for FAP detects a mutation in the APC (adenomatous polyposis coli) in up to 90% of people who carry it. Testing for HNPCC mutation is somewhat more complex.
Screening for ICF-2. A gene that regulates insulin-like growth factor (IGF-2) is functional during fetal development and then becomes inactive. Some evidence now suggests that people who have IGF-2 in adulthood have a higher risk for colon cancer. Blood tests for detecting IGF-2, then, may be helpful in identifying patient who should have more intensive screening. Currently, however, this is only used as a research tool.
Stool DNA Testing.A promising technique for colorectal cancer screening is the detection of altered DNA in cancer cells that have shed from the colon and are excreted in the stool. Such tests may prove to detect both inherited and noninherited genetic mutations. This may become a widely used tool in the future; however, larger clinical studies are needed.
Experimental Screening and Diagnostic Methods
Virtual Colonoscopy. A promising experimental technique called virtual colonoscopy allows three-dimensional imaging of the colon without using invasive instruments. As with standard colonoscopy, the patient takes a laxative first to clear out the intestine. The procedure itself involves pumping air into the colon and scanning the intestine using computed tomography (CT). It is very safe and takes only about 10 minutes. The procedure is similar in accuracy to conventional colonoscopy for detection of larger polyps (6 mm or more in diameter) and is also potentially less expensive. Colonoscopy is required, however, if suspicious areas are found, which may occur frequently with the CT procedure, since it erroneously identifies a high number of nonexistent polyps.
A study published in April 2004 in the Journal of the American Medical Association (JAMA) compared results of standard colonoscopy versus virtual colonoscopy in over 600 patients at nine major medical centers. Virtual colonoscopy had much lower rates of successfully finding polyps than standard colonoscopy. Virtual colonoscopy detected polyps of at least 6 mm in 39% of patients and polyps of at least 10 mm in 55% of patients. By contrast, standard colonoscopy detected 99% of polyps of at least 6 mm, and 100% of polyps of at least 10 mm. In addition, accuracy rates varied widely among the different hospitals. The authors advised that until more improvement in training and technique is achieved, virtual colonoscopy "is not yet ready for widespread clinical application."
Magnetic Resonance Colonography (MRC). Magnetic resonance colonography (MRC) is another non-invasive technique for visualizing the colon. The patient receives an enema containing a contrast agent, and then magnetic resonance images are taken. MRC is fast, comfortable, and less invasive than colonoscopy. Currently, however, there is a poor detection rate for flat tumors and for polyp tumors less than 10 mm in diameter.
Encapsulated Video Camera. Researchers have developed a video camera that is small enough to be swallowed. It works its way through the digestive tract, beaming data to a receiver worn on the patients waist, and is excreted in eight to 72 hours. The camera was not designed to replace standard visualization procedures and is currently being used to assess problems in the hard-to-reach small intestine. More testing is needed to determine whether it has value in colon cancer screening as well.