Colon and Rectal Cancers

Description

Alternative Names

Diagnosis

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.

There are 3 basic tests for colon cancer: a stool test (to check for blood); sigmoidoscopy (inspection of the lower colon; and colonoscopy (inspection of the entire colon). All 3 are effective in catching cancers in the early stages, when treatment is most beneficial.

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:

• The levels of iron in the blood can affects results. Patients should not take iron supplements or eat red meats several days before the test.
• Certain raw fruits and vegetables, including cauliflower, horseradish, radishes, melons, and turnips, that contain the chemical peroxidase can cause a positive test reaction even if no blood is present.
• Aspirin and other NSAIDs can cause minor bleeding and should not be taken for a week before the test.
• Vitamin C and foods rich in this vitamin may cause a false negative reaction and should be avoided a few days before the test.
• Bleeding from other causes, such as menstruation, hemorrhoids, gingivitis, or urinary infections, can produce blood in the stools and affect results.

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 can only view the rectum and the left side of the colon, while colonoscopy and barium enemas allow a view of the entire large intestine.
• Both flexible sigmoidoscopy and colonoscopy involve snaking a fiber optic tube through regions of the rectum and colon to view the walls of the intestine. The tube contains a tiny camera that transmits the image to a video screen. The use of an ultrasound (sound wave) scanner is proving to enhance viewing quality. Barium enemas simply use x-rays.
• During either sigmoidoscopy or colonoscopy, the physician is able to remove polyps or other abnormalities revealed by these procedures with surgical instruments inserted through the tube. It is not possible to remove polyps with a barium enema, which is not invasive.

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.

• The procedure employs a flexible fiberoptic tube (it is thus referred to as flexible sigmoidoscopy) that contains a tiny camera and surgical instruments.
• It lasts about 10 minutes and may be mildly uncomfortable, but it is not painful and is generally very safe. In one study, 70% of patients reported that the procedure was far less unpleasant than they had expected.

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.

• For about a day before the procedure the patient eats nothing and drinks a laxative solution that cleans out the colon. The taste of the solution is unpleasant, although it has improved in recent years.
• The procedure typically uses a sedative that produces a twilight sleep and often makes the procedure more comfortable than sigmoidoscopy.
• Air may be introduced into the intestine to widen it and allow the tube to navigate curves. A colonoscopy avoids the risk of radiation associated with a barium enema, but it is important to note that even a colonoscopy does not detect all cancers.

Complications are rare, but include the following:

• Hyponatremia. Hyponatremia is a low concentration of sodium in the blood. The complication may be caused by the effects of bowel cleaning before the procedure that can result in water retention and reductions in sodium. When severe, it can cause temporary neurological symptoms, such as confusion, lethargy, unsteadiness, and slurred speech. Researchers suggest that sodium concentrations be measured in patients who develop such symptoms after colonoscopy.
• Bowel perforation (very low risk, about two cases per 1,000 procedures).

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.

The barium enema is a valuable diagnostic tool that helps detect abnormalities in the large intestine (colon). The barium enema, along with colonoscopy, remain standards in the diagnosis of colon cancer, ulcerative colitis, and other diseases of the colon.

Genetic Screening

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.