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Colon Cancer
  Screening Procedures

By Miriam Komaromy, MD

Reviewed by Peggy Conrad, MS, CGC and Jonathan Terdiman, MD


 

There are four commonly recommended procedures for both preventing and detecting colon cancer: (For recent news about upcoming techniques for detecting colon cancer, see Related News below.)

 

 
 
 

Fecal Occult Blood Testing (FOBT)

This procedure, which is also referred to as stool guaiac testing after the chemical that is used in the test, is employed to detect microscopic amounts of blood in the stool, which may indicate a bleeding polyp or cancer in the colon. Since this test is usually performed on three successive stool samples, your doctor will likely give you a kit to collect the samples at home and then return them to his or her office. The test can show blood in your stool if you've ingested red meat prior to obtaining the samples, so it's important that you follow your doctor's diet instructions prior to taking the test. If one or more of the samples tests positive for blood, you should undergo further testing (colonoscopy) to find out where the blood is coming from as well as to rule out the presence of a polyp or cancer.

Regular (yearly) use of FOTB has been proven to reduce colon cancer death by as much as 30 percent.
Regular (yearly) use of FOBT has been proven to reduce colon cancer death by as much as 30 percent. It is also the only test that has been shown to actually prevent colon cancer from developing in the first place. However, it's important to understand that not all polyps or other types of malignant growths will bleed. Therefore, this test may produce normal results even if you have colon cancer. Because of this, doctors often will not rely on this test alone to eliminate the possibility of cancer in a particular patient, especially if there are other symptoms or significant risk factors. Most commonly, FOBT is performed every year and combined with flexible sigmoidoscopy (see below) performed every five years. If both tests are regularly used, the chance of colon cancer being detected at a treatable stage is much greater than if either test were relied upon alone.

For news about the effectiveness of fecal occult blood testing, see Related News below.

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Colonoscopy

This procedure, which is typically performed by a gastroenterologist, is designed to examine the entire length of the colon using a long, thin, flexible tube.
By directly visualizing the inside of the colon, the gastroenter-ologist can detect and remove polyps.
The tube has a light at the end and is connected to a video monitor. By directly visualizing the inside of the colon, the gastroenterologist can detect and remove polyps (mushroom-shaped growths that may have the potential to become cancerous) as well as other types of cancerous growths or abnormalities.

If your physician recommends that you undergo a colonoscopy, he or she will instruct you to first "prep" your colon by cleaning it out the day before the procedure. Usually the prep requires that you adhere to a liquid diet the day before your procedure, and drink a special cleansing solution the evening before the exam. The cleansing solution allows you to pass all of the contents of your colon before the exam (you'll spend a few hours in the bathroom), so that the colon is completely empty at the time of the exam.

You should be aware that different doctors may have different "prep" instructions for you to follow, but their goal is the same: to empty your colon (as much as possible) before they begin the colonoscopy procedure.

When you arrive for your colonoscopy, you will be given medication (a sedative and pain medication) to make you comfortable during the examination. Your doctor will insert in your anus a thin, flexible tube with a light and a viewing camera at one end. He or she will then advance the tube through your rectum and colon to the place where your large bowel ends and your small bowel begins. At that point, he or she will begin slowly pulling back the tube in order to look for abnormalities on the inside walls of the colon. Your doctor will also inflate your bowel slightly (by pumping air into it) to make viewing easier.

Patients are usually sedated but awake during the procedure, and typically experience little pain. The major risk of this procedure is perforation of the bowel wall, which is very rare (occurs in less than 1 of every 1,000 procedures). If your doctor finds a polyp during the procedure, he or she can remove it using the same tube that was employed to examine the colon. If any other type of abnormal growth is found in the colon, your physician can biopsy it to determine whether it is cancerous. (For recent news about risk of perforation, see Related News below.)

Regular screen by colonoscopy probably has the capacity to reduce the rate of colon cancer by 70 to 90 percent in the general population.
Regular screening by colonoscopy probably has the capacity to reduce the rate of colon cancer by 70 to 90 percent in the general population. Indeed, many experts believe that colonoscopy screening may prove to be the best strategy to prevent colon cancer. However, many practical barriers remain to routinely screening the entire adult population — most prominently, the high cost and inconvenience — with this test. Colonoscopy is therefore not yet widely used as a routine screening test for colon cancer but rather as a means of providing a more complete evaluation of the colon if other tests that screen for polyps (FOBT, sigmoidoscopy, barium enema) have come back positive.

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Sigmoidoscopy

Sigmoidoscopy, which is also called flexible sigmoidoscopy to distinguish it from the older and more painful rigid sigmoidoscopy, is similar to colonoscopy. However, it only explores the lower third to half of the colon. Even though this procedure is a simpler undertaking than colonoscopy, patients are more likely to experience pain or discomfort — mostly commonly gaslike pains that stem from the air used to inflate the bowel — since they are rarely given an intravenous sedation for the procedure. As is the case with colonoscopy, the procedure carries with it the risk of bowel perforation, though it is even less common with sigmoidoscopy than it is with colonoscopy.

Sigmoidoscopy is an easier procedure than colonoscopy, but only detects polyps in the lower part of the colon.
A sigmoidoscopy can be more quickly and easily performed in a doctor's own office and is therefore less expensive than full colonoscopy. For these reasons and more, a sigmoidoscopy is more commonly used than colonoscopy to look for signs of colon cancer. The drawback is that it cannot detect polyps or cancers that are located deep within the colon beyond the reach of the sigmoidoscope — an important consideration for patients who have been diagnosed with hereditary nonpolyposis colorectal cancer, or HNPCC, since the polyps associated with this syndrome typically appear higher up in the colon. (In other types of colon cancer, polyps and cancers most often occur in the lower third of the colon.)

Sigmoidoscopy reduces the rates of colon cancer by 60 to 80 percent for a patient who has polyps detectable within the scope of its diagnostic reach. The problem, of course, is that it will not reduce rates of colon cancer for people who have polyps that have developed beyond the reach of this scope.

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Double-Contrast Barium Enema

This procedure, which is typically performed by a radiologist and does not involve sedation, is designed to look for polyps, cancers, and other abnormalities in your large bowel (colon). If you undergo this procedure, the radiologist will start by giving you a barium enema — white liquid that's pumped into the colon through a thin tube inserted into the rectum. (The barium coats the inside of the colon so that it is visible on x-ray.) Next, the radiologist will pump air into your rectum to separate the walls of the colon and allow any polyps or masses to be seen on x-ray. Finally, he or she is ready to perform the x-ray.

The major limitation of a double-contrast barium enema, or DCBE, is that it doesn't allow your doctor to remove or perform a biopsy on any detected polyps or masses directly. Instead, it requires a follow-up colonoscopy. What's more, it doesn't always do a good job of evaluating the lowest part of your colon — the lower part of the recto-sigmoid colon — which means that you may have to undergo a sigmoidoscopy anyway once the DCBE has been completed. Barium enema is good at detecting cancers and large polyps but will miss a substantial proportion of small and medium-size polyps. As a result, barium enema is rarely used these days as an effective screening procedure for colon cancer.

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Related News
In order to view these articles you will need to have a MyGeneticHealth account. If you are not already a member, selecting the article will automatically take you to a page where you can sign up.
New colorectal cancer screen wins FDA premarket clearance
Stool assay for altered DNA may revolutionize screening for colorectal cancers
Colonoscopy perforation risk factors identified

References

Selby, J. et al. (1992). A case-control study of screening sigmoidoscopy and mortality from colorectal cancer. New England Journal of Medicine. 326(10): 700-2.

Mandel, J. et al. (1993). Reducing mortality from colorectal cancer by screening for fecal occult blood. Minnesota Colon Cancer Control Study. New England Journal of Medicine. 328(19): 1365-71.

Mandel, J. et al. (2000) The effect of fecal occult blood screening on the incidence of colorectal cancer. New England Journal of Medicine. 343(22):1603-1607.

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