DIAGNOSIS OF COLON CANCER

colon-cancer-diagnosis-1This page discusses the various tests which may be used to identify problems in the colon, such as polyps, or cancer, or other abnormalities of the lining of the colon.  Colonoscopy is often the test of choice, since so much information can be gained from it.  Other tests are still used depending on the circumstances, and these are reviewed as well.

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colon-cancer-diagnosis-2For any of the symptoms discussed on the “Symptoms” page, and for screening purposes as well, a colonoscopy is often a part of the evaluation process.  Your doctor will gather pertinent information from you to help decide what is the most likely explanation for your symptoms.  Depending on your age, your specific symptoms, duration of symptoms, time since any previous colonoscopy, and any pertinent family history, among other things, your doctor may recommend a colonoscopy.  This procedure involves the use of a long, thin, flexible tube with a bright light on the end, optical fibers, which transmit an image from the tip of the scope to a video output, and a channel for passing small biopsy devices to the tip.  In most cases, the source of your symptoms can be identified.  Any polyps (which could transform into cancer in the future if not removed) or cancers in the colon are almost always identified with this procedure.

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Bowel Preparation for Colonoscopy

In order to have a clear view of the inner lining of the colon, your doctor will give you a “bowel prep”, which is one of a variety of recipes which will wash out your colon over a day or so.  You will need to take only clear liquids by mouth during the bowel prep.  If there is any stool remaining in your colon, some of the colon lining may be obscured, so it is important to make sure to follow the instructions for the bowel prep carefully.

There are a few different options for bowel preparation, varying in duration, potential side effects, and cost.  Most physicians have a preference based on their own experience.  The first priority is to use a prep that is safe for you, and all of the options usually are.  If you have any chronic illnesses such as congestive heart failure, diabetes, or kidney disease, special consideration should be given to the ideal prep.  Whichever is selected, the goal is to have a completely clean colon so that all of the lining can be clearly seen.  If there is some stool remaining, it can obscure some of the colon, and polyps or other pathology might be hidden from view.

The procedure requires some sedation, almost always given by vein, so you will have a small plastic tubing inserted into a vein in your arm just before the procedure.  You will not be able to drive just after the procedure, so you will need to have someone with you to drive you home afterward.  It usually only takes about 15 minutes.  If any biopsies are needed, the results from the biopsy usually take a few days to get.  But if the doctor sees a cancer, they may know what the biopsy is going to show, since some cancers have such an obvious appearance.

Screening

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Since colon cancer in its early stages is usually without symptoms, we need to look for it and find it before it advances to a symptomatic stage.  A number of different options exist to do this.  We primarily rely on colonoscopy to screen for polyps and colon cancer currently, though there are some other options available.

As you are probably aware, there are a variety of screening tests recommended by physicians for the general population, such as mammography for women over age 40, skin exams to look for melanoma and other skin cancers, Pap smears for women, looking for female cancers, and colonoscopy for colon polyp and cancer screening.  There are many factors taken into account when making a recommendation for a screening test.  These factors include: accuracy of a “positive” result, accuracy of a “normal” result, cost, availability, convenience, and risk, among others.  A test may be extremely sensitive at detecting the condition being screened for, but if the test is too expensive, its use as a screening measure is limited.  On the other hand, a test may be very simple and inexpensive, but it may not be accurate.  There is no ideal screening test for colon cancer.  Colonoscopy is considered the most sensitive and accurate test, and has the advantage of allowing biopsies of suspicious areas.  But it is relatively expensive, and inconvenient in comparison with the other options. Currently in the US, this is still considered by most experts to be the best choice.

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Barium Enema

Occasionally, colonoscopy cannot be completed safely, and in these cases, other methods might be considered.  One method, called a barium enema, involves the instillation of barium through the rectum, so that it backfills the colon all the way around to the small intestine, while the abdomen is being viewed with real time x-ray, called fluoroscopy.  Although the colon is not directly visualized, the outline of the inner wall of the colon can be seen, allowing medium to large polyps, cancers, and other abnormalities of the colon to be seen.  This test is done much less often these days, but there are advantages in certain cases.

Rigid sigmoidoscopy

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There are other “scopes” that can be used to look at the inside of the colon.  A 12” inch hollow tube with a light and insufflation (pumping air in), called a rigid sigmoidoscope, gives visualization of the last segment of colon, primarily the rectum.  As you might imagine, it can be somewhat uncomfortable to insert this device into the rectum, and these days it is only occasionally used for screening for cancer.  It does play an important role for surgeons in the context of planning surgery for a cancer very close to the anus.  This device is the best for determining accurately precisely how close to the anus a cancer might be, information that is crucial for planning surgery for a rectal cancer (one which is close to the anus).

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Flexible Sigmoidoscopy

There is also a shorter flexible scope, called a flexible sigmoidoscope, which allows visualization of the last 3-4 feet of the colon.  In principle, it works just like the longer colonoscope.  Typically when this scope is used, no anesthesia or sedation is given to the patient.  For this reason, it actually may be a more uncomfortable procedure to go through than a complete colonoscopy.  The main advantage has to do with the fact that there are many more physicians who have the expertise to this procedure, and the cost is much less.

Virtual Colonoscopy

Recently, a new technique has been developed for colon screening, called virtual colonoscopy.  This test is not widely available, and there is no consensus on whether it is a reasonable substitute for performing a colonoscopy.  The test requires a bowel prep, just like for colonoscopy.  Then a CAT scan is done while at the same time, dye is inserted into the colon as an enema, using a flexible tube inserted through the anus.  A special software package is required to process the images taken, which reconstructs images to give a virtual three dimensional view of the inside of the colon.  This allows polyps to be seen almost as if one is undergoing a colonoscopy.

The test takes about 30 minutes, and does not require any sedation.  This makes it less inconvenient, since you don’t need to have someone else accompany you, and you could return to all your normal activities almost immediately. The test is less sensitive in seeing smaller polyps, and for this reason, if one relies on this test instead of colonoscopy, the test should be repeated every five years.  If you have a regular colonoscopy, and no polyps are seen, it usually does not need to be repeated for ten years.  The other drawback is, if significant polyps are seen, then you will need to undergo another bowel prep and a regular colonoscopy in order to have the polyp removed.  Polyps are seen in about 10% of patients who undergo screening colonoscopy.

It is likely that there will be continued refinements to this screening technique, but at present it is not considered to be the best screening test for the general population.

Fecal Occult Blood Testing (FOBT)/ Fecal DNA

One of the oldest screening tests for colon cancer is called the “fecal occult blood test”.  This basically means that a sample of stool is tested with a chemical which can detect the presence of microscopic amounts of blood in the stool.  As discussed on the “Symptoms” page, the presence of blood in the stool, whether visible to the naked eye, or in microscopic amounts, can be an early sign of a colon cancer.  Before colonoscopy was so readily available, this test was relied on more heavily to decide which patients should undergo more screening.   It is still considered part of a routine physical examination, though it has less importance these days.

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Fecal DNA

There is another test that can be done using a sample of stool, which has become available more recently.  This test involves the identification of DNA material in the stool that gives evidence of cancer cells somewhere in the intestinal tract.  The lining cells of the intestinal tract are constantly dividing, and being replaced by newer cells.  Older cells are sloughed off and excreted within the stool.  If there are any cancer cells somewhere in the colon, these cells also can show up in the stool.  There are certain differences in the DNA of the cancer cells compared to the normal colon cells, and these differences can be detected using a very sophisticated analysis of the stool.

The test has some drawbacks.  Rather than just a swab of stool as a specimen, an entire bowel movement must be collected, and it must be sent to a central laboratory.  It doesn’t sound like much fun, does it?  In addition, the test is not effective for identifying someone who has polyps, which are precursors to colon cancer.  It can only find evidence of a cancer once a polyp has transformed into a cancer.  So it really isn’t helping with prevention, only earlier detection of a cancer.  The test is also moderately expensive, and probably not covered by your insurance company.  But perhaps with more refinements, this test may have better utility in the future.

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Suggested next pages

Etiology of Colon Cancer

Symptoms and Signs of Colon Cancer

Incidence of Colon Cancer

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