Colon Cancer: Screening And Surveillance
Medical Author: Dennis Lee, M.D.
Medical Editor: Jay W. Marks, M.D.
Introduction
The colon, also known as the large intestine or large bowel, constitutes the last part of the digestive tract. The colon is a long, muscular tube that receives undigested food from the small intestine. It removes water from the undigested food, stores it and then finally eliminates it from the body through bowel movements. The rectum is the last part of the colon adjacent to the anus.
Cancer of the colon and rectum (colorectal cancer) is a malignant tumor arising from the inner wall of the large intestine. These malignant tumors invade nearby tissue and spread to other parts of the body. Benign tumors of the colon are called polyps. Benign polyps do not invade nearby tissue or spread to other parts of the body like malignant tumors do. Benign polyps can be removed easily during colonoscopy and are not life threatening. However, if benign polyps are not removed from the large intestine, they can become malignant (cancerous) over time. In fact, most of the cancers of the large intestine are believed to have evolved from benign polyps that are pre-cancerous, that is, they are benign at first but later become cancerous.
Cancer of the colon and rectum invades and damages adjacent tissues and organs. Cancer cells also can break away and spread to other parts of the body (such as the liver and lung) where new tumors grow. The sprocess whereby colon cancer preads to distant organs is called metastasis, and the new tumors are called metastases. Once metastasis has occurred in colorectal cancer, a complete cure of the cancer is unlikely.
Colorectal cancer is both preventable and curable. Colorectal cancer is prevented by removing precancerous colon polyps . It is cured if it is found early and is surgically removed before it spreads to other parts of the body. The National Polyp Study showed in its surveillance program that individuals who had their polyps removed experienced a 90% reduction in the incidence of colorectal cancer. The few patients in the study who did develop colorectal cancer had their cancer discovered at early, surgically or endoscopically curable stages. Since most colon polyps and early cancers are silent (produce no symptoms), it is important to do screening and surveillance for colon cancer in patients without symptoms or signs of the polyps or cancers. Recommendations for cost-effective public screening and surveillance have been promulgated and endorsed by numerous societies including the American Cancer Society, the National Cancer Institute, American College of Gastroenterology, American Medical Association, American College of Physicians, etc.
Screening recommendations for individuals with average risk
The life-time risk for an adult American to develop colorectal cancer is approximately 6%. Fecal occult blood tests and flexible sigmoidoscopic examinations are the recommended screening tests for these individuals at average risk for developing colorectal cancer. These tests are designed to detect and remove precancerous polyps and identify early cancers in order to decrease mortality from colorectal cancer. Fecal occult blood testing and sigmoidoscopy are affordable, easy to perform, and comfortable for healthy individuals.
Fecal occult blood tests
Fecal occult blood tests are chemical tests that are performed on samples of stool to detect the presence of "occult" blood (amounts of blood that are so small that they cannot be seen with the naked eye). These tests usually are begun at age 40 and then are repeated annually along with a digital rectal examination that is performed by a doctor. The use of fecal occult blood tests is based on the observation that slow bleeding from colon polyps or cancers can cause small amounts of blood to mix with the colonic contents. (This sometimes can lead to an iron deficiency anemia .) Since the small amounts of blood are not visible to the naked eye, sensitive chemical tests are needed to detect the traces of blood in the stool.
Fecal occult blood testing consists of checking for occult blood in 3 stool specimens collected on special cards at home. To properly prepare for collecting the specimens, individuals are asked to abstain (for 3-5 days before stool collections begin) from certain foods, medications and vitamins that can interfere with the accuracy of the test. These include certain meats, vitamins (especially vitamin C), iron, aspirin, and other antiinflammatory medicines (NSAIDs) such as ibuprofen that are used in treating arthritis and other painful inflammatory conditions.
An individual whose stool specimen tests positive for occult blood then undergoes a colonoscopic examination of the entire colon to look for polyps, cancers, or other conditions that cause bleeding (such as abnormal blood vessels and colitis). The majority (greater than 90%) of the polyps detected at colonoscopy can be removed painlessly and safely during the colonoscopic examination. Polyps so removed are examined later under the microscope by a pathologist to determine if they are precancerous. Individuals with precancerous polyps have a higher than average risk for developing colon cancer, and are advised to return for periodic surveillance colonoscopies (see below). Colon cancers that are detected at colonoscopy usually are removed surgically though under certain circumstances they may be removed at colonoscopy. Precancerous polyps that are too large or technically not possible to remove during colonoscopy also are removed surgically. Several studies have shown that fecal occult blood testing can reduce death rates (mortality) from colorectal cancer by 30-40%.
If no colonic abnormalities are found in an individual whose stool contains occult blood, consideration then is given to examining the stomach and the small intestine as sources of bleeding.
Flexible sigmoidoscopy
Flexible sigmoidoscopy utilizes a flexible sigmoidoscope, a flexible, fiberoptic viewing tube with a light at the tip. It is inserted through the anus and is used by the doctor to examine the rectum and the part of the colon adjacent to the rectum. It is a shorter version of a colonoscope. Approximately 50% of colorectal cancers and polyps are found to be within the reach of a flexible sigmoidoscope. It is recommended that individuals of average risk for colon cancer undergo a flexible sigmoidoscopy examination at age 50 and every 3-5 years thereafter. If polyps are found during a flexible sigmoidoscopic examination, a colonoscopy to examine the entire colon is recommended to remove the polyps as well as to find and remove additional polyps in other parts of the colon. The removed polyps are examined by a pathologist under a microscope to determine if the polyps are benign, malignant or pre-cancerous. Individuals with precancerous polyps (adenomas and villous adenomas) have a higher than average risk of developing colon cancer, and it is recommended that they return periodically for surveillance colonoscopies (see below). For more information about this procedure, please see the Flexible Sigmoidoscopy article.
Screening colonoscopy
Many doctors in the US are recommending screening colonoscopies rather than flexible sigmoidoscopies for healthy subjects with an average risk for developing colon cancer. Colonoscopies are recommended beginning at the age of 50 and thereafter every 7-10 years if no colon polyps or cancers are found. The rationale for this recommendation is: - Colonoscopy examines the entire colon while flexible sigmoidoscopy only examines the rectum and the colon adjacent to the rectum.
- Approximately 50% of colon polyps (and colon cancers) are found in the upper colon (cecum, ascending colon, and transverse colon) and, therefore, are beyond the reach of sigmoidoscopes and would be missed by flexible sigmoidoscopy.
- The National Polyp Study, a large, scientific study, has shown that colonoscopy with removal of all colon polyps reduces deaths from colon cancer.
For more information about this procedure, please see the Colonoscopy article.
Virtual colonoscopy
Virtual colonoscopy is a new technique that uses CT to construct virtual images of the colon that are similar to the views of the colon obtained at colonoscopy. The virtual colonoscopic images are produced by computerized manipulation of two-dimensional images obtained by a CT scanner rather than direct observation through the colonoscope. The colon is cleaned-out using laxatives the day prior to the virtual colonoscopy examination. A tube then is inserted into the anus and is used to inject air into the colon. The CT scans then are performed, and the scans are analyzed and manipulated to form a virtual image of the colon.
Properly performed virtual colonoscopy can be very good. It can even find polyps "hiding "behind folds that occasionally are missed by colonoscopy. Nevertheless, virtual colonoscopy has several limitations. They are: - Virtual colonoscopy cannot find small polyps (less than 5 mm in size) that are easily seen at colonoscopy.
- Virtual colonoscopy is not as accurate as colonoscopy at finding flat cancers or premalignant lesions that are not protruding, that is, are not polyp-like.
- Small pieces of stool can look like polyps on virtual colonoscopy and lead to a diagnosis of polyps when there are none.
- Virtual colonoscopy cannot remove polyps. Thirty to forty percent of people have colon polyps. If polyps are found by virtual colonoscopy, then colonoscopy must be done to remove the polyps, and, therefore, many individuals having virtual colonoscopy will have to undergo a second procedure, colonoscopy.
- There have not been studies to compare the discomfort levels of colonoscopy versus virtual colonoscopy, and comparisons will be difficult to do. The discomfort of colonoscopy is from the insertion of the colonoscope and air insufflation. The discomfort of virtual colonoscopy is from air insufflations. Patients' perceptions of discomfort from both procedures are highly variable. What makes the discomfort difficult to compare is that patients undergoing colonoscopy usually are sedated intravenously, while patients undergoing virtual colonoscopy are not sedated. As a result, patients may actually find colonoscopy more comfortable than virtual colonoscopy. On the other hand, sedation increases the risk of complications from colonoscopy.
Because of these limitations, virtual colonoscopy has not replaced colonoscopy as the primary screening tool for individuals at either normal or high risk for polyps or colon cancer. It is currently a good option for individuals who cannot or will not undergo colonoscopy
Air contrast (double contrast barium enema)
Even though double contrast barium enema has been included in screening guidelines, it is not as accurate as colonoscopy or, perhaps, virtual colonoscopy in detecting small polyps or cancers. Like virtual colonoscopy, it cannot remove polyps. Also like virtual colonoscopy, it may mistake particles of stool for polyps. In addition, as the numbers of barium enema examinations decreases, radiologists are have less experience doing them, and their ability to do good examinations is decreaseing. For these reasons, double contrast barium enemas are not widely used for colon cancer or polyp screening. For more information, please see the Barium Enema article.
Surveillance recommendations for individuals with higher than average risk
Many individuals are at higher than average risk for developing colon cancer because of a family history of colon cancer, history of chronic ulcerative colitis, rare hereditary forms of colorectal cancer, or a history of colon polyps or cancer. Periodic surveillance colonoscopies are recommended for these individuals to remove precancerous polyps, and /or to detect early cancers.
Patients with a history of colon polyps
Patients with history of colon polyps often develop polyps subsequently. Therefore, periodic surveillance colonoscopies are recommended. In individuals with only precancerous polyps that are completely removed, the usual recommendation is to repeat the colonoscopy after 3 years. If the colonoscopy at 3 years shows no recurrence of polyps, then the interval between subsequent colonoscopies is extended to 5 years.
Sometimes, doctors are not confident that all polyps have been completely removed. Examples include individuals with multiple pre-cancerous polyps, polyps that are technically difficulty to completely excise, or less than optimal visualization of the colon due to inadequate cleansing of the colon. Under these circumstances, the decision regarding the interval between surveillance colonoscopies is best arrived at jointly between the patient and the doctor. For more information, please see the Colon Polyps article.
Patients with history of colorectal cancer
Individuals who have undergone colon cancer surgery are at higher risk of developing another colon cancer in the future. It usually is recommended that they undergo a repeat colonoscopy after 6 to 12 months and every 3 years thereafter. Early detection and treatment of future polyps and early cancers can significantly improve chances of survival. The annual testing of stool for occult blood continues.
Patients with ulcerative colitis
Patients with long standing ulcerative colitis also have a higher risk of developing colorectal cancer. The risk of developing colon cancer is proportional to the duration of disease and to the extent of colon involved by colitis. Thus, patients with chronic ulcerative colitis involving the entire colon should have a colonoscopy every 1 to 2 years after having the colitis for 10 years or more. During the procedure, biopsies are taken from the colon to look for early, microscopic precancerous changes in the cells. If precancerous cells are detected, colonoscopy is repeated 3 months later. If still present, doctors may discuss with the patient the benefits of surgically removing the colon to prevent colon cancer. If the colitis is limited to only the left colon, the same surveillance program is started 15 years after the onset of colitis. For more information, please see the Ulcerative Colitis article.
Family history of colorectal cancer
Colorectal cancer may run in families. Colon cancer risk to an individual is even higher if more than one immediate family member (parents, siblings or children) has had colorectal cancer, and/or the family member developed the cancer at a young age (less than 55). Under these circumstances, it is recommended that individuals undergo a colonoscopy every three years starting at an age that is 7-10 years younger than the age at which the family member who developed colorectal cancer at the youngest age developed his or her cancer.
If only one immediate family member developed colorectal cancer at an advanced age, the colon cancer risk to the individual is still higher than average but not as high as if two immediate family members developed colorectal cancer or if a family member developed colorectal cancer at an early age. Whether and when to perform screening colonoscopies in these individuals are best decided jointly by the individuals and their doctors.
Other groups in need of surveillance
There are other rare conditions that can increase the risk of colorectal cancer. These conditions are often hereditary, such as familial polyposis, familial nonpolyposis syndromes, the cancer family syndrome, hereditary site-specific colon cancer, etc. While uncommon, these conditions require specialized surveillance and treatment. Individuals suspected of having these familial conditions should consult their doctors, a gastroenterologist or an oncologist (specialist in cancer treatment) for proper surveillance and treatment.
Summary
Colon cancer is both preventable and curable. Colon cancer is preventable by removing precancerous colon polyps. It is curable if early cancer is surgically removed before cancer spreads to other parts of the body. Therefore, if screening and surveillance program were practiced universally, there would be a major reduction in the incidence and mortality of colorectal cancer.
Ongoing genetic research will help doctors better understand the genetic basis of colorectal cancer formation. Genetic blood tests and tests for premalignant or malignant cells in stool also may have a role in colorectal cancer screening. Screening programs other than those discussed here also are being evaluated. For example, one such screening program involves colonoscopy at age 50-55 that is repeated once, in approximately 10 years, instead of periodic flexible sigmoidoscopy. Regardless of what new screening methods become available, viewers should remember to discuss with their doctors colon cancer screening and/or surveillance as it relates to their own situations.
For further information, please read the Cancer of the Colon and Rectum article.
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