Colon Cancer Screening
INTRODUCTION— Colorectal cancer (cancer of the large portion of the bowel [colon] or rectum) is a common, lethal, and preventable disease. Approximately one-third of people who develop it die of the disease, making it the second leading cause of cancer death. However, screening tests now make it possible to detect existing cancers at an early, treatable stage and even to prevent the development of colorectal cancer.
In contrast to some other cancers, such as prostate cancer, there is general agreement by experts that all adults undergo screening beginning at age 50, or even earlier for people who are at high risk for colorectal cancer. Several different tests are currently available and several new tests are being developed; all of these have advantages and disadvantages. The optimal screening test depends on each person's preferences. It is important to review each test's effectiveness, safety, convenience, and cost with your doctor. By working with your doctor, you can design the screening plan that is best for you.
WHY IS SCREENING EFFECTIVE? — Most colorectal cancers develop gradually over many years. They begin as small, benign tumors called adenomatous polyps. These polyps grow, develop precancerous changes, eventually become cancerous, and later spread and become incurable. This progression takes at least 10 years in most people.
The screening tests described below all work by detecting cancers at the polyp stage before they become cancerous, or by detecting cancers themselves while they are still curable. Regular screening for and removal of polyps can reduce a person's risk of developing colorectal cancer by up to 90 percent. In addition, early detection of cancers that are already present in the colon often allows for successful treatment.
WHO IS AT INCREASED RISK FOR COLORECTAL CANCER? — Several factors increase an individual's risk of developing colorectal cancer. The presence of these factors will determine the age at which screening should begin, the frequency of screening, and the screening tests that are most appropriate.
Family history of colorectal cancer — The occurrence of colorectal cancer in a family member increases the risk of getting the cancer, especially if it is a first degree relative (a parent, brother or sister, or child), several family members are affected, or if the cancers have occurred at an early age (eg, before age 55 years).
Familial adenomatous polyposis — Familial adenomatous polyposis (FAP) is an inherited condition associated with an increased risk of colorectal cancer. Almost 100 percent of people with this condition will experience colorectal cancer during their lifetime, and most of these cancers occur before the age of 50 years. This condition is recognized by the occurrence of hundreds of polyps throughout the colon.
Hereditary nonpolyposis colon cancer — Hereditary nonpolyposis colon cancer (HNPCC) is another inherited condition associated with an increased risk of colorectal cancer. About 70 percent of people with HNPCC will experience colorectal cancer by the age of 65. Cancer also tends to occur at younger ages and in the part of the colon on the right side of the body (the ascending colon). This condition is recognized by the presence of a very strong family history: several family members from different generations affected, some of whom develop the cancer relatively early in life.
Prior colorectal cancer or polyps — People who have previously had colorectal cancer have an increased risk of developing a new colorectal cancer. People who have had adenomatous polyps before the age of 60 years are also at increased risk for colorectal cancer.
Inflammatory bowel disease — The risk of colorectal cancer is increased in people with either Crohn's disease of the colon or ulcerative colitis. The risk increases as the amount of inflamed colon increases and as the duration of disease increases; pancolitis (inflammation of the entire colon) and colitis of 10 years' duration or longer are associated with the greatest risk for colorectal cancer. Risk is not increased in irritable bowel disease.
Other risk factors — Other factors affect risk, but not so much as to change the approach to screening. Modifying the ones that can be changed can reduce your chances of getting colorectal cancer.
Increasing age — Although the average person has a 5 percent lifetime risk of developing colorectal cancer, 90 percent of these cancers occur in people older than 50 years of age.
Race — Black Americans have a higher risk of dying from colorectal cancer than white Americans. This risk is also high in native Alaskans and low in American Indians.
Lifestyle factors — Several lifestyle factors have been linked to the risk of colorectal cancer. Factors that appear to increase risk include:
Factors that appear to decrease risk include:
SCREENING TESTS — Four tests are currently available for colorectal cancer screening: the fecal occult blood test, sigmoidoscopy, barium enema, and colonoscopy. Some emerging tests may be used in the future.
Fecal occult blood test — Colorectal cancers (and, more rarely, polyps) often bleed, releasing microscopic amounts of blood into the stool. The blood is frequently not visible to the naked eye, requiring specialized tests for detection. The fecal occult blood test can be used to detect blood in the stool.
Procedure — This simple test is performed by putting small amounts of stool on chemically coated cards. Usually two samples from three consecutive stools are applied to the cards at home and returned to your doctor. The sample on the card is then treated with a clear solution that turns blue when blood is present.
Some simple dietary restrictions for two days prior to testing can improve the accuracy of the test. These include:
Effectiveness — The fecal occult blood test, when performed once every year, may reduce the risk of dying from colorectal cancer by up to one-third .
Risks and disadvantages — Because polyps seldom bleed, the fecal occult blood test is less likely to detect polyps than other screening tests (see below). In addition, only 2 to 5 percent of people with a positive test actually have colorectal cancer; thus, for every patient with cancer, 50 patients are unnecessarily distressed and undergo tests that eventually reveal no cancer. Following the procedures discussed above will reduce the chance of a false-positive test.
Additional testing — If a fecal occult blood test has a positive result, your doctor will recommend that the entire colon be examined, usually with colonoscopy.
Sigmoidoscopy — Sigmoidoscopy allows direct viewing of the lining of the rectum and the lower part of the colon (the descending colon). This area accounts for about one-half of the total area of the rectum and colon.
Procedure — During sigmoidoscopy, a thin, lighted tube is advanced along the rectum and the left-sided colon to check for polyps and cancer. Biopsies (small samples of tissue) can be removed during sigmoidoscopy. Preparation for sigmoidoscopy entails using an enema a few hours before the procedure. Sigmoidoscopy may be performed in a doctor's office; because this procedure produces only mild cramping, most people undergoing the procedure do not need sedative drugs and are able to return to work or other activities the same day.
Effectiveness — Physicians who perform sigmoidoscopy can identify polyps and cancers in the descending colon and rectum with a high degree of accuracy. Studies suggest that sigmoidoscopy, performed as infrequently as every five to ten years, reduces death from cancers in the upper half of the colon and rectum by 66 percent .
Risks and disadvantages — The risks of sigmoidoscopy are low. The procedure can create a small tear in the intestinal wall in about 2 per every 10,000 people; death from this complication is rare. A major disadvantage of sigmoidoscopy is that it cannot detect polyps or cancers located in the right side of the colon.
Additional testing — Certain changes in the left-sided colon increase the likelihood of polyps or cancer in the remaining part of the colon. Thus, if sigmoidoscopy reveals suspicious findings in the left-sided colon, such as many small polyps or polyps with certain microscopic features, your doctor may recommend colonoscopy to view the entire length of the colon.
Combination of fecal occult blood test and sigmoidoscopy — Combined screening with a fecal occult blood test and sigmoidoscopy is a common practice and may be more effective than screening with either test alone .
Barium enema test — A barium enema test provides a detailed x-ray picture of the rectum and the entire colon. Doctors usually recommend a specific type of test called a double-contrast barium enema.
Procedure — During a barium enema test, liquid barium is used to coat the inside of the colon. The barium outlines the profile of the colon on x-rays and can reveal structural abnormalities such as polyps and cancers. Preparation for a barium enema entails cleansing the colon with a saline laxative. People may experience mild cramping during the procedure, but sedative drugs are usually not necessary, and most people can return to work or other activities on the same day.
Effectiveness — The barium enema test detects about one-half of large polyps and about 40 percent of all polyps in the colon and rectum . Most experts suspect that the screening barium enema helps reduce the risk of dying from colorectal cancer, but this has not been definitively proven.
Risks and disadvantages — The barium enema test is relatively safe when compared with other screening tests for colorectal cancer.
Additional testing — If a barium enema test reveals an abnormality, your doctor may recommend colonoscopy or other tests.
Colonoscopy — Colonoscopy allows direct viewing of the lining of the rectum and the entire colon.
Procedure — During colonoscopy, a thin, lighted tube is used to directly view the lining of the rectum and the entire colon. This test can therefore detect polyps and cancers that are beyond the reach of the sigmoidoscope. Preparation for colonoscopy entails cleansing the colon with a saline laxative before the procedure. People are usually given a mild sedative drug during the procedure.
Effectiveness — Colonoscopy detects most small polyps and almost all large polyps and cancers . Polyps and some cancers can be removed during this procedure.
Risks and disadvantages — The risks of colonoscopy are greater than those of other screening tests. Colonoscopy leads to serious bleeding or a tear of the intestinal wall in about 1 in 1,000 people. Because the procedure requires sedation, most people must be accompanied home after the procedure and are unable to return to work or other activities on the same day.
New tests — Several new screening tests for colorectal cancer are currently being developed and evaluated. These tests include improved fecal occult blood tests, fecal tests for genetic abnormalities linked to colorectal cancer, and a type of computed tomography (CT) scan called a virtual colonoscopy. These tests are still being studied, and their value is not yet well enough defined to be used for routine screening.
SCREENING PLANS — Different screening plans are recommended for people with an average risk of colorectal cancer and people with an increased risk of colorectal cancer.
Screening plans for people with an average risk of colorectal cancer — Doctors usually recommend that people with an average risk of colorectal cancer begin screening at age 50. No single screening test has been identified as the best test. You should discuss the available options with your doctor and design a screening plan that is best for you. The most important thing is that you follow through with the screening plan.
Some doctors recommend a fecal occult blood test once every year and a sigmoidoscopy once every 5 years; they may also recommend a combination of these screening tests. Other doctors recommend a barium enema test once every 5 years or colonoscopy once every 10 years.
If any of these screening tests has a positive result, your doctor will probably recommend more frequent examinations with colonoscopy; this is referred to as surveillance.
Screening plans for people with an increased risk of colorectal cancer — When compared to screening programs for people with an average risk, programs for people with an increased risk may entail screening at a younger age, more frequent screening, and the use of more sensitive screening tests (like colonoscopy). For people who have an increased risk of colorectal cancer, the optimal screening plan depends upon the cause of increased risk.
Family history of colorectal cancer — People who have a first degree relative (a parent, brother, sister, or child) who has experienced colorectal cancer or polyps at a young age (before the age of 60 years) should begin screening earlier, typically at age 40, or 10 years younger than the earliest diagnosis in their family, whichever comes first. People with a second-degree relative (grandparent, aunt, or uncle) or third-degree relative (great-grandparent or cousin) with colorectal cancer should be screened as average risk persons.
Familial adenomatous polyposis — People with a family history of FAP should consider genetic counseling and genetic testing to determine if they carry the affected gene. People who carry the gene or who do not know if they carry the gene should begin screening with sigmoidoscopy once every year, beginning at puberty. When this screening reveals many polyps, a person should begin to plan colectomy (surgical removal of the colon) with their doctor; this surgery is the only certain way to prevent colorectal cancer in people with FAP.
Hereditary nonpolyposis colon cancer — People with a family history of HNPCC should consider genetic counseling and genetic testing to determine if they carry the affected gene. People who carry the gene or who do not know if they carry the gene should be screened with colonoscopy or barium enema because HNPCC is associated with cancers of the right-sided colon. This screening should be scheduled once every one to two years between the ages of 20 and 30, and once every year after the age of 40. Because polyps can progress more rapidly to cancer in people with HNPCC, more frequent screening may also be recommended.
Inflammatory bowel disease — In people with Crohn's disease of the colon or with ulcerative colitis, the optimal screening plan will depend on the amount of colon inflamed by the disease and the duration of the disease. Screening usually entails colonoscopy once every 1 to 2 years beginning after 8 years of disease in people with pancolitis (inflammation of the entire colon) or after 15 years in people with colitis only of the left-sided colon.