Tests to Detect Colorectal Cancer and Polyps
What is colorectal cancer?
Most colorectal cancers begin as a polyp, a growth in the tissue that lines the inner surface of the colon or rectum. Polyps may be flat, or they may be raised. Raised polyps may grow on the inner surface of the colon or rectum like mushrooms without a stalk (sessile polyps), or they may grow like a mushroom with a stalk (pedunculated polyps). Polyps are common in people older than 50 years of age, and most are not cancers. However, a certain type of polyp known as an adenoma may have a higher risk of becoming a cancer.
Colorectal cancer is the third most common type of non-skin cancer in both men (after prostate cancer and lung cancer) and women (after breast cancer and lung cancer). It is the second leading cause of cancer death in the United States after lung cancer. The rates of new colorectal cancer cases and deaths among adults aged 50 years or older are decreasing in this country. However, the incidence is increasing among younger adults (1). It is estimated that a total of 136,830 people in the United States will be diagnosed with colorectal cancer and 50,310 people will die from it in 2014 (1).
Although the major risk factors for colorectal cancer are a family history and older age, several other factors have been associated with increased risk, including excessive alcohol use, obesity, being physically inactive, cigarette smoking, and, possibly, diet.
In addition, people with a history of inflammatory bowel disease (such as ulcerative colitis or Crohn disease) have a higher risk of colorectal cancer than people without such conditions. And people who have a family history of colorectal cancer or certain inherited conditions (such as Lynch syndrome and familial adenomatous polyposis) also have an increased risk of colorectal cancer. (More information about risk factors for colorectal cancer is available in NCI’s PDQ summary on Colorectal Cancer Screening.)
Several screening tests have been developed to help doctors find colorectal cancer early, when it may be more treatable. Some tests that detect adenomas and polyps can actually prevent the development of cancer because these tests allow growths that might otherwise become cancer to be detected and removed. That is, colorectal cancer screening may be a form of cancer prevention, not just early detection.
What methods are used to screen people for colorectal cancer?
Expert medical groups, including the U.S. Preventive Services Task Force, strongly recommend screening for colorectal cancer. Although minor details of the recommendations may vary, these groups generally recommend that people at average risk of colorectal cancer get screened at regular intervals with high-sensitivity fecal occult blood tests (FOBT), sigmoidoscopy, or colonoscopy beginning at age 50 (2). People at increased risk because of a family history of colorectal cancer or polyps or because they have inflammatory bowel disease or certain inherited conditions may be advised to start screening before age 50 and/or have more frequent screening.
- High-sensitivity fecal occult blood tests (FOBT): Both polyps and colorectal cancers can bleed, and FOBT checks for tiny amounts of blood in feces (stool) that cannot be seen. (Blood in stool may also indicate the presence of conditions that are not cancer, such as hemorrhoids.) Currently, two types of FOBT are approved by the Food and Drug Administration (FDA) to screen for colorectal cancer: guaiac FOBT (gFOBT) and the fecal immunochemical (or immunohistochemical) test (FIT, also known as iFOBT). With both types of FOBT, stool samples are collected by the patient using a kit, and the samples are returned to the doctor.
- Guaiac FOBT uses a chemical to detect heme, a component of the blood protein hemoglobin. Because the guaiac FOBT can also detect heme in some foods (e.g., red meat), people have to avoid certain foods before having this test.
- FIT uses antibodies to detect human hemoglobin specifically (3, 4). Dietary restrictions are typically not required for FIT.
- Sigmoidoscopy: In this test, the rectum and sigmoid colon are examined using a sigmoidoscope, a flexible lighted tube with a lens for viewing and a tool for removing tissue. This instrument is inserted through the anus into the rectum and sigmoid colon as air (or carbon dioxide) is pumped into the colon to expand it so the doctor can see the colon lining more clearly. During sigmoidoscopy, abnormal growths in the rectum and sigmoid colon can be removed for analysis (biopsied). The lower colon must be cleared of stool before sigmoidoscopy, but the preparation is less involved than that required for colonoscopy. People are usually not sedated for this test.
Studies have shown that people who have regular screening with sigmoidoscopy after age 50 years have a 60 to 70 percent lower risk of death due to cancer of the rectum and lower colon than people who do not have screening (5, 6). One randomized controlled clinical trial found that even just one sigmoidoscopy screening between 55 and 64 years of age can substantially reduce colorectal cancer incidence and mortality (7). The U.S. Preventive Services Task Force recommends sigmoidoscopy every 5 years along with FOBT every 3 years for people at average risk who have had negative test results (2).
- Standard (or optical) colonoscopy: In this test, the rectum and entire colon are examined using a colonoscope, a flexible lighted tube with a lens for viewing and a tool for removing tissue. Like the shorter sigmoidoscope, the colonoscope is inserted through the anus into the rectum and the colon as air (or carbon dioxide) is pumped into the colon to expand it so the doctor can see the colon lining more clearly. During colonoscopy, any abnormal growths in the colon and the rectum can be removed, including growths in the upper parts of the colon that are not reached by sigmoidoscopy. A thorough cleansing of the entire colon is necessary before this test. Most patients receive some form of sedation during the test.
Studies suggest that colonoscopy reduces deaths from colorectal cancer by about 60 to 70 percent. Additional studies are currently being done to better evaluate how effective colonoscopy screening methods are (8). The U.S. Preventive Services Task Force recommends colonoscopy every 10 years for people at average risk as long as their test results are negative (2).
- Other methods: Although most expert groups generally recommend high-sensitivity FOBT, sigmoidoscopy, and colonoscopy as standard colorectal cancer screening tests, several other types of tests exist.
Cologuard®. This test detects tiny amounts of blood in stool (with an immunochemical test similar to FIT) as well as nine DNA biomarkers in three genes that have been found in colorectal cancer and precancerous advanced adenomas. The DNA comes from cells from the lining of the colon and rectum that collect in stool as it passes through the large intestine and rectum. As with both types of FOBT, the stool sample for the Cologuard test is collected by the patient using a kit; the sample is mailed to a laboratory for testing. A computer program analyzes the results of the two tests (blood and DNA biomarkers) and provides a finding of negative or positive. People who have a positive finding with this test are advised to undergo a colonoscopy.
In one study of people who were at average risk for developing colon cancer and had no symptoms of colon problems (9), this test detected more cancers and adenomas than the FIT test (that is, it was more sensitive). However, the Cologuard test also was more likely to identify an abnormality when none was actually present (that is, it had more false-positive results).
In August 2014, the FDA approved the Cologuard test. At the same time, in a pilot program known as parallel review, the Centers for Medicare & Medicaid Services proposed national coverage for this test. This test is still being evaluated to see where it fits in screening guidelines. To date, it has not been incorporated into clinical practice guidelines or recommended by the U.S. Preventive Services Task Force as a method to screen for colorectal cancer. The Task Force is currently updating its guideline and is examining recent evidence on the Cologuard test.
Virtual colonoscopy. This screening method, also called computed tomographic [CT] colonography), uses special x-ray equipment (a CT scanner) is to produce a series of pictures of the colon and the rectum from outside the body. A computer then assembles these pictures into detailed images that can show polyps and other abnormalities. Virtual colonoscopy is less invasive than standard colonoscopy and does not require sedation. As with standard colonoscopy, a thorough cleansing of the colon is necessary before this test, and air (or carbon dioxide) is pumped into the colon to expand it for better viewing of the colon’s lining. The accuracy of virtual colonoscopy is similar to that of standard colonoscopy, and virtual colonoscopy has a lower risk of complications. However, if polyps or other abnormal growths are found during a virtual colonoscopy, a standard colonoscopy is usually performed to remove them.
Whether virtual colonoscopy can help reduce deaths from colorectal cancer is not yet known, and Medicare and some insurance companies currently do not pay for the costs of this procedure. Studies are ongoing to compare virtual colonoscopy with other screening methods.
Double-contrast barium enema. This test, also called DCBE, is another method of visualizing the colon from outside the body. In DCBE, a series of x-ray images of the entire colon and rectum is taken after the patient is given an enema with a barium solution. The barium helps to outline the colon and the rectum on the images. DCBE is rarely used for screening because it is less sensitive than colonoscopy for detecting small polyps and cancers. However, it may be used for people who cannot undergo standard colonoscopy—for example, because they are at particular risk for complications.
Single-specimen guaiac FOBT done in a doctor's office. Doctors sometimes perform a single-specimen guaiac FOBT on a stool sample collected during a digital rectal examination done as part of a routine physical examination. However, this approach has not been shown to be an effective way to screen for colorectal cancer (10).
How can people and their health care providers decide which colorectal cancer screening test(s) to use?
People should talk with their health care provider about when to begin screening for colorectal cancer, what test(s) to have, the advantages and disadvantages of each test, how often to undergo colorectal cancer screening, and when to stop.
The decision about which test to have usually takes into account several factors, including:
- The person’s age, medical history, family history, and general health
- The potential harms of the test
- The preparation required for the test
- Whether sedation may be needed for the test
- The follow-up care needed after the test
- The convenience of the test
- The cost of the test and the availability of insurance coverage
The commonly used colorectal cancer screening tests all have advantages and disadvantages:
- No cleansing of the colon is necessary.
- No dietary restrictions are needed before FIT.
- Samples can be collected at home.
- Cost is low compared with other colorectal cancer screening tests.
- There is no risk of damage to the lining of the colon.
- No sedation is needed.
- The test does not detect some polyps and cancers.
- False-positive test results (i.e., the test suggests an abnormality when none is present) are possible.
- Dietary restrictions are needed before guaiac FOBT.
- Additional procedures, such as colonoscopy, may be needed if the test result shows blood in the stool.
- For most patients, discomfort is minimal, and complications are rare.
- The doctor can perform a biopsy or polypectomy (removal of a polyp or adenoma) during the test, if necessary.
- Less extensive cleansing of the colon is necessary for this test than for a colonoscopy.
- Sedation is often not required.
- Abnormal growths in the upper part of the colon will be missed because the test allows the doctor to view only the rectum and the lower part of the colon.
- Bowel cleansing is needed before the test.
- Medication and diet changes may be needed before the test.
- There is a very small risk of bleeding or of tearing or perforation of the lining of the colon.
- Additional procedures, such as colonoscopy, may be needed if the test finds an abnormality.
- The availability of sigmoidoscopy has decreased substantially in the United States in recent years (11).
- This test is one of the most sensitive currently available.
- It allows the doctor to view the rectum and the entire colon.
- The doctor can perform a biopsy or polypectomy during the test if necessary.
- Even though this test is highly sensitive, it still may not detect all small polyps, nonpolypoid lesions, or cancers.
- A thorough cleansing of the colon is required before the test.
- Diet changes are needed before the test, and medications may need to be adjusted.
- Some form of sedation is almost always used. As a result, the patient must have someone accompany them to the procedure and drive them home afterward, and they may not be able to work the day of the procedure.
- There is a small risk of bleeding or of tearing or perforation of the lining of the colon; this risk increases with age, the presence of other health problems, and when polyps are removed.
Does health insurance pay for colorectal cancer screening?
What happens if a colorectal cancer screening test finds an abnormality?
If a screening test finds an abnormality, additional tests may be recommended. These tests may include x-rays of the gastrointestinal tract, a sigmoidoscopy, or, most often, a colonoscopy if it has not already been done. If a polyp or tumor is found during a sigmoidoscopy, a biopsy or polypectomy may be performed during the test, and a colonoscopy may be recommended. If a polyp or tumor is found during a standard colonoscopy, a biopsy or polypectomy may be performed during the test to determine whether cancer is present. If a polyp or tumor is detected during virtual colonoscopy, the patient will be referred for a standard colonoscopy.
What new tests are being developed for colorectal cancer screening?
Colorectal polyps and tumors can release cells and DNA into the bloodstream as well as into stool (12). Researchers are studying whether the presence of an altered gene called SEPT9 in blood can be used to screen for early-stage colorectal cancer and advanced adenomas (13). In clinical studies, a biomarker test that detects altered SEPT9 DNA in blood (Epi proColon®) was no less sensitive than FIT, but it was less specific (14). In March 2014, the FDA recommended premarket approval, a process of scientific and regulatory review to evaluate safety and effectiveness, for this test.
New approaches to visualize the colon that avoid the need for thorough cleansing of the colon that is currently required for virtual colonoscopy are being studied and refined. One approach is "fecal tagging" with a contrast agent that is ingested over several days before the procedure. This technique allows fecal material in the colon to be differentiated from colon tissue (15); computer software can be used to electronically remove the tagged fecal material from images (16).
Information about ongoing clinical trials that are studying methods for colorectal cancer screening can be found in NCI's clinical trials database. You may also contact NCI’s Cancer Information Service at 1–800–4–CANCER (1–800–422–6237) for assistance with searching the clinical trials database or for other cancer information needs.