Screening for colorectal cancer
This section covers the following topics:
- Types of colorectal cancer screening
- Colorectal cancer screening guidelines for high risk people
- Colorectal cancer screening guidelines for average risk people
Most colorectal cancers start as an abnormal growth known as a polyp. Although most polyps will never turn into a colorectal cancer, some polyps do have the potential to grow into cancers. The goal of colorectal cancer screening is to find polyps and growths and remove them before they have the chance to turn into cancer. If cancer has already formed, early detection can still help improve a person’s chance of detecting cancer at an earlier, more treatable stage.
The tests used to screen for colorectal cancer include:
- flexible sigmoidoscopy
- stool tests
For this test, the doctor uses a long, flexible tube with a small video camera attached to look at the entire length of the colon and rectum. The patient is placed under sedation while the doctor inserts the tube through the anus into the rectum and colon. The scope has openings for instruments the doctor can use to remove any polyps, which can be examined under a microscope to look for cancer.
Colonoscopy requires preparation beginning 1-2 days before the exam to empty the colon of any stool. This allows the doctor to get a good look at the lining of the entire colon during the exam. The doctor prescribes a combination of a liquid diet, laxatives (usually taken as pills or large amount of liquid) and in some cases, enemas.
Flexible sigmoidoscopy is similar to colonoscopy using a flexible tube with a video camera to examine the colon and rectum. Similar to the colonoscopy, the scope has openings for instruments the doctor can use to remove any polyps, which can be examined under a microscope to look for cancer. However, the scope used for sigmoidoscopy is shorter and can only examine the rectum and a small portion of the colon. Most patients don't require sedation for sigmoidoscopy.
As with colonoscopy, sigmoidoscopy requires preparation to empty the colon beginning 1 day before the exam.
This test uses a type of xray scan known as a CT scan to take detailed, 3-dimensional pictures of the colon and rectum. This test is not as invasive as a colonoscopy, however, it does involve filling the colon and rectum with air using a small, flexible tube. The goal of the scan is to find any polyps. If polyps or abnormalities are found during colonography, a followup colonoscopy is needed to remove the polyps.
This test does not require any sedation. However, the same type of bowel preparation needed for a colonoscopy or sigmoidoscopy is needed 1-2 days before the exam.
Stool tests look for possible signs of colorectal cancer—such as blood or abnormal DNA—in stool samples. There are several different tests available. These tests require a doctor's prescription. The patient is mailed a test kit, which comes with simple instructions on how to collect the sample and mail it back to the lab. If abnormalities are found during stool tests, a followup colonoscopy is needed to find and remove any polyps.
Experts have different recommendations for screening for people of average risk and high risk for colorectal cancer. The National Comprehensive Cancer Network (NCCN), is an organization of cancer experts that creates guidelines on cancer care and updates them yearly. NCCN recommends that doctors collect personal and family history in order to assess risk for colorectal cancer and make screening recommendations. NCCN has separate guidelines for colorectal cancer screening in people with average risk for colorectal cancer, people with Lynch syndrome and other inherited mutations.
NCCN has separate guidelines for people at increased risk for colorectal cancer due to inherited mutations or other risk factors. Screening guidelines for high risk people involve colonoscopy. The recommended age for beginning screening depends on the gene mutation and, in some cases, personal or family history of cancer or polyps. The NCCN guidelines include screening recommendations for people with inherited mutations in the following genes:
- Genes associated with Lynch syndrome
- Other genes associated with colorectal cancer
- MUTYH (guidelines are different for people with a mutation in one vs. both copies of MUTYH)
- STK11 (Peutz-Jegher syndrome)
- APC (mutations associated with Familial Adenomatous Polyposis or FAP)
- APC I1307K (this specific variant of the APC gene is found frequently in people of Ashkenazi Jewish descent. The variant increases the risk for colorectal cancer but not as high as for APC mutations associated with FAP).
- NTHL1 (guidelines are for people with a mutation in both copies of the gene)
- MSH3 (guidelines are for people with a mutation in both copies of the gene)
Several organizations publish colorectal cancer screening guidelines for average-risk people. The National Comprehensive Cancer Network guidelines recommend that average risk people begin colorectal cancer screening at age 50, while the American Cancer Society guidelines recommend beginning at age 45. According to the guidelines, screening can begin with any of the four types of tests listed above. Each test has benefits and limitations. The frequency of testing depends on the type of test used and whether any polyps or other abnormalities were found on previous tests.