Colon Cancer
Screening Tests for Early Detection
It is recommended that all adults begin regular colon cancer screening when they turn 50. Deciding which screening test to use and how often ultimately depends on a person's individual risk of colon cancer. If a first-degree relative has had colon cancer, for instance, screening should start 10 years prior to the age that relative was diagnosed to help identify possible pre-cancerous polyps.
In March 2008, screening guidelines for the early detection of pre-cancerous polyps and colon cancer were released jointly by the American Cancer Society, the US Multi-Society Task Force on Colorectal Cancer, and the American College of Radiology. These guidelines divide screening options into two categories: 1) full or partial structural exams that inspect the colon itself and can detect both cancer and precancerous polyps and 2) laboratory tests on stool samples that detect blood that is possibly caused by existing cancer or detect cancerous cells shed in the stool. Another difference between these screening options is that direct examinations such as sigmoidoscopy and colonoscopy allow for removal of polyps at the time the test is done. All other tests must be followed up with another procedure to remove any suspected growths.
Imaging Tests
| Test | Description | Recommended Screening Interval | Pros | Cons |
|---|---|---|---|---|
| Sigmoidoscopy |
Examination of the rectum and lower colon with a rigid or flexible lighted instrument | Every 5 years | Minimal preparation ahead of time Does not usually need sedation Fairly quick and safe |
Only examines about 30% of colon Can’t remove all polyps Small risk of bleeding, infection or bowel tear May need to have colonoscopy if abnormal result found |
| Colonoscopy | Examination of the rectum and entire colon with a lighted instrument | Every 10 years | Can examine the entire colon Can remove polyps and take biopsies for pathological testing |
Extensive full bowel preparation ahead of time Sedation needed to perform Takes at least one to two days for prep and recovery Risk of bleeding, infection or bowel tears |
| Double contrast barium enema | Series of x-rays of the colon and rectum; patient is given an enema with a white, chalky solution that outlines the colon and rectum on the x-rays; tube inserted in rectum, bowel is inflated with air | Every 5 years | Does not require sedation Can view entire colon Relatively safe; minimal risk of tear to colon |
Same full bowel preparation needed as for colonoscopy Cannot remove polyps May need to have colonoscopy if suspicious results found |
| Virtual colonoscopy (CTC, or Computed Tomographic Colonography) | Examination of the rectum and entire colon to the small intestine using x-rays and computers; tube inserted in rectum and bowel is inflated with air | Every 5 years | No sedation required Can view entire colon Relatively safe; minimal risk of tear to colon |
Full bowel preparation required Cannot remove polyps May need colonoscopy if abnormal results |
Laboratory Tests
| Purpose | Description | Recommended Screening Interval | Pros | Cons |
|---|---|---|---|---|
| Fecal occult blood test (FOBT) |
Test to detect hidden blood in stool sample | Annually | No bowel preparation No direct risk to bowel Sample can be collected at home |
Dietary restriction before testing Cannot detect precancerous changes Detects any blood in colon, not just from cancers but from food or dental procedures |
| Fecal Immuno-chemical test (FIT) | Test to detect hidden blood in stool sample; different collection technique than FOBT | Annually | No dietary or drug restrictions No bowel preparation No direct risk to bowel Sample can be collected at home |
Cannot detect precancerous changes May miss some cancers; one time testing not effective |
| DNA test | Detects mutations in a specific gene associated with colon cancer in DNA isolated from a stool sample | Not defined at this time; more scientific evidence needed to develop a recommendation | No bowel preparation or dietary restrictions Sample can be collected at home No risk of bowel tear |
Cannot detect precancerous changes Adequate stool sample must be obtained Special handling needed |
In addition to these, a physician may perform a digital rectal examination (DRE) to feel for a rectal mass with a gloved finger. Most colon cancers, however, are beyond the reach of a finger and have no symptoms.
If a test other than colonoscopy gives a result suggestive of polyps or cancer, a full colonoscopy is often done to examine the full colon and collect tissue samples (biopsies) of polyps or potentially cancerous areas.
Tests to Diagnose and Stage Colon Cancer
When a suspected cancer is found during a colonoscopy, a biopsy is taken, removing some tissue from the suspicious site for examination under a microscope by a pathologist. If the tissue is cancerous, the next step is to determine the stage (or extent) of disease. Treatment will depend in part on the “stage” of the colon or rectal cancer; it is categorized by how far it has spread from its original site. Staging systems for colon cancer vary in different parts of the world, and some use letters instead of numbers. One common system used to describe colon cancer stages is:
- Stage 0: Very early cancer on the innermost layer
- Stage I: Tumor in the inner layers of the colon
- Stage II: Tumor in the outer layers of the colon and/or nearby tissue
- Stage III: Tumor that has spread to the lymph nodes
- Stage IV: Tumor that has spread to distant organs
A laboratory test called carcinoembryonic antigen (CEA) may be ordered to help in staging. This protein is increased in many patients with colon cancer, and blood levels correlate with the stage of the disease. CEA testing also can help in determining prognosis, as increased concentrations are associated with an increased risk of recurrence and development of metastatic disease.
