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Colon Cancer

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Screening Tests for Early Detection

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. The US Preventive Services Task Force released similar recommendations in 2008 and they are currently under review. The guidelines divide screening options into two categories:

  • Full or partial structural exams that inspect the colon itself and can detect both cancer and precancerous polyps
  • Laboratory tests on stool samples that detect blood (which may be caused by existing cancer) or that 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 by another procedure to remove any suspected growths.

High risk:
People with a family history of colon cancer or polyps or a personal history of inflammatory bowel disease or certain inherited conditions may be advised to start screening before age 50. A colonoscopy is usually recommended because it is the most accurate and thorough. Also, the recommended screening interval for high-risk individuals is shorter than for people with average risk, such as every 1-2 years compared to every 10 years.

Average risk:
It is recommended that people at average risk for colon cancer begin regular screening when they turn 50 years old. The following tables summarize the screening tests that are options for people with average risk.

Imaging Tests

These tests visualize the colon and can detect both pre-cancerous polyps and existing cancers.
TestDescriptionRecommended Screening Interval Starting at age 50 for people at average riskProsCons
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 day 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

Effectiveness as a screening tool is not fully accepted

Laboratory Tests

These tests are performed on stool samples and detect mainly existing cancers.
Stool TestsDescriptionRecommended Screening Interval Starting at Age 50 for People at Average RiskProsCons
Guaiac-based fecal occult blood test (gFOBT) 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 or iFOBT) Test to detect hidden blood in stool sample; different collection technique than gFOBT 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 specific genes associated with colon cancer in DNA isolated from a stool sample Every three years, according to the American Cancer Society 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 tests, a health practitioner 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 for Diagnosis, Staging, and Prognosis
Biopsy:
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. (For in-depth information, see the article on Anatomic Pathology.)

Drawing of a colon polyp and two normal intestinal folds             Drawing of a colon polyp being removed

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 of the colon or rectum (carcinoma in situ)
  • Stage I: Tumor in the inner layers of the colon but has not grown through the wall of the colon
  • Stage II: Tumor in the outer layers of the colon and/or nearby tissue but has not spread to lymph nodes
  • Stage III: Tumor that has spread to the lymph nodes but not to distant organs of the body
  • Stage IV: Tumor that has spread to distant organs, such as the lungs, bone, or liver (metastatic)

Other laboratory tests:

  • A laboratory test for carcinoembryonic antigen (CEA) may be ordered to help in staging. This protein is increased in many people with colon cancer, and blood levels generally correlate with the stage of the disease. CEA testing also may be used to evaluate the success of surgery or other treatments.
  • Genetic tests to detect KRAS, BRAF and/or NRAS gene mutations in tumor tissue may be used to guide cancer treatment and to evaluate prognosis in people with metastatic colon cancer. The presence of certain mutations indicates that anti-EGFR drug therapy, such as cetuximab and panitumumab, will not be effective in treating the cancer and a likely poorer prognosis.
  • Microsatellite instability (MSI) testing—this is another type of genetic test used to help guide treatment. About 15% of colon cancers have microsatellite instability. Studies have shown that tumors with MSI have better prognosis than tumors with no MSI, but MSI tumors may not respond to certain chemotherapies.

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