Colorectal Cancer, which screening test is best?

By April 28, 2011Health Tips
Colorectal cancer (CRC) is the second leading cancer killer in the U.S. That is an alarming statistic, considering that, for the most part, CRC is a preventable disease. CRC usually begins as benign growths called polyps in the colon or rectum, and removing these growths effectively eliminates the possibility that they will develop into cancer.

The importance of early detection is underscored by these statistics from the American Gastroenterological Association:

  • When CRC is diagnosed at the early localized stage, the five-year survival rate is 90 percent.
  • When CRC is not diagnosed until the late distant stage, the five-year survival rate is only 10 percent.

There are a number of ways to screen for CRC. The two main types of tests are: 1) those that look for signs of colon cancer from the stool, and 2) those that visually inspect the colon looking for pre-malignant lesions such as polyps.

Stool Tests for CRC:

  • CalorieFecal occult blood test(FOBT). This is the most common type of stool test, often known by its commercial name “Hemoccult”. The FOBT is used to find hidden (occult) blood in feces coming from damaged blood vessels on the surface of polyps or colon cancers. Only rarely is there enough bleeding to be visible in the stool, but by placing a special chemical (guaiac) on a small sample of stool, the presence of blood can be detected. To be most accurate, the FOBT should be done on several stool samples on an annual basis.
  • Fecal immunochemical test (FIT). The FIT is a newer kind of test that also detects occult blood in the stool. This test reacts to part of the human hemoglobin protein, which is found in red blood cells. It is recommended that this test be done on an annual basis.
  • Stool DNA test (sDNA). This is the newest of the stool tests and has not yet gained wide acceptance. Its works by detecting DNA markers (genetic changes) that are common to colon cancer or precancerous polyps from a stool specimen. Barriers to the use of the stool DNA test include its high rate of false positives (a positive test for cancer when no cancer is present) and expense.

All of the stool tests for CRC have several serious drawbacks. First, if a positive stool test is detected, a follow-up test, such as a colonoscopy will need to be performed to determine why the test was abnormal. Secondly, blood in the stool can be a sign of colon cancer, but it is more often caused by something else, such as hemorrhoids, ulcers, or taking aspirin. These other conditions can cause a “positive” result even when you don’t have cancer. In addition, stool tests may only become positive after CRC has developed. They are not as good at picking up premalignant lesions (e.g. polyps).

Tests that visually inspect the colon:

  • Sigmoidoscopy. A sigmoidoscope is a flexible, tube-like instrument with a light and a lens for viewing the walls of the colon. This instrument is approximately 2 feet long, which only allows it to inspect the rectum and lower portion of the colon. Some physicians who perform sigmoidoscopy are able to remove polyps or tissue samples during this procedure, which can then be checked by a Pathologist for signs of cancer.
  • Colonoscopy. This test is similar to sigmoidoscopy although the instrument is longer and able to view the entire colon. Biopsies or removal of polyps can be performed at the time of the study.
  • CalorieCT colonography (virtual colonoscopy). This is a special type of x-ray test that uses a computer program to create a 3-dimensional picture of the lining of the colon. If detected during this procedure, however, the polyps or cancer cannot be biopsied or removed. A second procedure (usually a colonoscopy) must be performed to have this done.
  • Air contrast barium enema. The air-contrast barium enema is a type of x-ray test that was used for years before sigmoidoscopy or colonoscopy were in widespread use. In performing this test, a chalky liquid called barium sulfate is introduced into the colon and x-rays are performed. If suspicious areas are seen on this test, a colonoscopy will be needed to explore them further.

Screening recommendations: According to the American Cancer Society, if everyone were screened for CRC, tens of thousands of lives could be saved each year. Screening recommendation, however, vary among experts. The U.S. Preventive Services Taskforce (USPSTF) recommends screening for colorectal cancer in adults aged 50 to 75 years using FOBT (annually), sigmoidoscopy (every 5 years), or colonoscopy (every 10 years). They do not recommend screening after the age of 75 since the development of colon cancer from a polyp is so slow. The USPSTF does not support the use of computed CT colonography or fecal DNA testing to screen for CRC at this time. The American Cancer Society, the U.S.Task Force on Colorectal Cancer, and the American College of Radiology recommend screening for colorectal cancer beginning at 50 years of age by 1) FOBT or FIT testing annually, 2) flexible sigmoidoscopy every 5 years, 3) double-contrast barium enema every 5 years, 4) CT colonography (virtual colonoscopy) every 5 years, or 5) colonoscopy every 10 years. With few exceptions, authorities agree that of all the screening tests available, colonoscopy is considered the “gold standard” against which the others can be compared.

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