Definition of colon cancer: Cancer that forms in the tissues of the colon (the longest part of the large intestine). Most colon cancers are adenocarcinomas (cancers that begin in cells that make and release mucus and other fluids).
Unfortunately, colon cancers can be well advanced before they are detected. The most effective prevention of colon cancer is early detection and removal of precancerous colon polyps before they turn cancerous. Even in cases where cancer has already developed, early detection still significantly improves the chances of a cure by surgically removing the cancer before the disease spreads to other organs.

Multiple world health organizations have suggested general screening guidelines.

Digital rectal examination and stool occult blood testing

It is recommended that all individuals over the age of 40 have yearly digital examinations of the rectum and their stool tested for hidden or "occult" blood. During digital examination of the rectum, the doctor inserts a gloved finger into the rectum to feel for abnormal growths. Stool samples can be obtained to test for occult blood (see below). The prostate gland can be examined at the same time.

An important screening test for colorectal cancers and polyps is the stool occult blood test. Tumors of the colon and rectum tend to bleed slowly into the stool. The small amount of blood mixed into the stool is usually not visible to the naked eye. The commonly used stool occult blood tests rely on chemical color conversions to detect microscopic amounts of blood. These tests are both convenient and inexpensive. A small amount of stool sample is smeared on a special card for occult blood testing. Usually, three consecutive stool cards are collected. A person who tests positive for stool occult blood has a 30% to 45% chance of having a colon polyp and a 3% to 5% chance of having a colon cancer. Colon cancers found under these circumstances tend to be early and have a better long-term prognosis.

It is important to remember that having stool tested positive for occult blood does not necessarily mean the person has colon cancer. Many other conditions can cause occult blood in the stool. However, patients with a positive stool occult blood should undergo further evaluations involving barium enema x-rays, colonoscopies, and other tests to exclude colon cancer, and to explain the source of the bleeding. It is also important to realize that stool which has tested negative for occult blood does not mean the absence of colorectal cancer or polyps. Even under ideal testing conditions, at least 20% of colon cancers can be missed by stool occult blood screening. Many patients with colon polyps are tested negative for stool occult blood. In patients suspected of having colon tumors, and in those with high risk factors for developing colorectal polyps and cancer, flexible sigmoidoscopies or screening colonoscopies are performed even if the stool occult blood tests are negative.

Flexible sigmoidoscopy and colonoscopy

Beginning at age 50, a flexible sigmoidoscopy screening tests is recommended every three to five years. Flexible sigmoidoscopy is an exam of the rectum and the lower colon using a viewing tube (a short version of colonoscopy). Recent studies have shown that the use of screening flexible sigmoidoscopy can reduce mortality from colon cancer. This is a result of the detection of polyps or early cancers in people with no symptoms. If a polyp or cancer is found, a complete colonoscopy is recommended. The majority of colon polyps can be completely removed by colonoscopy without open surgery. Recently doctors are recommending screening colonoscopies instead of screening flexible sigmoidoscopies for healthy individuals starting at ages 50-55. Please read the Colon Cancer Screening article.

Patients with a high risk of developing colorectal cancer may undergo colonoscopies starting at earlier ages than 50. For example, patients with family history of colon cancer are recommended to start screening colonoscopies at an age 10 years before the earliest colon caner diagnosed in a first-degree relative, or five years earlier than the earliest precancerous colon polyp discovered in a first-degree relative. Patients with hereditary colon cancer syndromes such as FAP, AFAP, HNPCC, and MYH are recommended to begin colonoscopies early. The recommendations differ depending on the genetic defect, for example in FAP; colonoscopies may begin during teenage years to look for the development of colon polyps. Patients with a prior history of polyps or colon cancer may also undergo colonoscopies to exclude recurrence. Patients with a long history (greater than 10 years) of chronic ulcerative colitis have an increased risk of colon cancer, and should have regular colonoscopies to look for precancerous changes in the colon lining.

Genetic counseling and testing

Blood tests are now available to test for FAP, AFAP, MYH, and HNPCC hereditary colon cancer syndromes. Families with multiple members having colon cancers, members with multiple colon polyps, members having cancers at young ages, and having other cancers such as cancers of the ureters, uterus, duodenum, etc., should be referred for genetic counseling followed possibly by genetic testing. Genetic testing without prior counseling is discouraged because of the extensive family education that is involved and the complicated nature of interpreting the test results.

The advantages of genetic counseling followed by genetic testing include: (1) identifying family members at high risk of developing colon cancer to begin colonoscopies early; (2) identifying high risk members so that screening may begin to prevent other cancers such as ultrasound tests for uterine cancer, urine examinations for ureter cancer, and upper endoscopies for stomach and duodenal cancers; and (3) alleviating concern for members who test negative for the hereditary genetic defects.

Diet and colon cancer to prevent colon cancer

People can change their eating habits by reducing fat intake and increasing fiber (roughage) in their diet. Major sources of fat are meat, eggs, dairy products, salad dressings, and oils used in cooking. Fiber is the insoluble, nondigestible part of plant material present in fruits, vegetables, and whole-grain breads and cereals. It is postulated that high fiber in the diet leads to the creation of bulky stools which can rid the intestines of potential carcinogens. In addition, fiber leads to the more rapid transit of fecal material through the intestine, thus allowing less time for a potential carcinogen to react with the intestinal lining.