The colon, also known as the large intestine or large bowel, constitutes the last part of the digestive tract. The colon is a long, muscular tube that receives still-
undigested food from the small intestine. It removes water from the undigested food, stores it and then finally eliminates it from the body as stool or feces through
bowel movements. The rectum is the last part of the bowel adjacent to the anus through which stool passes to the outside.
Cancer of the colon and rectum (colorectal cancer) is a type of malignant tumor arising from the inner wall of the large intestine. These malignant tumors are called
cancers and can invade nearby tissue and spread to other parts of the body. Benign tumors of the colon are usually called polyps. Benign polyps do not invade nearby
tissue or spread to other parts of the body like malignant tumors do. Benign polyps can be removed easily during colonoscopy and are not life-threatening. However, if
benign polyps are not removed from the large intestine, they can become malignant (turn into cancer) over time. In fact, most of the cancers of the large intestine are
believed to have evolved from benign polyps that are precancerous, that is, they are benign at first but later become cancerous.
Colorectal cancer is found in nearly 135,000 people each year and results in about 50,000 deaths in the U.S. It is the second most common cause of death due to cancer
in the U.S. after lung cancer. It is the second most common cancer in women and the third most common cancer in men. The lifetime risk for an adult American to develop
colorectal cancer is 4.4%.
Cancer of the colon and rectum can invade and damage adjacent tissues and organs. Cancer cells also can break away and spread to other parts of the body (such as the
liver and lung) where new tumors grow. The process whereby colon cancer spreads to distant organs is called metastasis, and the new tumors are called metastases.
Direct extension to or invasion of adjacent organs is a sign of a more advanced cancer, and the chance of cure in the treatment of a cancer which has directly extended
into an adjacent tissue is less, even with surgery, as hidden cancer cells may also have spread elsewhere. If a colon or rectal cancer is found to have spread through
the lymph channels to adjacent lymph nodes, it is increasingly likely that even the removal of the portion of the colon and lymph nodes will not cure the patient.
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Finding lymph node metastases suggests that undetectable microscopic cancer cells may be more likely to still be present elsewhere in the body. If the cancer spreads
through the bloodstream to the liver, lungs, bones, or other organs, or through lymph channels to distant lymph nodes, then it is unlikely that a permanent cure will
be obtained with treatment.
Colorectal cancer is both preventable and curable when found early. Colorectal cancer is prevented by removing precancerous colon polyps. It is cured if cancerous
change is found early and is surgically removed before the cancer cells spread to other parts of the body. The National Polyp Study showed in its surveillance program
that individuals who had their polyps removed experienced a 90% reduction in the incidence of colorectal cancer. The few patients in the study who did develop
colorectal cancer had their cancer discovered at early, surgically or endoscopically curable stages. Since most colon polyps and early cancers are silent (produce no
symptoms), it is important to do screening and surveillance for colon cancer in patients without symptoms or signs of the polyps or cancers. Recommendations for cost-
effective public screening and surveillance have been promulgated and endorsed by numerous societies including the U.S. Preventive Services Task Force (USPSTF),
American Cancer Society, the National Cancer Institute, American College of Gastroenterology, American Medical Association, American College of Physicians, etc.
Screening recommendations for individuals with average risk of colon cancer
Fecal (stool) occult blood tests (FOBT) performed on stool samples, stool DNA tests, flexible sigmoidoscopic examinations, and colonoscopy are among the recommended
screening tests for individuals at average risk for developing colorectal cancer. These tests are designed to detect and to prompt removal of precancerous polyps and
identify early cancers in order to decrease deaths from colorectal cancer. Most guidelines recommend beginning screening at 50 years of age for people at average risk
of developing colorectal cancer. Some groups recommend that African Americans begin screening at a slightly younger age due to an increased risk.
Fecal occult blood tests are chemical tests that are performed on samples of stool to detect the presence of “occult” blood (amounts of blood that are so small that
they cannot be seen with the naked eye). These tests usually are performed along with a digital rectal examination (DRE) that is performed by a doctor. The use of
fecal occult blood tests is based on observations that slow bleeding from colon polyps or cancers can cause small amounts of blood to mix with the colonic contents.
Since the small amounts of blood are not visible to the naked eye, sensitive tests are needed to detect the traces of blood in the stool. The newest form of the test
is known as a fecal immunochemical, or FIT test and is even more sensitive than the older chemical or guaiac-based tests (gFOBT) for diagnosing colorectal cancer.
A stool DNA test (Cologuard or FIT-DNA test) has also been developed as a screening tool. DNA testing identifies abnormal genes in stool that are characteristic of
colorectal cancer from cells that have broken off from colorectal cancers and some premalignant tumors. The FIT-DNA is a stool DNA test (MT-sDNA), combining fecal DNA,
fecal immunochemical test (FIT), and DNA methylation assays. (DNA methylation is a type of genetic abnormality that controls the expression of the abnormal DNA.)
An individual whose stool specimen tests positive for occult blood then undergoes a colonoscopy of the entire colon to look for polyps, cancers, or other conditions
that cause bleeding (such as abnormal blood vessels, diverticuli, or colitis). The majority (greater than 90%) of the polyps detected at colonoscopy can be removed
painlessly and safely during the colonoscopic examination. Polyps so removed are examined later under the microscope by a pathologist to determine if they are
precancerous. Individuals with precancerous polyps have a higher than average risk for developing colon cancer and are advised to return for periodic surveillance
Colon cancers that are detected at colonoscopy usually are removed surgically though under certain circumstances they may be removed at colonoscopy. Precancerous
polyps that are too large or technically not possible to remove during colonoscopy also are removed surgically. Several studies have shown that fecal occult blood and
related testing can reduce death rates (mortality) from colorectal cancer by 30%-40%.
If no colonic abnormalities are found in an individual whose stool contains occult blood, consideration then is given to examining the stomach and the small intestine
as sources of bleeding.