Colo, malignancy of the large intestine, the lower portion of the intestinal tract, which consists of the colon and rectum. Although colon cancer can occur in any segment of the colon, it is most common in the sigmoid colon, the section closest to the rectum. The American Cancer Society estimates that 95, 000 new cases of colon cancer and 35, 000 cases of rectal cancer are diagnosed annually in the United States. An estimated 47, 900 people with colon cancer and 8, 700 with rectal cancer die from the diseases each year. These cancers are the third most common of all cancers, as well as the third most frequent cause of cancer death in both men and women. The risk of colorectal cancer increases significantly with age.
About 90 percent of all colorectal cancers are diagnosed in people over the age of 50. A family history of colorectal cancer, the presence of polyps (abnormal but usually benign growths) in the large intestine, or inflammatory bowel disease are also risk factors. Mutated versions of several genes have been linked to colon cancer. For example, in their normal form the genes MSH 2, MLH 1, PMS 1, and PMS 2 correct tiny errors that occur when cells divide and grow. Mutated versions of these genes cannot make such repairs, and eventually an accumulation of many such errors interferes with a cell’s ability to resist the uncontrolled division and growth that characterize cancer. Research has linked the consumption of certain foods to colorectal cancer.
The more red meat and animal fat that people eat, the greater their risk of developing colorectal cancer. On the other hand, some studies indicate that diets high in fiber (indigestible roughage) may reduce the risk of the disease. These factors are believed to explain why rates of colon cancer are higher in the United States where a high-fat, high-protein, low-fiber diet is the norm than in Japan where a low-fat, low-protein, high-fiber diet is common. Studies suggest that some drugs may decrease the risk of colorectal cancer. For instance, estrogen replacement therapy after menopause and the use of nonsteroidal anti-inflammatory drugs such as ibuprofen appear to reduce the chances of developing this cancer. Colorectal cancer usually develops slowly and may not present apparent symptoms in its early stages.
Some individuals with undiagnosed colorectal cancer may detect blood in their bowel movements (feces). They may also experience persistent constipation or diarrhea, abdominal pain, or unexplained weight loss. Two simple tests can detect most colorectal tumors while they are still in an early, easy-to-treat stage. The first test is the digital rectal examination, during which the physician uses a gloved finger to gently check the smoothness of the rectal lining. The second test is the fecal occult blood test, in which a small sample of the patient’s feces is smeared on a card coated with a chemical called guai ac, which reacts with blood. The card is analyzed in a laboratory for occult (hidden) blood.
A positive result does not necessarily indicate the presence of cancer. Although most colorectal cancers bleed, so do benign conditions such as hemorrhoids. A more definitive test is fiberoptic sigmoidoscopy, in which a flexible instrument is inserted into the lower intestinal tract through the anus. This instrument has light-conducting fibers that enable a physician to visually examine the interior of the colon and rectum. A biopsy (removal of tissue samples) can be performed simultaneously with a special biopsy tool attached to the end of the sigmoidoscopy.
The tissue is then examined under a microscope for signs of cancerous cells. If these tests reveal a possible problem, more extensive tests are used. Colonoscopy uses a much longer flexible instrument than fiberoptic sigmoidoscopy, enabling a physician to view the entire length of the large intestine. The patient may be given a barium enema followed by an X-ray examination of the large intestine to detect unusual growths. Early diagnosis is a major factor in surviving colorectal cancer. The American Cancer Society recommends that people aged 50 years and older have a yearly fecal occult blood test and also follow one of the following screening options: a sigmoidoscopy every five years, a colonoscopy every ten years, or a double contrast barium enema every five to ten years.
A digital rectal examination should be performed at the time of each screening sigmoidoscopy, colonoscopy, or barium enema examination. The primary treatment for colorectal cancer is surgery to remove the tumor. The surgery may be combined with radiation, chemotherapy, or both. Using a combination of high-dose radiation and chemotherapy prior to surgery now make it possible to avoid permanent colostomies in many patients who previously would have needed this procedure.
A colostomy is a surgical procedure to create an artificial opening through the abdominal wall to the exterior of the body for elimination of wastes into a plastic bag. If cancer has spread from the colorectal area to the lymph nodes or liver, surgery or chemotherapy with a drug called fluorouracil prolongs the lives of some patients. According to the American Cancer Society, 83 percent of people diagnosed with colorectal cancer survive one year after diagnosis and 64 percent survive five years. If the cancer is discovered while still localized, the five-year relative survival rate (a measure used to monitor persons who are living five years after diagnosis) is 93 percent. If the cancer has metastasized, or spread, to adjacent organs or lymph nodes, the rate is 64 percent. If it has metastasized to distant organs, the rate of survival is less than 7 percent.
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