Food that is swallowed goes from the stomach into the small intestine and then the large intestine. What’s left after food is digested leaves the body as waste (stool) through the rectum. The colon is the last part of the large intestine, just before the rectum. Colon cancer is a tumor of the large intestine, often the lower rectum and sigmoid colon but can also involve the beginning (cecum). It’s also called colorectal cancer or adenocarcinoma of the colon. It’s the most common cancer of the digestive tract.
Colon cancer usually starts as a small growth, called a polyp, on the surface of the colon. Some polyps are harmless, but some can turn into cancer.
Symptoms include a change in bowel habits, such as constipation or diarrhea, change in stool size (e.g., pencil thin) or appearance (e.g., black, tarry-looking), rectal bleeding, and pain in the abdomen (belly). Sometimes people have no symptoms, but iron deficiency anemia may occur from chronic blood loss and can be detected by blood tests ordered by your physician.
Early diagnosis is critical, because undetected or untreated cancer usually spreads (metastasizes) through the intestine wall into nearby areas, liver, and sometimes lungs and bones.
The health care provider may suspect the diagnosis from the physical examination, including a digital rectal examination (DRE), and from blood tests showing low blood and iron deficiency anemia. Stool samples are tested for hidden (occult) blood. Tests to look for abnormal DNA found in cancer cells in the stool are also available. If cancer is suspected, a specialist (gastroenterologist surgeon) then looks inside the colon with a flexible tube called a colonoscope and obtains tissue samples of abnormal areas (biopsy). A computed tomography (CT) scan and other tests may be done to determine if the cancer has spread outside of the colon. A newer test called "virtual colonoscopy" or "CT colongraphy" involves taking CT scans of the colon to look for abnormalities. If abnormal colonoscopy will be done. The bowel preparation for both colonoscopy and virtual colonoscopy is identical.
Treatment depends on how far the cancer spread. Abdominal and pelvic CT can tell whether it spread outside the colon. This information is important because it’s used to decide the kind of treatment. Surgery is best for complete tumor removal. If cancer hasn’t spread past the colon, surgery offers the best chance for cure. Even if cancer can’t be cured because it spread to other parts of the body, surgery can make people more comfortable by preventing obstruction of the colon lumen by the cancer.
Medicine (chemotherapy) and radiation can be used to try to kill cancer cells not removed by surgery. Both may have side effects, including inflammation with diarrhea and bloody stools (radiation) and bone marrow problems leading to infections, bleeding, and anemia (chemotherapy).
More than one half of people with colon cancer can be cured. The success rate depends on the cancer’s stage.
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