Almost all cancers of the large intestine and rectum (colorectal) are adenocarcinomas, which develop from the lining of the large intestine (colon) and rectum. Colorectal cancer usually begins as a buttonlike swelling on the surface of the intestinal or rectal lining or on a polyp. As the cancer grows, it begins to invade the wall of the intestine or rectum. Nearby lymph nodes also may be invaded. Because blood from the wall of the intestine and much of the rectum is carried to the liver, colorectal cancer usually spreads (metastasizes) to the liver soon after spreading to nearby lymph nodes.
In Western countries, cancer of the large intestine and rectum is one of the most common types of cancer and the second leading cause of cancer death. The incidence of colorectal cancer begins to rise at age 40 and peaks between the ages of 60 and 75. Each year, about 143,000 people in the United States develop colorectal cancer and about 51,000 die. Colon cancer is more common among women, and rectal cancer is more common among men. About 5% of people with colon cancer or rectal cancer have cancer in two or more sites in the colon and rectum that do not seem to simply have spread from one site to another.
People with a family history of colorectal cancer have a higher risk of developing the cancer themselves. A family history of polyps (see Tumors of the Digestive System: Polyps of the Colon and Rectum) also increases the risk of colorectal cancer.
People with ulcerative colitis or Crohn disease of the colon are at greater risk as well. This risk is related to the person's age when the disease developed, the amount of intestine or rectum that is affected, and the length of time the person has had the disease.
People at highest risk tend to consume a diet that is high in fat, animal protein, and refined carbohydrates and low in fiber. Greater exposure to air and water pollution, particularly to industrial cancer-causing substances (carcinogens), may play a role.
Hereditary nonpolyposis colorectal carcinoma (HNPCC):
HNPCC comes from an inherited gene mutation that causes cancer in 70 to 80% of the people with that mutation. People with HNPCC often develop colorectal cancer before age 50. They are also at increased risk of other types of cancer, particularly endometrial cancer and ovarian cancer, but also stomach cancer, cancer of the small intestine, and kidney cancer.
Colorectal cancer grows slowly and does not cause symptoms for a long time. Symptoms depend on the type, location, and extent of the cancer.
Fatigue and weakness resulting from occult bleeding (bleeding not visible to the naked eye) may be the person's only symptoms. A tumor in the left (descending) colon is likely to cause obstruction at an earlier stage, because the left colon has a smaller diameter and the stool is semisolid. Cancer tends to encircle this part of the colon, causing alternating constipation and frequent bowel movements before obstruction. The person may seek medical treatment because of crampy abdominal pain or severe abdominal pain and constipation. A tumor in the right (ascending) colon does not cause obstruction until later in the course of the cancer, because the ascending colon has a large diameter and the contents flowing through it are liquid. By the time the tumor is discovered, therefore, it may be so large that a doctor can feel it through the abdominal wall.
Most colon cancers bleed, usually slowly. The stool may be streaked or mixed with blood, but often the blood cannot be seen. Testing the stool for occult blood is needed (see Diagnosis of Digestive Disorders: Occult Blood Tests). The most common first symptom of rectal cancer is bleeding during a bowel movement. Whenever the rectum bleeds, even if the person is known to have hemorrhoids or diverticular disease, doctors must consider cancer as part of their diagnosis. Painful bowel movements and a feeling that the rectum has not been completely emptied are other symptoms of rectal cancer. Sitting may be painful, but otherwise the person usually feels no pain from the cancer itself unless it spreads to tissue outside the rectum.
Early diagnosis depends on routine screening, which should typically begin at age 50. The stool can be tested for blood that cannot be seen by the naked eye (occult blood). Usually, the stool is tested every year. To help ensure accurate test results, the person eats a high-fiber diet that is free of red meat for 3 days before providing a stool sample. Some newer stool tests for blood do not require people to avoid red meat. Alternatively, a doctor can test stool obtained during a digital rectal examination, in which a gloved finger is inserted in the person's rectum. If blood is detected, further testing is needed.
Sigmoidoscopy (examination of the lower portion of the large intestine with a viewing tube) is another diagnostic procedure performed for screening. Unlike occult blood testing, sigmoidoscopy needs to be done only every 5 years. Some doctors use colonoscopy, in which the entire large intestine is examined, to screen all people. With colonoscopy, the test needs to be done only every 10 years. People at high risk of cancer should usually have colonoscopy for screening. In such cases, colonoscopy is done more frequently. Some growths that appear cancerous (malignant) are removed using surgical instruments passed through the scope. Other growths must be removed during regular surgery.
Computed tomography (CT) colonography (virtual colonoscopy) generates three-dimensional images of the colon by using a special CT scan technique. In this technique, people drink a contrast agent and their colon is inflated with gas from a tube inserted in the rectum. Viewing the high-resolution three-dimensional images somewhat simulates the appearance of regular endoscopy, hence the name. Virtual colonoscopy may be an option for people who are unable or unwilling to undergo the regular colonoscopy procedure, but it is less sensitive and highly interpreter-dependent. Virtual colonoscopy avoids the need for sedation but still requires a thorough bowel preparation, and the gas may be uncomfortable. Additionally, unlike with regular colonoscopy, lesions cannot be removed for examination under a microscope (biopsied) during the procedure.
Capsule endoscopy (see Diagnosis of Digestive Disorders: Capsule Endoscopy) offers promise for the future but currently has many technical problems that limit its usefulness for evaluating the colon.
People with HNPCC require ongoing screening for other cancers. Such screening includes ultrasonography of the female organs (done through the vagina), examination of cells taken from the endometrium with a suction device, and tests of the blood and urine. Close family members of people with HNPCC should have colonoscopy every 1 to 2 years beginning in their 20s, and women should be tested every year for endometrial and ovarian cancer.
People with blood in their stool require colonoscopy, as do those with abnormalities seen during a sigmoidoscopy or an imaging study. Any lesions seen should be completely removed during the colonoscopy.
Once cancer is diagnosed, doctors usually perform an abdominal CT, chest x-ray, and routine laboratory tests to look for cancer that has spread, to detect a low blood count (anemia), and to evaluate the person's overall condition.
Blood tests are not used to diagnose colorectal cancer, but they can help the doctor monitor the effectiveness of treatment after a tumor has been removed. For example, if levels of carcinoembryonic antigen (CEA) are high before surgery to remove a known cancer but are low after surgery, monitoring for another increase in the CEA level may help detect an early recurrence of the cancer. Two other cancer markers, CA 199 and CA 125, are similar to CEA and are sometimes elevated in colorectal cancer.
People with HNPCC undergo genetic testing.
Colon cancer is most likely to be cured if it is removed early, before it has spread. Cancers that have grown deeply or through the wall of the colon have often spread, and sometimes these cancers cannot be detected. The 10-year survival rate is about 90% when the cancer is only in the lining of the bowel wall, about 70 to 80% when the cancer extends through the bowel wall, only about 30 to 50% when the cancer has spread to the lymph nodes in the abdomen, and less than 20% when the cancer has metastasized.
In most cases of colon cancer, the cancerous segment of the intestine and any nearby lymph nodes are removed surgically, and the remaining ends of the intestine are joined. When people have colon cancer that has penetrated the wall of the large intestine and spread to a very limited number of nearby lymph nodes, chemotherapy after surgical removal of all visible cancer may lengthen survival time, although the effects of these treatments are often modest.
For rectal cancer, the type of operation depends on how far the cancer is located from the anus and how deeply it has grown into the rectal wall. The complete removal of the rectum and anus leaves the person with a permanent colostomy, which is a surgically created opening between the large intestine and the abdominal wall. The contents of the large intestine empty through the abdominal wall into a colostomy bag. If possible, however, only part of the rectum is removed, leaving a rectal stump and the anus intact. Then the rectal stump is rejoined to the end of the large intestine.
When rectal cancer has penetrated the rectal wall and spread to a very limited number of nearby lymph nodes, giving chemotherapy plus radiation therapy after surgical removal of all visible cancer may lengthen survival time. Some doctors give chemotherapy and radiation therapy before surgery.
When cancer has spread to lymph nodes far from the colon or rectum, to the lining of the abdominal cavity, or to other organs, the cancer cannot be cured by surgery alone. However, surgery is sometimes done to relieve any intestinal obstruction and ease symptoms. Survival time is typically only about 7 months. Chemotherapy with a single drug or combination of drugs may shrink the cancer and prolong life for several months. The doctor usually discusses end-of-life care with the person, the family, and other health care practitioners (see Death and Dying: Introduction to Death and Dying).
|Staging Colon Cancer
STAGE 0: Cancer is limited to the inner layer (lining) of the large intestine (colon) covering the polyp. More than 95% of people with cancer at this stage survive at least 5 years.
STAGE 1: Cancer spreads to the space between the inner layer and muscle layer of the large intestine. (This space contains blood vessels, nerves, and lymph vessels.) More than 90% of people with cancer at this stage survive at least 5 years.
STAGE 2: Cancer invades the muscle layer and outer layer of the colon. About 55 to 85% of people with cancer at this stage survive at least 5 years.
STAGE 3: Cancer extends through the outer layer of the colon into nearby lymph nodes. About 20 to 55% of people with cancer at this stage survive at least 5 years.
STAGE 4 (not shown): Cancer spreads to other organs, such as the liver, lungs, or ovaries, or to the lining of the abdominal cavity (peritoneum). Fewer than 1% of people with cancer at this stage survive at least 5 years.
When the cancer has spread only to the liver and there are 3 or fewer tumors (metastases), doctors sometimes remove these tumors surgically. Alternatively, doctors may inject chemotherapy drugs directly into the artery supplying the liver. A small pump inserted surgically beneath the skin or an external pump worn on a belt allows the person to be mobile during the treatment. This treatment may provide more benefit than ordinary chemotherapy, but more research is needed. When cancer has spread beyond the liver, this approach has no advantage.
If the cancer is obstructing the colon in a person who cannot tolerate surgery because of poor health, doctors may try to relieve symptoms in other ways. One treatment involves shrinking the tumor with a probe that applies an electric current (electrocautery) or with a laser. Alternatively, doctors may use an expandable wire mesh tube to hold the obstructed area open. All of these treatments can be done through a colonoscope. Although the person often feels better for a while, these treatments do not lengthen survival time.
After surgery, colonoscopy should be done every year for 5 years and every 3 years after that if no polyps or tumors are found. Doctors also do blood tests such as a complete blood count and liver function tests at intervals after surgery. Imaging tests such as CT or MRI are useful mainly when doctors find abnormalities during the examination or people have abnormal blood test results.
Last full review/revision February 2013 by Elliot M. Livstone, MD