Colorectal Cancer
Almost all colorectal cancers 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 the second most common type 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. The rate for the entire U.S. population is about 50 new instances of colorectal cancer for every 100,000 people each year. Colon cancer is more common among women; rectal cancer is more common among men. About 5% of the people with colon or rectal cancer have cancer in two or more sites in the colon and rectum that do not appear to simply be spread from one site to another.
Risk Factors
People with a family history of colorectal cancer have a higher risk of developing the cancer themselves. A family history of polyps (see Section 9, Chapter 131) also increases the risk of colorectal cancer. People with ulcerative colitis or Crohn's disease are at greater risk as well; this risk is related to the person's age when the disease developed and the length of time the person has had the disease.
People at highest risk tend to consume a high-fat, low-fiber diet. Greater exposure to air and water pollution, particularly to industrial cancer-causing substances (carcinogens), may play a role.
Symptoms
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 late 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; a test of the stool for occult blood is needed to detect it (see Section 9, Chapter 119). 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 differential 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.
Diagnosis
Early diagnosis depends on routine screening. The stool can be tested for occult blood. 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. Alternatively, a doctor can test stool obtained during a digital rectal examination, in which he places a gloved finger 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. People at high risk may undergo colonoscopy, in which the entire large intestine is evaluated. Some growths that appear cancerous are removed using surgical instruments passed through the scope; other growths must be removed during regular surgery.
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 and 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 19-9 and CA 125, are similar to CEA and are sometimes elevated in colorectal cancer.
Prognosis and Treatment
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, even if metastases (spread) cannot be detected. Surgery, the main treatment for colorectal cancer, is curative in about 70% of cases.
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.
 See the figure Understanding Colostomy.
When rectal cancer has penetrated the rectal wall and spread to a very limited number of nearby lymph nodes, chemotherapy after surgical removal of all visible cancer may lengthen survival time. Also, radiation therapy after surgical removal of visible rectal cancer may help control the growth of any residual tumors, delay a recurrence, and lengthen survival time.
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. Survival time is typically only about 7 months. Chemotherapy with fluorouracil (sometimes also with another drug) may be given after surgery as part of the treatment for colorectal cancer that has spread widely, but the chemotherapy usually has little effect on how long the person survives. The doctor usually discusses end-of-life care with the person, the family, and other health care practitioners (see Section 1, Chapter 8). Even when the cancer has spread widely, surgery is sometimes performed to relieve the intestinal obstruction and ease symptoms.
 See the figure Staging Colon Cancer.
When the cancer has spread only to the liver, chemotherapy drugs can be injected 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.
For people who cannot tolerate surgery because of poor health, treatment may involve drying out and shrinking the tumor in a procedure called desiccation. Desiccation is performed either with a probe that applies an electrical charge to the surface of the tumor (cautery device) or with a device that dries the tumor with electrified Argon gas (Argon plasma coagulator); both devices can be passed through a colonoscope. Desiccation may relieve symptoms and lengthen survival time modestly by reducing tumor mass but rarely cures the cancer.
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