Colorectal cancer develops in the colon (the first 4–5 feet [0.6–0.9 meter] of the large intestine) or the rectum (the last few inches of the large intestine). Colorectal cancer affects the lower part of the digestive tract. It occurs most often in people over 50 but may develop in younger adults with a family history of colorectal cancer. Most colorectal cancers develop out of polyps, tissue growths that arise out of the tissue that lines the large intestine. Most polyps are benign, but some undergo changes in their genetic makeup that cause them to eventually become cancerous.
Colorectal cancer usually develops over a period of several years. In many cases the patient has no symptoms but is diagnosed as the result of screening for the disease. About half of patients go to their doctor because they have abdominal pain; a third notice a change in bowel habits; and 15 percent have an obstruction (blockage) in their intestines. In some cases these patients may notice that their bowel movements are unusually thin in shape. As a rule, the larger the cancer and the closer it is to the anus, the more likely the patient is to have noticeable changes in bowel habits.
Colorectal cancer is the fourth most common cancer in the United States. According to the American Cancer Society (ACS), about 112,000 people are diagnosed with colon cancer annually; about 41,000 new cases of rectal cancer are diagnosed each year; and about 57,000 persons die each year from colorectal cancer. According to the World Health Organization (WHO) there are about 940,000 new cases of and 500,000 deaths from colorectal cancer reported worldwide each year. Colorectal cancer is most common in older adults; the average age at the time of diagnosis is 72. Colon cancer in teenagers or young adults is unusual. Rates are equal for men and women. African Americans appear to have higher rates of colon cancer than members of other racial groups in the United States, but the reasons are unclear.
The lifetime risk of developing colon cancer in the United States is about 7 percent. Researchers at the National Cancer Institute have identified several factors that increase a person’s risk of colon cancer:
• Age over 50.
• A family history of colorectal cancer. Parents, siblings, or children of a person diagnosed with colon cancer have an increased risk of developing it, particularly if the relative was diagnosed at a young age.
• Having either of two specific genes that increase the risk of colon cancer. These genes are associated with 3 percent of all colon cancers, and can lead to colon cancer by age forty. They can be detected by genetic testing.
• In women, a personal history of breast cancer.
• A history of ulcerative colitis or Crohn’s disease.
• A history of polyps in the colon or rectum. Polyps are growths that develop along the inner wall of the colon or rectum, most often in people over fifty. Most are benign (not cancerous), but some may develop into cancerous tumors.
• A diet high in fat and low in fiber.
• A history of heavy smoking or alcohol consumption.
• Obesity and diabetes.
• Gigantism and other disorders involving growth hormone.
• Previous radiation treatment for cancers elsewhere in the abdomen.
Causes and Symptoms of Colorectal Cancer
The cause of most cases of colorectal cancer is the change in normally benign intestinal polyps to cancerous tumors. There are several different types of intestinal polyps, but only two carry a risk of developing into cancers. These two types can be removed during screening tests for colorectal cancer. The triggers that cause some polyps to become cancerous are not completely understood.
In addition to changes in bowel habits, abdominal cramping, and signs of intestinal blockage, patients with colorectal cancer may have the following symptoms:
• General tiredness, unexplained weight loss, and lack of appetite
• Bleeding from the rectum or blood or mucus on the stools
• Pain when passing a bowel movement
• Nausea and vomiting
• A feeling that the bowel hasn’t completely emptied following a bowel movement
How to Diagnose Colorectal Cancer
Doctors may use several methods to screen for colorectal cancer. The simplest are a digital rectal examination (DRE) and a fecal occult blood test (FOBT). In a DRE, the doctor inserts a gloved finger into the lower part of the rectum to feel for tumors. The FOBT requires the patient to take a kit home from the doctor’s office and collect a stool sample, which is then returned to the doctor or a laboratory to be tested for occult (hidden) blood. Patients must avoid eating rare meat and other foods that can affect the test results before using the kit.
Other tests that may be used to diagnose colorectal cancer include:
• Blood tests for tumor markers, substances that can be analyzed to detect the presence of cancer.
• Barium enema. Barium in enema form is given to coat the lining of the colon and rectum. Air is then blown into the colon in order to fill it. The resultant x-ray can be used to detect precancerous polyps as well as cancerous tumors.
• Sigmoidoscopy. A sigmoidoscope is a flexible lighted tube that can be inserted into the rectum and used to examine the last 2 feet (0.6 meter) of the colon. It can be done in a doctor’s office but does not provide a view of the entire colon. If a polyp or tumor is found, the doctor will recommend a colonoscopy in order to check the upper part of the colon.
• Colonoscopy. A colonoscope is a long flexible tube attached to a video camera and monitor that allows the doctor to examine the entire length of the patient’s colon and rectum. The patient must take a laxative the night before to cleanse the bowel and may be given a sedative in the doctor’s office to make them more comfortable.
The doctor can remove polyps during a colonoscopy or take tissue samples for analysis.
• Virtual colonoscopy. This technique uses computed tomography (a CT scan) to take images of the patient’s colon but is not yet available in all medical centers. Although virtual colonoscopy does not involve inserting a tube into the patient’s rectum, the patient must still take a laxative the night before to empty the bowel.
Treatment for Colorectal Cancer
The first step in treating colorectal cancer is called staging. Staging describes the location of the cancer, its size, how far it has penetrated into healthy tissue, and whether it has spread to other parts of the body. Colorectal cancers are classified into five stages:
• Stage 0: The cancer has not grown beyond the lining of the colon or rectum.
• Stage I: The cancer has penetrated through the lining of the colon or rectum into the underlying tissues but has not spread beyond the colon wall.
• Stage II. The cancer has grown through the wall of the colon or rectum but has not yet spread to nearby lymph nodes.
• Stage III. The cancer has spread to nearby lymph nodes but has not yet affected other organs.
• Stage IV. The cancer has spread to other organs. This process of spread is called metastasis. The most common locations of metastases from colorectal cancer are the liver, the lungs, the inside of the abdomen, or the ovaries.
The next steps in treatment depend on the stage of the cancer. Most colorectal cancers are first treated by some type of surgery.
• Small Stage 0 cancers may be completely removed during a colonoscopy. Some larger polyps can also be removed by inserting surgical instruments through the abdominal wall in a procedure called a laparoscopy.
• Stage I or Stage II cancers may be treated by removing the section of the colon that contains the tumor and then reconnecting the cut ends of the bowel. If reconnection is not possible, or if the cancer is at the lower end of the rectum, the doctor may have to perform a colostomy, in which an opening called a stoma is made in the wall of the abdomen and a portion of the remaining colon is attached to the stoma. The person’s body wastes pass through the stoma and are collected in a special bag attached to the outside of the body.
• If the cancer is advanced, surgery is unlikely to cure it. However, the surgeon can remove some of the tumor in order to relieve pain and bleeding.
• If the colorectal cancer has spread only to the liver and the patient’s health is otherwise good, the surgeon can remove the cancerous part of the liver with the colorectal tumor.
Radiation therapy or chemotherapy may be used following surgery to lower the risk of recurrence. Chemotherapy is often used to treat patients with Stage III or Stage IV cancer for a period of six to eight months after surgery. Radiation therapy is used more often to treat Stage III rectal cancer, although it may also be given to patients with colon cancer to relieve pain or to shrink tumors before surgery.
In spite of progress in early identification and treatment of colorectal cancer, it remains the third leading cause of death from cancer in the United States. Prognosis for recovery depends on the stage at which the disease is detected and treated. If it does not reappear (recur) within five years, it is considered cured. Stage I, II, and III colorectal cancers are considered potentially curable, but most doctors do not consider Stage IV cancer to be curable.
According to the NCI, 93 percent of colon cancer patients and 93 percent of rectal cancer patients who were diagnosed with Stage I cancer are still alive five years after diagnosis, but only 39 percent of colorectal cancers are detected at this early stage. The five-year survival rate drops to 8 percent for those diagnosed with Stage IV cancer.
How to Prevent Colorectal Cancer
Screening tests for colon cancer in adults over age 50 are good preventive measures. The death rate for colon cancer has dropped since 1990, in part because of increased awareness and screening by colonoscopy. Colon cancer can almost always be caught in its earliest and most curable stages by colonoscopy.