Colorectal Cancer
What Is Colorectal Cancer?
Colorectal cancer is a term used to refer to cancer that develops in the colon or the rectum. The colon and rectum are parts of the digestive system, which is also called the gastrointestinal, or GI, system. The digestive system processes food for energy and rids the body of solid waste matter (fecal matter or stool).
Signs and Symptoms of Colorectal Cancer
If you have any of the following you should check with your doctor for prompt diagnosis and treatment:
- A change in bowel habits such as diarrhea, constipation, or narrowing of the stool that lasts for more than a few days
- A feeling that you need to have a bowel movement that is not relieved by doing so
- Rectal bleeding or blood in the stool (often, though, the stool will look normal)
- Cramping or steady abdominal (stomach area) pain
- Weakness and fatigue
Do We Know What Causes Colorectal cancer?
Although we do not know the exact cause of most colorectal cancers, there are certain known risk factors, and there is a great deal of research going on to find answers to the question.
A small percentage of colorectal cancers are known to be caused by inherited gene mutations (changes in DNA). Recently, scientists have discovered many of these DNA changes, learned how they change the growth control of cells, and determined how the changes can be detected in people before colorectal cancers develop.
Changes in a gene called APC, for example, are responsible for familial adenomatous polyposis (FAP) and Gardner syndrome. This gene is normally responsible for slowing the growth of cells. In patients who have inherited changes in the APC gene, this "brake" on cell growth is turned off and hundreds of polyps develop in the colon. Over time, cancer will nearly always develop in one or more of these polyps because of new gene mutations in the cells of the polyps. We all have these new gene mutations. But they rarely lead to cancer because the cells die instead of continuing to grow as they do when the APC "brake" is turned off.
In addition, a defective DNA repair mechanism is responsible for hereditary nonpolyposis colon cancer (HNPCC). Cells must make a new copy of their DNA each time they divide. Occasional errors are made in copying the DNA code. Fortunately, cells have DNA repair enzymes that act like proofreaders or "spell checkers." Mutations in the DNA repair enzyme genes in HNPCC allow DNA errors to go uncorrected. Mutations in at least four different genes can lead to errors in repair. These errors will sometimes affect growth-regulating genes. This can lead to the development of cancer.
Tests are available that can detect gene mutations associated with FAP and HNPCC. If you have a family history of colorectal cancer or any of the associated cancers discussed above, you should ask your doctor about genetic counseling and genetic testing. The American Cancer Society recommends discussing genetic testing with a qualified genetic counselor before genetic testing is done.
Most people with colorectal cancer do not have an inherited gene mutation. Instead, the gene mutations develop spontaneously. Many doctors think the first mutation occurs in the APC gene. This leads to an increased growth of colorectal cells because of the loss of this “brake” molecule. Another mutation then occurs in the gene called K-RAS and causes this gene to become an “accelerator” of cell growth. Many other mutations eventually occur and lead the cells to grow uncontrollably.
Can Colorectal Cancer Be Prevented?
Even though we do not know the exact cause of most colorectal cancer, it is possible to prevent many colorectal cancers.
Screening: One of the most powerful weapons in preventing colorectal cancer is regular colorectal cancer screening or testing. From the time the first abnormal cells start to grow, it usually takes about 10 to 15 years for them to develop into colorectal cancer. Regular colorectal cancer screening can, in many cases, prevent colorectal cancer altogether. (See the American Cancer Society screening guidelines in the next section "Can Colorectal Polyps and Cancer Be Found Early?"). This is because polyps, or growths, can be detected and removed before they have the chance to turn into cancer. Screening can also result in finding colorectal cancer early, when it is highly curable.
People who have no identified risk factors (other than age) should begin regular screening at age 50. Those who have a family history or other risk factors for colorectal cancer polyps or cancer need to talk with their doctor about starting screening at a younger age and more frequent intervals.
Diet and exercise: People can lower their risk of developing colorectal cancer by managing the risk factors that they can control, such as diet and physical activity. It is important to eat plenty of fruits, vegetables, and whole grain foods and to limit intake of high-fat foods. Physical activity is another area that people can control. The American Cancer Society recommends at least 30, preferably 45 to 60 minutes of physical activity on 5 or more days of the week. If you are overweight, you can ask your doctor about a weight loss plan that will work for you. For more information about diet and physical activity, refer to the American Cancer Society document, American Cancer Society Guidelines for Nutrition and Physical Activity for Cancer Prevention.
Vitamins, calcium, magnesium: Some studies suggest that taking a daily multivitamin containing folic acid, or folate, can lower colorectal cancer risk. Other studies suggest that increasing calcium intake may lower risk. Some have suggested that vitamin D, which you can get from sun exposure or in a vitamin pill, can lower colorectal cancer risk. Of course, excessive sun exposure can cause skin cancer and is not recommended as a way to lower colorectal cancer risk. Calcium and vitamin D may work together to reduce colorectal cancer risk, as vitamin D aids in the body’s absorption of calcium. In addition, one recent study suggested that a diet high in magnesium may also reduce colorectal cancer risk in women.
Nonsteroidal anti-inflammatory drugs: Many studies have found that people who regularly use aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen (Motrin, Advil) and naproxen (Aleve), have a 20% to 50% lower risk of colorectal cancer and adenomatous polyps. Most of these studies, however, are based on observations of people who took these medications for reasons such as treatment of arthritis or prevention of heart attacks. Two recent studies have provided even stronger evidence regarding the ability of aspirin to prevent the growth of polyps. The advantage of these recent studies is that people were randomly selected by the researchers to receive either aspirin or an inactive placebo. One study included people who were previously treated for early stages of colorectal cancer, and the other study included people who previously had polyps removed.
But NSAIDs can cause serious or even life-threatening bleeding from stomach irritation. Currently available information suggests that the risks of serious bleeding outweigh the benefits of these medicines for the general public. For this reason, experts do not recommend NSAIDs as a cancer-prevention strategy for people at average risk of developing colorectal cancer. However, the value of these drugs for people at increased colorectal cancer risk is being actively studied. Celecoxib (Celebrex) has been approved by the US Food and Drug Administration for reducing polyp formation in people with FAP. One advantage of this drug is that it causes less bleeding in the stomach. However, celecoxib may increase the risk of heart attacks and strokes. A similar drug, rofecoxib (Vioxx), was taken off the market because people who took it had an increased number of heart attacks and strokes. Please check with your doctor before beginning to take aspirin and other NSAIDs on a regular basis.
Female hormones: Hormone replacement therapy (HRT) in postmenopausal women may reduce their risk of developing colorectal cancer. But those women on HRT who do develop colorectal cancer may have a fast growing cancer.
HRT also lowers the risk of developing osteoporosis, but it may increase the risk of heart disease, blood clots, and breast and uterine cancers. For these reasons, the decision to use HRT should be based on a careful discussion of benefits and risks with your doctor.
Other factors: There are other risk factors that can't be controlled, such as a strong family history of colorectal cancer. But even when people have a history of colorectal cancer in their family, they may be able to prevent the disease. For example, people with a family history of colorectal cancer may benefit from starting screening tests when they are younger and having them done more often than people without this risk factor.
Genetic tests can help determine which members of certain families have inherited a high risk for developing colorectal cancer. Without genetic testing, all members of a family known to have an inherited form of colorectal cancer should be screened early and frequently. However, with genetic testing, family members who are found not to have inherited the mutated gene can be screened with the same frequency as people at average risk.
People with FAP should start colonoscopy during their teens. Most doctors recommend they have their colon removed when they are in their 20s to prevent cancer from developing.
The lifetime risk of developing colorectal cancer for people with HNPCC is about 80% compared to near 100% for those with FAP. Doctors recommend that people with HNPCC start colonoscopy screening during their 20s to remove any polyps and find any cancers at the earliest possible stage. People known to carry the genetic mutation associated with HNPCC may be offered the option of yearly screening with colonoscopy or removal of most of the colon.
Since some colorectal cancers can't be prevented, finding the disease early is the best way to improve the chance of a cure and reduce the number of deaths caused by this disease.
Credit: National Institute of Health.