Fact: Colorectal cancer is the fourth most common form of cancer in both men and women in the United States.
What is colorectal cancer?
Colorectal cancer is cancer that develops in the tissues of the colon and/or rectum. The colon and the rectum are both found in the lower part of the gastrointestinal (digestive) system. They form a long, muscular tube called the large intestine (or large bowel). The colon absorbs food and water and stores waste. The rectum is responsible for passing waste from the body.
If the cancer began in the colon, which is the first four to five feet of the large intestine, it may be referred to as colon cancer. If the cancer began in the rectum, which is the last several inches of the large intestine leading to the anus, it is called rectal cancer.
Colorectal cancer starts in the inner lining of the colon and/or rectum, slowly growing through some or all of its layers. It typically starts as a growth of tissue called a polyp. A particular type of polyp, called an adenoma, can then develop into cancer.
The chance of a polyp changing into cancer depends on the type of polyp it is. The 2 main types of polyps are:
- Adenomatous polyps (adenomas): These polyps sometimes change into cancer. Because of this, adenomas are called a pre-cancerous condition.
- Hyperplastic polyps and inflammatory polyps: These polyps are more common, but in general they are not pre-cancerous.
Other factors that can make a polyp more likely to contain cancer or increase someone’s risk of developing colorectal cancer include:
- If a polyp larger than 1 cm is found
- If more than 2 polyps are found
- If dysplasia is seen in the polyp after it's removed. Dysplasia is another pre-cancerous condition. It means there's an area in a polyp or in the lining of the colon or rectum where the cells look abnormal, but they don't look like true cancer cells.
How colorectal cancer spreads
If cancer forms in a polyp, it can grow into the wall of the colon or rectum over time. The wall of the colon and rectum is made up of many layers. Colorectal cancer starts in the innermost layer (the mucosa) and can grow outward through some or all of the other layers.
When cancer cells are in the wall, they can then grow into blood vessels or lymph vessels (tiny channels that carry away waste and fluid). From there, they can travel to nearby lymph nodes or to distant parts of the body.
The stage (extent of spread) of a colorectal cancer depends on how deeply it grows into the wall and if it has spread outside the colon or rectum.
Where does colorectal cancer grow?
To understand colorectal cancer, it helps to understand the parts that make up the colon and rectum. The colon and rectum make up the large intestine (or large bowel), which is part of the digestive system, also called the gastrointestinal (GI) system (see illustration below).
Most of the large intestine is made up of the colon, a muscular tube about 5 feet long. The parts of the colon are named by which way the food is traveling through them.
- The first section is called the ascending colon. It starts with a pouch called the cecum, where undigested food is comes in from the small intestine. It extends upward on the right side of the abdomen (belly).
- The second section is called the transverse colon. It goes across the body from the right to the left side.
- The third section is called the descending colon because it descends (travels down) on the left side.
- The fourth section is called the sigmoid colon because of its “S” shape. The sigmoid colon joins the rectum, which connects to the anus.
The ascending and transverse sections together are called the proximal colon. The descending and sigmoid colon are called the distal colon.
In most cases, it's not clear what causes colon cancer. Doctors know that colon cancer occurs when healthy cells in the colon develop errors in their genetic blueprint, the DNA.
Healthy cells grow and divide in an orderly way to keep your body functioning normally. But when a cell's DNA is damaged and becomes cancerous, cells continue to divide — even when new cells aren't needed. As the cells accumulate, they form a tumor.
With time, the cancer cells can grow to invade and destroy normal tissue nearby. And cancerous cells can travel to other parts of the body to form deposits there (metastasis).
Inherited gene mutations that increase the risk of colon cancerInherited gene mutations that increase the risk of colon cancer can be passed through families, but these inherited genes are linked to only a small percentage of colon cancers. Inherited gene mutations don't make cancer inevitable, but they can increase an individual's risk of cancer significantly.
The most common forms of inherited colon cancer syndromes are:
- Hereditary nonpolyposis colorectal cancer (HNPCC). HNPCC, also called Lynch syndrome, increases the risk of colon cancer and other cancers. People with HNPCC tend to develop colon cancer before age 50.
- Familial adenomatous polyposis (FAP). FAP is a rare disorder that causes you to develop thousands of polyps in the lining of your colon and rectum. People with untreated FAP have a greatly increased risk of developing colon cancer before age 40.
FAP, HNPCC and other, rarer inherited colon cancer syndromes can be detected through genetic testing. If you're concerned about your family's history of colon cancer, talk to your doctor about whether your family history suggests you have a risk of these conditions.
Association between diet and increased colon cancer risk
Studies of large groups of people have shown an association between a typical Western diet and an increased risk of colon cancer. A typical Western diet is high in fat and low in fiber.
When people move from areas where the typical diet is low in fat and high in fiber to areas where the typical Western diet is most common, the risk of colon cancer in these people increases significantly. It's not clear why this occurs, but researchers are studying whether a high-fat, low-fiber diet affects the microbes that live in the colon or causes underlying inflammation that may contribute to cancer risk. This is an area of active investigation and research is ongoing.
Factors that may increase your risk of colon cancer include:
- Older age. The great majority of people diagnosed with colon cancer are older than 50. Colon cancer can occur in younger people, but it occurs much less frequently.
- African-American race. African-Americans have a greater risk of colon cancer than do people of other races.
- A personal history of colorectal cancer or polyps. If you've already had colon cancer or adenomatous polyps, you have a greater risk of colon cancer in the future.
- Inflammatory intestinal conditions. Chronic inflammatory diseases of the colon, such as ulcerative colitis and Crohn's disease, can increase your risk of colon cancer.
- Inherited syndromes that increase colon cancer risk. Genetic syndromes passed through generations of your family can increase your risk of colon cancer. These syndromes include familial adenomatous polyposis and hereditary nonpolyposis colorectal cancer, which is also known as Lynch syndrome.
- Family history of colon cancer. You're more likely to develop colon cancer if you have a parent, sibling or child with the disease. If more than one family member has colon cancer or rectal cancer, your risk is even greater.
- Low-fiber, high-fat diet. Colon cancer and rectal cancer may be associated with a diet low in fiber and high in fat and calories. Research in this area has had mixed results. Some studies have found an increased risk of colon cancer in people who eat diets high in red meat and processed meat.
- A sedentary lifestyle. If you're inactive, you're more likely to develop colon cancer. Getting regular physical activity may reduce your risk of colon cancer.
- Diabetes. People with diabetes and insulin resistance have an increased risk of colon cancer.
- Obesity. People who are obese have an increased risk of colon cancer and an increased risk of dying of colon cancer when compared with people considered normal weight.
- Smoking. People who smoke may have an increased risk of colon cancer.
- Alcohol. Heavy use of alcohol increases your risk of colon cancer.
- Radiation therapy for cancer. Radiation therapy directed at the abdomen to treat previous cancers increases the risk of colon and rectal cancer.
Get screened for colon cancerPeople with an average risk of colon cancer can consider screening beginning at age 50. But people with an increased risk, such as those with a family history of colon cancer, should consider screening sooner.
Several screening options exist — each with its own benefits and drawbacks. Talk about your options with your doctor, and together you can decide which tests are appropriate for you.
Make lifestyle changes to reduce your riskYou can take steps to reduce your risk of colon cancer by making changes in your everyday life. Take steps to:
- Eat a variety of fruits, vegetables and whole grains. Fruits, vegetables and whole grains contain vitamins, minerals, fiber and antioxidants, which may play a role in cancer prevention. Choose a variety of fruits and vegetables so that you get an array of vitamins and nutrients.
- Drink alcohol in moderation, if at all. If you choose to drink alcohol, limit the amount of alcohol you drink to no more than one drink a day for women and two for men.
- Stop smoking. Talk to your doctor about ways to quit that may work for you.
- Exercise most days of the week. Try to get at least 30 minutes of exercise on most days. If you've been inactive, start slowly and build up gradually to 30 minutes. Also, talk to your doctor before starting any exercise program.
- Maintain a healthy weight. If you are at a healthy weight, work to maintain your weight by combining a healthy diet with daily exercise. If you need to lose weight, ask your doctor about healthy ways to achieve your goal. Aim to lose weight slowly by increasing the amount of exercise you get and reducing the number of calories you eat.
Colon cancer prevention for people with a high risk
Some medications have been found to reduce the risk of precancerous polyps or colon cancer. However, not enough evidence exists to recommend these medications to people who have an average risk of colon cancer. These options are generally reserved for people with a high risk of colon cancer.
For instance, some evidence links a reduced risk of polyps and colon cancer to regular use of aspirin or aspirin-like drugs. But it's not clear what dose and what length of time would be needed to reduce the risk of colon cancer. Taking aspirin daily has some risks, including gastrointestinal bleeding and ulcers, so doctors typically don't recommend this as a prevention strategy unless you have an increased risk of colon cancer.