The colon is what we commonly call the last 6 feet of the intestines, which leads from the small intestine to the rectum. The colon is sometimes referred to as the large intestine. The last part of the large intestine is the rectum.
Colon and rectal cancer, also called colorectal cancer, is the third most common cancer in Canada.
The risk of colorectal cancer increases as a person gets older. After 30, the risk at least doubles with each passing decade. Most doctors start screening at age 50. Men appear to be at greater risk for developing colon cancer.
Screening is crucial because colorectal cancer responds best to treatment if caught early. Treatment is most effective for people with localized colorectal cancer. People who may be at higher risk for colorectal cancer should speak with their doctor to decide on the best screening schedule for them.
The stage of a tumour is usually the best way of knowing its curability. Most cancers are staged or graded according to their size and, above all, the degree to which they have spread to other parts of the body. The stage depends heavily on whether lymph nodes are involved. Lymph nodes are bean-shaped concentrations of immune system cells that are found all over the body. They are interconnected by their own network of tubes, called the lymphatic system. Cancer often spreads through the lymphatic system to reach other organs – this jumping from one part of the body to another is called metastasis.
In colon cancer, stages 1 and 2 represent increasing degrees of penetration of the colon's wall. Stage 3 cancer has reached the nearby lymph nodes. Stage 4 has spread to other parts of the body (metastasized). Recurrent cancer has come back after an apparently successful treatment, often reappearing in the liver or lungs instead of the colon.
The causes of cancer are still not understood. Sometimes the genes that control a cell's activity mutate and start giving instructions that lead to runaway growth of tissue. A single cancerous cell divides millions of times, producing a tumour. The tumour builds its own blood vessels to ensure its supply of oxygen. See the general article on cancer for more on the causes and development of cancer.
By observing cancer rates, experts can predict who is most at risk of colon cancer, and it also helps them find ways to predict it in individuals.
The greatest risk factor, and the clearest warning sign, is colorectal polyps. These are benign growths (not in themselves cancerous) in the inside of the colon. However, many benign growths are bad for you, and some are likely to turn into cancer. Polyps belong to this last category.
A polyp usually looks like a ball on a stalk, protruding from the inner wall of the colon. It may cause rectal bleeding, though this is often hidden in the stool. The average new polyp has a 2.5% chance of becoming cancerous in its first 5 years, and a 24% chance of becoming cancerous after 20 years. Larger polyps are more likely to become cancerous. Some people have more than one polyp.
You are more at risk for polyps and colon cancer if you:
- are over 50 years of age
- drink – people who smoke and drink are at 4 times the average risk
- don't get enough exercise
- eat a diet heavy in red meat or processed meat
- have history of ulcerative colitis or Crohn's disease
- have family history of polyps, colon cancer, or cancers of female reproductive organs (ovarian cancer, endometrial cancer, breast cancer)
- are obese
- have diabetes
- have received abdominal radiation (e.g., are an adult survivor of childhood cancer who received abdominal radiation)
Like most cancers, colon cancer appears to be partly genetic and partly environmental, and may be partly random. Some families have genes that cause them to develop thousands of polyps and give them a high probability of colon cancer. Other families lack this gene, but still have a higher incidence of colon cancer than the general population. At the same time, people whose families have never known the disease can be struck by colon cancer.
Symptoms and Complications
Polyps and colon cancer may cause slow, steady bleeding in the colon. The blood emerges with the stool but is often invisible. Many other conditions can cause blood in the stool, but if you experience it you should consult a doctor. Often, the blood loss isn't seen, but its effect is felt as iron-deficient anemia. Men and postmenopausal women with symptoms of anemia need a checkup.
Colon cancer can cause vague symptoms like weight loss or change in bowel habits, or it can cause no symptoms at all. More often, colon cancer has no symptoms until the disease is advanced. That's why screening is so vital.
Making the Diagnosis
A stool test for blood is one of the screening tests for colorectal cancer. Polyps or tumours can release small amounts of blood onto stool and there are several tests available to check for this. These tests require you to provide a stool sample to look for hidden blood. Hidden blood in the stool doesn't automatically mean you have colon cancer. Your doctor will need to do further tests to find out why there is blood in your stool.
The colonoscopy is the primary screening method used for people at high risk of developing colon cancer. An endoscope, a flexible fibre optic tube, is inserted into the colon to look for bleeding, polyps, or tumours. If any growths are found, the doctor will take a tissue sample (biopsy). Alternatively, a polyp may be completely removed or destroyed during the procedure. The procedure is not painful, though it can be a little uncomfortable. You may be offered medication to help you relax during this procedure.
Treatment and Prevention
There are 3 possible treatments for colon cancer: surgery, radiation, and chemotherapy.
Surgery is the treatment most likely to cure colon cancer, but it's only used for people with stage 1 to stage 3 cancer. If you're diagnosed with stage 3 cancer, you will have to undergo radiation therapy or chemotherapy in addition to surgery. The same is sometimes true of stage 2 tumours.
Polyps can be removed without surgery. An endoscope is fitted with special cutters for the task.
The best way to prevent colon cancer is to avoid risky behaviours like smoking, and to improve your diet. If you correct unhealthy lifestyle factors – bring your weight closer to a healthy weight, quit smoking, or get more exercise – you are also reducing your risk.
Diets high in vegetables, fruits, and fibre have been shown to help prevent colon cancer. There is also evidence to suggest that calcium may have a protective effect.
Above all, get screened for polyps if you have risk factors. People who have had polyps removed are at more than twice the average colon cancer risk for their age. People who leave them in, however, are at 8 times the normal risk.