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 meat, fat, and protein
- 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 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.