The colon is the longest part of the large intestine and the lowest part of the digestive system. Inside the colon, water and salt from solid wastes are extracted before the waste moves through the rectum and exits the body through the anus.
What is colon cancer?
Cancer is a class of diseases characterized by out-of-control cell growth, and colon cancer forms when this uncontrolled cell growth initiates with cells in the large intestine. Most colon cancers originate from small, noncancerous (benign) tumors called adenomatous polyps that form on the inner walls of the large intestine. Some of these polyps may grow into malignant colon cancers over time if they are not removed during colonoscopy. Colon cancer cells will invade and damage healthy tissue that is near the tumor causing many complications.
After malignant tumors form, the cancerous cells may travel through the blood and lymph systems, spreading to other parts of the body. These cancer cells can grow in several places, invading and destroying other healthy tissues throughout the body. This process itself is called metastasis, and the result is a more serious condition that is very difficult to treat.
Colon cancer is not necessarily the same as rectal cancer, but they often occur together in what is called colorectal cancer. Rectal cancer originates in the rectum, which is the last several inches of the large intestine, closest to the anus.
According to the American Cancer Society, there will be 93,090 new cases of colon cancer in the United States in 2015.1
Causes of colon cancer
Cancer is ultimately the result of cells that uncontrollably grow and do not die. Normal cells in the body follow an orderly path of growth, division, and death. Programmed cell death is called apoptosis, and when this process breaks down, cancer results. Colon cancer cells do not experience programmatic death, but instead continue to grow and divide. Although scientists do not know exactly what causes these cells to behave this way, they have identified several potential risk factors.
The large intestine
Colon cancer usually derives from precancerous polyps that exist in the large intestine. The most common types of polyps are:
- Adenomas: can become cancerous but are usually removed during colonoscopy
- Hyperplastic polyps: rarely become colon cancer
- Inflammatory polyps: usually occur after inflammation of the colon (colitis) and may become cancerous
2) Genes - the DNA type
Cells can experience uncontrolled growth if there are damages or mutations to DNA, and therefore, damage to the genes involved in cell division. Four key types of genes are responsible for the cell division process: oncogenes tell cells when to divide, tumor suppressor genes tell cells when not to divide, suicide genes control apoptosis and tell cells to kill themselves if something goes wrong, and DNA-repair genes instruct cells to repair damaged DNA.
Cancer occurs when a cell's gene mutations make the cell unable to correct DNA damage and unable to commit suicide. Similarly, cancer is a result of mutations that inhibit oncogene and tumor suppressor gene functions, leading to uncontrollable cell growth. If you have DNA mutations of oncogenes or tumor suppressor genes that lead to colon cancer, it is likely that the mutation was a result of factors that affect DNA after you were born rather than a result of inheritance from parents.
3) Genes - the family type
Cancer can be the result of a genetic predisposition that is inherited from family members. It is possible to be born with certain genetic mutations or a fault in a gene that makes one statistically more likely to develop cancer later in life. About 20% of colon cancers are thought to be caused by inherited gene mutations. Genetic syndromes that are associated with colon cancer include familial adenomatous polyposis, attenuated adenomatous polyposis, and hereditary nonpolyposis colon cancer.
4) Traits, habits, and diet
Age is an important risk factor for colon cancer, as some 90% of those diagnosed are over 50. Colon cancers are more likely to occur in people with sedentary lifestyles, obese people, and those who smoke tobacco.
Diet is an important factor associated with colon cancer. Diets that are low in fiber and high in fat, calories, and red meat increase the risk of developing colon cancer. In fact, Western diets that are commonly followed in the United States and Europe increase the risk of colon cancer compared to diets found in developing countries. Heavy alcohol consumption may also increase the risk of colon cancer.
5) Other medical factors
There are several diseases and conditions that have been associated with an increased risk of colon cancer. Diabetes, acromegaly (a growth hormone disorder), radiation treatment for other cancers, ulcerative colitis, and Crohn's disease all increase the risk of colon cancer.
On the next page we look at the symptoms of colon cancer and how it is diagnosed. On the final page we discuss the available treatments for colon cancer and prevention methods, including regular screening.