This disease is also called colorectal or rectal cancer, depending where it starts. Most start as a growth—called a polyp—on the inner lining of the colon or rectum.
Colon cancer: learn more
Not all polyps found in the colon become cancer, but we do know that some types of polyps can change into cancer over several years. The chance of this happening depends on the type of polyp. There are two main types:
Adenomatous polyps or adenomas. Sometimes change into cancer, so adenomas are considered a pre-cancerous condition.
Hyperplastic polyps and inflammatory polyps. More common, but in general they are not pre-cancerous.
We also look for other factors that can make a polyp more likely to contain cancer or increase the risk of developing colorectal cancer:
- The polyp is larger than one centimeter.
- More than two polyps are found.
- If dysplasia is present. Dysplasia is another pre-cancerous condition, when 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.
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 other parts of the body.
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. The colon makes up most of the large intestine, a muscular tube about five feet long. The parts of the colon are named by which way the food is traveling through them:
- Ascending colon. The first section. It starts with a pouch called the cecum, where undigested food comes in from the small intestine. It extends upward on the right side of the abdomen.
- Transverse colon. The second section. It goes across the body from the right to the left side.
- Descending colon. The third section. It descends on the left side.
- Sigmoid colon. The final section, named 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.
Types of cancer in the colon and rectum
- Adenocarcinomas. The vast majority of colorectal cancers—about 96% of cases. These cancers start in cells that make mucus to lubricate the inside of the colon and rectum. Sub-types of adenocarcinoma include signet rin, mucinous, intestinal and diffuse.
- Carcinoid tumors. Start from special hormone-making cells in the intestine.
- Gastrointestinal stromal tumors (GISTs). Start from special cells in the wall of the colon called the interstitial cells of Cajal.
- Lymphomas. These are cancers of immune system cells that mostly start in lymph nodes, but they can also start in the colon, rectum, or other organs.
- Sarcomas. Can start in blood vessels, muscle layers, or other connective tissues in the wall of the colon and rectum. Sarcomas of the colon or rectum are rare.
Causes and risk factors of colon cancer
We know of some genetic mutations and risk factors that can increase your chance of getting colorectal cancer, but we still don’t fully understand the cause.
Genetic mutations. We do know that cancer is caused by mutations in the DNA inside our cells, some of which can be passed on in families and which are found in all of a person’s cells. These are called inherited mutations, and a very small number of colorectal cancers are caused by inherited gene mutations, including:
- Familial adenomatous polyposis (FAP), attenuated FAP (AFAP), and Gardner syndrome. Causes hundreds of polyps to form in the colon—over time, cancer will nearly always develop in one or more of these polyps.
- Lynch syndrome (hereditary non-polyposis colon cancer, or HNPCC).
- Peutz-Jeghers syndrome.
- MYH-associated polyposis (MAP).
Acquired mutations. Most gene mutations that lead to cancer are acquired mutations, meaning they happen during a person’s lifetime due to environmental or other factors. Most cases of colorectal cancer are due to acquired mutations rather than having been inherited—in many cases, the first mutation occurs in the APC gene, which leads to an increased growth of colorectal cells because of the loss of this brake on cell growth.
Certain risk factors probably play a role in causing these acquired mutations, but so far it’s not known what causes most of them.
Many lifestyle-related factors have been linked to colorectal cancer. In fact, the links between diet, weight, and exercise and colorectal cancer risk are some of the strongest for any type of cancer. You can reduce the risk of getting colorectal cancer by avoiding these risk factors:
- Being overweight or obese. Raises the risk of colon and rectal cancer in both men and women, but the link seems to be stronger in men.
- Physical inactivity.
- Certain types of diets. A diet that’s high in red meats and processed meats raises your colorectal cancer risk.
- Heavy alcohol use. Limiting alcohol use to no more than 2 drinks a day for men and 1 drink a day for women could have many health benefits, including a lower risk of many kinds of cancer.
Additional risk factors
- Aging. Younger adults can get colorectal cancer, but it’s much more common after age 50.
- A personal history of colorectal polyps or colorectal cancer. If you have a history of adenomas, you are at increased risk of developing colorectal cancer. This is especially true if the polyps are large, if there are many of them, or if any of them show dysplasia. If you’ve had colorectal cancer, even though it was completely removed, you are more likely to develop new cancers in other parts of the colon and rectum. The chances of this happening are greater if you had your first colorectal cancer when you were younger.
- A personal history of inflammatory bowel disease (IBD). This includes either ulcerative colitis or Crohn’s disease. IBD is different from irritable bowel syndrome (IBS), which does not increase your risk for colorectal cancer.
- A family history of colorectal cancer or adenomas. Most colorectal cancers are found in people without a family history of it, but nearly one in three people who develop it have other family members who have had it. If you have a family history of adenomatous polyps or colorectal cancer, talk with your doctor about the possible need to start screening before age 45.
- Having an inherited syndrome.
- Your racial and ethnic background. African Americans have the highest colorectal cancer incidence and mortality rates of all racial groups in the U.S. The reasons for this are not fully understood. Jews of eastern European descent (Ashkenazi Jews) have one of the highest colorectal cancer risks of any ethnic group in the world.
- Having type 2 diabetes. People with type 2 (usually non-insulin dependent) diabetes have an increased risk of colorectal cancer.