Colon cancer (also called colorectal cancer) is cancer of the large intestine, including the colon and rectum. All cancers begin when cells grow uncontrollably to form a clump of cells referred to as a tumor. Benign tumors are not life-threatening and can be removed when they cause discomfort. Cancerous (or malignant) tumors, however, can spread to other organs (metastasize) via the body’s circulatory system. Colorectal cancer is 90% curable if detected early.
The intestinal wall consists of five layers: mucosa, submucosa, muscle, subserosa, and serosa. Over 95% of all colon cancers are adenocarcinomas, which occur in the mucus-producing cells of the mucosa lining. Adenocarcinomas begin in the form of an abnormal growth called a polyp; however not all polyps are pre-cancerous.
Colorectal cancer affects both men and women as the third most common cancer. In the United States, colon cancer is the second leading cause of cancer-related deaths.
Causes and Risk Factors
All cancers start with errors, or mutations, in the DNA of our cells. While some mutations are harmless, others can interrupt the processes that regulate cell division and maintenance, hence allowing uncontrolled cell growth. While the detailed mechanisms that lead to colon cancer onset are not fully understood, the following risk factors should be considered:
Genetic syndromes. Two inherited diseases that result from faulty gene function are associated with colon cancer. They are familial adenomatous polyps (FAP) and non-polyposis colorectal cancer (Lynch syndrome). FAP leads to hundreds of polyps in the colon, some of which become cancerous.
Chronic inflammation. Diseases that cause chronic inflammation in the colon lead to a higher chance of colon cancer. These include ulcerative colitis, which is a break in the lining of the colon, and Crohn’s disease, which is inflammation of the gastrointestinal tract. Both ulcerative colitis and Crohn’s are types of inflammatory bowel disease (IBD).
Lifestyle. A higher risk of developing colon cancer is associated with obesity; lack of physical activity; smoking; heavy alcohol use; and a diet low in fiber, fruits and vegetables, but high in fat.
Age. Colon cancer is most common in individuals over the age 50.
Race. In the Unites States, African Americans have the highest rates of colon cancer.
Personal and family history. The chance of developing colon cancer is greater in women who have had ovarian, uterine or breast cancers. A family history of colon cancer increases a person’s chances of developing the disease, due to shared genetic and environmental factors. Individuals who have had colon cancer are at a greater risk for reoccurrence.
As colon cancer advances, it can produce the following symptoms:
Bloody or narrow stools
Diarrhea or constipation
Sensation of rectal fullness
Feeling full or bloated
Gas pains and cramps
Nausea and vomiting
Unexplained weight loss.
Early screening and diagnosis of colon cancer are carried out using the following methods:
Fecal test. As stool passes through the large intestine, rubbing against polyps can cause bleeding. A fecal occult blood test detects blood in the stool that is otherwise not visible. Since a variety of conditions can lead to a positive test, follow-up tests are required to identify the exact cause.
Sigmoidoscopy. During this procedure, a flexible lighted tube with a camera is inserted into the rectum and lower colon to look for polyps. This instrument is also capable of removing tissue samples (biopsy) for further testing.
Colonoscopy. A colonoscope functions similar to a sigmoidoscope, but is used to visualize the entire length of the colon.
Imaging. Tumors can be detected using a type of computerized tomography (CT) scan that prepares both a 2- and 3-dimentional view of the colon. This technique is also called virtual colonography, and is the alternative to the more invasive colonoscopy.
Once a cancer is diagnosed, it is then “staged” to assess the extent of spread within the body. In order to stage cancer, various imaging techniques (CT scan or X-ray) and surgery are performed. Adenocarcinomas are typically staged from 1-4 (represented as Roman numerals I-IV). A stage 0 designations means the cancer has not spread beyond the mucus lining of the colon.
Stage I: The cancer has spread to the submucosa layer, but not the lymph nodes.
Stage II: The cancer has spread to the outermost layer and possibly neighboring tissues, but not the lymph nodes.
Stage III: The cancer has spread through all layers of the colon wall, to some lymph nodes, and/or possibly nearby organs.
Stage IV: The cancer may or may not have invaded the colon wall, or spread to lymph nodes, but has metastasized to distant organs.
Treatment options vary per patient and depend to a great extent on the disease stage; an early stage cancer has the best chances for a favorable outcome. Cancer is typically treated using a combination of approaches that are designed to kill and remove the affected cells. The 5-year survival rates for colon cancer ranges from 74% (stages I) to 6% (stage IV).
Surgery. Early-stage cancerous polyps can be removed completely - often during a diagnostic colonoscopy - and may not require further treatment. If cancer has spread through the colon wall, a colectomy is performed to remove the affected portion of the colon, as well as nearby tissues and lymph nodes. For advanced cancers, surgery is only performed to relieve the pain and discomfort associated with the growing tumor. Surgery is often combined with chemotherapy and radiation.
Chemotherapy. Chemotherapy drugs target and kill rapidly dividing cells, like cancer cells. Treatment also kills other rapidly dividing cells such as blood cells, and cells of the hair root and digestive tract, resulting in commonly known side effects like hair loss and nausea.
Radiation. Radiation is a non-targeted therapy that kills all cells using high-energy radiation. Typically radiation is applied locally to the affected area of the body.
Targeted drugs. Along with the above treatment paths, targeted drugs are available for treating advanced colon cancer. For instance, Avastin® (bevacizumab) is a humanized antibody that binds and inhibits a protein involved in blood vessel formation - a normal process that is also required for tumor growth.
Experimental therapies. Advanced cancer patients may consider participating in clinical trial studies of therapies that are not yet approved by the Food and Drug Administration (FDA) for public use. Information about ongoing clinical trials is available through health care providers.
Colon cancer screening in people over the age of 50, or those with known risk factors, is key to early detection. The risk of developing colon cancer can be reduced by refraining from smoking; maintaining a healthy weight; and eating a diet that is high in fruits and vegetables, but low in fat.
“Colorectal Cancer”. American Cancer Society. Jan 2014. Retrieved May 15, 2014. http://www.cancer.org/cancer/colonandrectumcancer/detailedguide/index.
“Colon Cancer”. Mayo Clinic. Mayo Foundation for Medical Education and Research. Aug 2013. Retrieved May 15, 2014. http://www.mayoclinic.org/diseases-conditions/colon-cancer/basics/definition/con-20031877.
“Colorectal (Colon) Cancer”. Centers for Disease Control and Prevention. CDC. Mar 2014. Retrieved May 15, 2014. http://www.cdc.gov/cancer/colorectal/basic_info/what-is-colorectal-cancer.htm.
Tina is a Life Science Writer for a number of online publications, including Innerbody.com. Her expertise is in conveying complex scientific topics to diverse audiences. Tina earned her PhD in Biochemistry from the University of California, San Francisco and her BS degree in Cell Biology from U.C. Davis. In her spare time, she enjoys drawing science-related cartoons.