Cancer is a disease in which cells grow in an abnormal way. Normally, the cells divide in a controlled manner to replace old or damaged cells. If cells keep dividing when new cells are not needed, a mass of tissue called a tumor forms.
A tumor can be benign or malignant. A benign tumor is not cancer and will not spread to other parts of the body. In the large intestine, most benign growths are called polyps. A malignant tumor is cancer. Cancer cells invade and damage tissue around them. They can also enter the lymph and blood streams, spreading to other parts of the body. Colorectal cancer is the development of malignant cells in the large intestine.
The gastrointestinal (GI) tract is one long tube that runs from the mouth to the anus. Its main function is to breakdown food and absorb nutrients into the bloodstream. The last part of the GI tract is the large intestine. The main function of the large intestine is to absorb water and salt from solid waste before it is eliminated from the body as stool. The large intestine is over 5 feet long and consists of the colon, rectum, and anal canal. Sections of the colon include:
Waste material moves through the colon. The rectum is the final part of the large intestine before it connects to the anal canal. The rectum is a pouch that stores stool until it is ready to be eliminated from the body through the anal canal.
Cell division and cell death are a normal process in the body to replace old or damaged cells. The inside of the large intestine may have a higher rate of cell turnover because of damage during normal digestive processes. The colon is also exposed to carcinogens (cancer-causing agents) from foods that are eaten or because of how bacteria in the gut react to the food. These changes may irritate the cells and increase turnover from old to new cells.
Polyps are small growths that can be found in the colon and rectum. There are several types of polyps. Though most are benign, some have the potential to develop into cancer. The highest risk of cancer development comes from adenomatous polyps.
Cancer can occur anywhere in the large intestine, but it is most commonly found in the last sections (sigmoid colon and rectum). Though it is fairly rare, cancer can also occur in the anal canal. Colorectal cancer may cause bleeding or interfere with normal function. If it grows beyond the colon or rectum, the cancer can penetrate nearby structures, such as the urinary tract, reproductive organs, or anus, and interfere with their function as well. It can also spread to lymph nodes or blood vessels, which can carry cancer cells to other areas of the body. The most common sites for colorectal cancer to spread to are lymph nodes in other parts of the body, the lungs, liver, or other organs in the abdominal and pelvic cavities.
The wall of the large intestine is made of 4 different layers. From the innermost to outermost layer, they are named mucosa, submucosa, muscle, and serosa. Almost all colorectal cancers are adenocarcinomas. That is, they start in the mucosa and spread outward through the serosa. Other types include:
Colorectal cancer. American Cancer Society website. Available at: http://www.cancer.org/acs/groups/cid/documents/webcontent/003096-pdf.pdf. Accessed January 3, 2017.
Colorectal cancer. EBSCO DynaMed Plus website. Available at: http://www.dynamed.com/topics/dmp~AN~T113642/Colorectal-cancer. Updated August 18, 2016. Accessed January 3, 2017.
Colorectal cancer. Merck Manual Professional Version website. Available at: http://www.merckmanuals.com/professional/gastrointestinal-disorders/tumors-of-the-gi-tract/colorectal-cancer. Updated July 2014. Accessed January 3, 2017.
General information about colon cancer. National Cancer Institute website. Available at: https://www.cancer.gov/types/colorectal/patient/colon-treatment-pdq. Updated June 30, 2016. Accessed January 3, 2017.
General information about rectal cancer. National Cancer Institute website. Available at: https://www.cancer.gov/types/colorectal/patient/rectal-treatment-pdq. Updated June 30, 2016. Accessed January 3, 2017.
Last reviewed December 2016 by Mohei Abouzied, MD
Please be aware that this information is provided to supplement the care provided by your physician. It is neither intended nor implied to be a substitute for professional medical advice. CALL YOUR HEALTHCARE PROVIDER IMMEDIATELY IF YOU THINK YOU MAY HAVE A MEDICAL EMERGENCY. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with any questions you may have regarding a medical condition.