Colorectal Cancer Association of Canada - CCAC
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Understanding Colorectal Cancer


The Colon and Rectum

The colon and rectum perform vital functions in the last phases of digestion. Digestion first begins in the mouth where food is chewed into smaller pieces and swallowed. The food travels down the esophagus to the stomach where it is further broken down by gastric juices and sent to the small intestine. The small intestine continues to break down the contents in addition to absorbing most of the nutrients, including carbohydrates, proteins and vitamins. Once the contents have passed through the small intestine, the material has become mostly liquid and is moved into the colon, which measures about 5 ½ feet long. The main function of the colon is to absorb water and dehydrate the leftover material, forming semi-solid matter, or stool. The colon moves the stool into the approximately 6-inch long rectum, which acts as a holding chamber, until it is ready to be expelled through the anus.

Colorectal Cancer

Because colon and rectal cancers arise from the same type of cell and have many similarities, they are often referred to collectively as “colorectal cancer”. The cells lining the colon or rectum can sometimes become abnormal and divide rapidly. These cells can form benign (non-cancerous) tumours or growths called polyps. Although not all polyps will develop into colorectal cancer, colorectal cancer almost always develops from a polyp. Over a period of many years, a polyp’s cells may undergo a series of DNA changes that cause them to become malignant (cancerous). At first, these cancer cells are contained on the surface of a polyp, but can grow into the wall of the colon or rectum where they can gain access to blood and lymph vessels. Once this happens, the cancer can spread to lymph nodes and other organs, such as the liver or lungs—this process is called metastasis, and tumours found in distant organs are called metastases.

Staging Colorectal Cancer

Staging describes the extent of the cancer based on how many layers of the bowel wall are affected, whether lymph nodes are involved, and if there is spread to other organs. For colorectal cancer, staging often can’t be completed until after surgery to remove the primary tumour along with surrounding tissue (containing lymph nodes), and possibly lesions found on other organs. A pathologist studies the surgical specimen(s) to determine the stage. The most frequently used, and most precise staging system for colorectal cancer, is the TNM system. TNM stands for Tumour Nodes Metastases and the system describes the extent of each using numbers; the higher the number, the more of the element that is affected:

After each element has been determined, they are combined to form and overall stage of the cancer in roman numerals, the higher the number, the more advanced the cancer. This is generally how the cancer is referred to between doctor and patient:

  • 0 - The cancer is confined to the innermost layer of the colon or rectum. It has not yet invaded the bowel wall.
  • I - The cancer has penetrated several layers of the colon or rectum wall.
  • II - The cancer has penetrated the entire wall of the colon or rectum and may extend into nearby tissue(s).
  • III - The cancer has spread to the lymph nodes.
  • IV- The cancer has spread to distant organs, usually the liver or lungs.