Colorectal Cancer
Colorectal cancer is a term used to refer to cancer that develops in the colon or the rectum. The colon and rectum are parts of the digestive system, which is also called the gastrointestinal, or GI, system. The digestive system processes food for energy and rids the body of solid waste matter (fecal matter or stool).
After food is chewed and swallowed, it travels through the esophagus to the stomach. There it is partly broken down and then sent to the small intestine, also known as the small bowel. The word "small" refers to the diameter of the small intestine, which is narrower than that of the large bowel. Actually the small intestine is the longest segment of the digestive system – about 20 feet. The small intestine continues breaking down the food and absorbs most of the nutrients. The small bowel joins the colon in the right lower abdomen. The colon (also called the large bowel or large intestine) is a muscular tube about 5 feet long. The colon continues to absorb water and mineral nutrients from the food matter and serves as a storage place for waste matter. The waste matter left after this process is feces and goes into the rectum, the final 6 inches of the digestive system. From there it passes out of the body through the anus.
The colon has 4 sections:
- The first section is called the ascending colon. It begins where the small bowel attaches to the colon and extends upward on the right side of the abdomen.
- The second section is called the transverse colon since it goes across the body from the right to the left side in the upper abdomen.
- The third section, the descending colon, continues downward on the left side.
- The fourth section is known as the sigmoid colon because of its “S” or “sigmoid” shape. The sigmoid colon joins the rectum, which in turn joins the anus, or the opening where waste matter, or stool, passes out of the body.
The wall of each of these sections of the colon and rectum has several layers of tissue. Colorectal cancer starts in the innermost layer and can grow through some or all of the other layers. Knowing a little about these layers is important, because the stage (extent of spread) of a colorectal cancer depends to a great degree on how deeply it invades into these layers. For more information, please refer to the staging section of this document.
Colon cancer and rectal cancer, collectively known as colorectal cancer, have many features in common. They will be discussed together in this document except for the section about treatment, where they will each be discussed separately.
In most people, colorectal cancers develop slowly over a period of several years. Before a true cancer develops, a growth of tissue or tumor usually begins as a non-cancerous polyp, which may eventually change into cancer. A polyp develops on the lining of the colon or rectum. Certain kinds of polyps, called adenomatous polyps or adenomas, are types that have the potential to become cancerous.
There are other kinds of polyps called hyperplastic and inflammatory polyps. Inflammatory polyps and hyperplastic polyps, in general, do not become pre-cancerous. But some doctors think that some hyperplastic polyps can become pre-cancerous or might be a sign of a greater likelihood of developing adenomatous polyps and cancer, particularly if they grow in the right or ascending colon. Another kind of pre-cancerous condition is called dysplasia. This is usually seen in people with diseases, such as ulcerative colitis or Crohn’s colitis, which cause chronic inflammation of the colon.
Once cancer forms within a polyp, it can eventually begin to grow into the wall of the colon or rectum. Once cancer cells are in the wall, they can grow into blood vessels or lymph vessels. Lymph vessels are thin, tiny channels that carry away waste and fluid. They first drain into nearby lymph nodes, which are bean-shaped structures that help fight against infections. When they spread into blood or lymph vessels, the cancer cells can travel to distant parts of the body. This process of spread is called metastasis.
More than 95% of colorectal cancers are adenocarcinomas. These are cancers of the glandular cells that line the inside layer of the wall of the colon and rectum. The information in this document is about this type of cancer. Other less common types of tumors may also develop in the colon and rectum, such as:
- carcinoid tumors – these tumors develop from specialized hormone-producing cells of the intestine.
- gastrointestinal stromal tumors – these tumors develop from specialized cells in the wall of the colon called the "interstitial cells of Cajal." Some are benign (non-cancerous); others are malignant (cancerous). Although these cancers can be found anywhere in the gastrointestinal tract, they are unusual in the colon.
- lymphomas – these are cancers of immune system cells that typically develop in lymph nodes but also may start in the colon and rectum or other organs.
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