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.

Cancer Cell Growth
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Normal Anatomy and the Development of Colorectal Cancer

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:

  • Ascending colon—The first part of the large intestine that attaches to the small intestine in the lower right part of the abdomen. From there, it runs up to the area just under the right rib cage near the liver.
  • Transverse colon—Runs across the top of the abdomen from the right side to the left side before turning down near the spleen.
  • Descending colon—After the turn, it runs down the left side of the abdomen.
  • Sigmoid colon—An s-shaped downturn in the colon before it connects to the rectum.

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.

Types of Colorectal Cancer

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:

  • Carcinoid tumors—Arise from the hormone-producing cells of the intestinal tract.
  • Gastrointestinal stromal tumors (GISTs)—Arise from specialized cells in the colon wall, but can be found anywhere in the GI tract.
  • Sarcoma—Rare and arise from the muscular tissue of the intestine.
Colon Cancer
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Rectal Cancer
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What are the risk factors for colorectal cancer?What are the symptoms of colorectal cancer?How is colorectal cancer diagnosed?What are the treatments for colorectal cancer?Are there screening tests for colorectal cancer?How can I reduce my risk of colorectal cancer?What questions should I ask my doctor?What is it like to live with colorectal cancer?Where can I get more information about colorectal cancer?

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