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 colon (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 colon.
Normal Anatomy and the Development of Colorectal Cancer
The colon is the last part of the GI tract. The main function of the colon is to absorb water and salt from solid waste before it is eliminated from the body as stool. The rectum is next. It stores stool until it is ready to be passed 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 walls of the colon have a higher rate of cell turnover because of digestive process. This area are also be exposed to carcinogens (cancer-causing agents). These carcinogens may come from waste expelled from the body, food breakdown, and gut bacteria. It causes damage to cells in the colon which can lead to a high level of cell turnover. Areas of rapid change like this are more at risk for cancer development.
Polyps are another factor that can affect development of cancer. Polyps are small growths that can be found in the colon and rectum. There are several types. 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 colon, but it is most commonly found in the last section. Colorectal cancer may cause bleeding or make it hard for the colon to work as it should. If the cancer grows beyond the colon or rectum, it can spread into nearby structures, such as the urinary tract, reproductive organs, or anus. It can make it hard for these organs to work as well, such as problems passing urine. 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 belly and pelvic areas.
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
