Colorectal cancer is cancer that develops in the tissues of the colon and/or rectum. The colon and the rectum are both found in the lower part of the gastrointestinal (digestive) system. They form a long, muscular tube called the large intestine (or large bowel). The colon absorbs food and water and stores waste. The rectum is responsible for passing waste from the body.
If the cancer began in the colon, which is the first four to five feet of the large intestine, it may be referred to as colon cancer. If the cancer began in the rectum, which is the last several inches of the large intestine leading to the anus, it is called rectal cancer. Colorectal cancer starts in the inner lining of the colon and/or rectum, slowly growing through some or all of its layers. It typically starts as a growth of tissue called a polyp. A particular type of polyp, called an adenoma, can then develop into cancer.
Colorectal cancer is the fourth-most common form of cancer (or third, excluding skin cancers) in both men and women in the United States. The American Cancer Society estimated that 143,460 people will be diagnosed with colorectal cancer in 2012.
The most common type of colorectal cancer is adenocarcinoma. A cancer of the intestinal gland cells, adenocarcinomas represent more than 95 percent of colon and rectal cancers. Adenocarcinomas typically start within the intestinal gland cells that line the inside of the colon and/or rectum. They tend to start in the inner layer and then spread deeper to other layers. Subtypes include:
Mucinous adenocarcinoma is made up of approximately 60 percent mucus. The mucus can cause cancer cells to spread faster and become more aggressive than typical adenocarcinomas. Mucinous adenocarcinomas account for 10 to 15 percent of all colon and rectal adenocarcinomas.
Signet ring cell adenocarcinoma is very uncommon. It accounts for less than one percent of adenocarcinomas. Named for its appearance under a microscope, signet ring cell adenocarcinoma is typically aggressive and may be more difficult to treat.
Other types of colorectal cancers (e.g., gastrointestinal carcinoid tumors, lymphomas, gastrointestinal stromal tumors) are rare and only account for a small percentage of diagnosed colorectal cancers.
In most cases, the actual cause of colorectal cancer is not known. Even if there are some factors that are beyond your control, managing your lifestyle-related factors may help you reduce your risk for this disease.
An inherited condition or family history may increase your chances of developing the disease, so it is important to discuss these colorectal cancer risk factors with your family and your doctor. He or she can provide you with more information, and let you know if you are a candidate for screenings that can aide in the early detection of the disease.