Colorectal cancer involves uncontrolled growth of abnormal cells in the colon or rectum. It is the third most common form of cancer in the United States. Colorectal cancer is the third leading cause of cancer deaths for both men and women, but when the deaths by gender are combined, it is the second leading cause of cancer deaths overall. Among men, colon cancer is surpassed by lung cancer and prostate cancer. In women, it is third behind lung and breast cancer in occurrence and mortality. However, the mortality rate has declined for the past 15 years, which is most likely due to improved screening and treatment methods.

The colon and rectum are part of the gastrointestinal system. The first 6 feet (1.8 meters) of the large intestine comprise the colon. The last 6 inches (15 centimeters) is the rectum, which ends at the anus. Although the large intestine includes the colon and rectum, the term colon is often used as a synonym for large intestine or large bowel.

The colorectal tract is a muscular tube at the lower end of the digestive system. It is shaped somewhat like a question mark. The colon consists of the:

• Ascending colon. Starts at the end of the small intestine and goes upward on the right side of the abdomen.
  
  
• Transverse colon. Continues across the body from the right to left side.
  
  
• Descending colon. Extends downward on the left side.
  
  
• Sigmoid colon. A small S-shaped section that connects to the rectum.

The small intestine absorbs most nutrients before digested food moves to the colon. The colon absorbs water and minerals and stores the rest as waste before expelling the waste through the rectum as feces, or stool.

The colon and rectum consist of several layers of tissue. Colorectal cancer starts in the inner lining, usually as abnormal growths called polyps. Polyps may remain benign, but some are precancerous and over the years turn malignant. The polyps most likely to advance to cancer are adenomatous polyps, or adenomas. The adenomatous cells are gland cells on the inner lining of the colorectal tract. Other types of polyps include inflammatory polyps, which are not precancerous, and hyperplastic polyps, which can be precancerous.

According to the National Cancer Institute (NCI), polyps are a cause for concern because:

• Precancerous polyps occur in about 40 percent of people over age 50.
  
  
• About 5 to 10 percent of polyps become cancer if not removed.
  
  
• Physicians cannot determine which polyps will lead to cancer.
  
  
• People who have had a polyp have up to a 50 percent increased risk of developing more polyps within three years.

Colorectal cancer can spread from the innermost mucosal layer to the wall of the colorectal tract and beyond. Cancer in the colorectal wall can grow into the blood vessels or lymph vessels. These vessels can allow metastasis (spread) of the cancer to distant parts of the body. Prognosis (outlook) is best when cancer is confined to the innermost layer and does not block or perforate the large intestine.

Unlike some forms of cancer, early detection of colorectal cancer is possible through screening tests (see Diagnosis methods). However, most Americans do not follow the recommended schedule of tests, according to the Centers for Disease Control and Prevention (CDC). A major reason is embarrassment with the type of tests and reluctance to undergo certain procedures. However, increased screening could further reduce the incidence and death rate of colorectal cancer.

The importance of screening is apparent in the survival rates for colorectal cancer. Ninety percent of people treated for early-stage (non-metastatic) colorectal cancer survive at least five years and often much longer, according to the American Cancer Society (ACS). However, only 39 percent of colorectal cancer is detected early. For later stages of colorectal cancer, the five-year survival rate declines to:

• 68 percent when colorectal cancer spreads to nearby organs or lymph nodes
  
  
• 10 percent when colorectal cancer spreads to distant organs or lymph nodes

The ACS predicts there will be about 112,000 new cases of colon cancer and nearly 42,000 new cases of rectal cancer in the United States in 2007. Colorectal cancer causes about 10 percent of all cancer deaths in the United States and is expected to kill more than 53,000 Americans in 2007. There are about 1 million Americans who have survived colorectal cancer.