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Our bodies are made up of billions of cells that grow, divide, and then die in a predictable manner. Cancer occurs when something goes wrong with this system, causing uncontrolled cell division and growth. The cancer cells lump together and form a mass of extra tissue, also known as a cancerous tumor. When cancer cells are present in the colon, it's referred to as colon cancer.

Symptoms

Colon cancer often causes no symptoms until it has reached a relatively advanced stage. Thus, many organizations recommend periodic screening for the disease with fecal occult blood testing and colonoscopy. When symptoms do occur, they depend on the site of the lesion. Generally speaking, the nearer the lesion is to the anus, the more bowel symptoms there will be, such as:

- Change in bowel habits
- change in frequency (constipation and/or diarrhea)
- change in the quality of stools
- change in consistency of stools
- Bloody stools or rectal bleeding
- Stools with mucus
- Tarry stools (melena) (more likely related to upper gastrointestinal eg stomach or duodenal disease)
- Feeling of incomplete defecation (Tenesmus) (usually associated with rectal cancer)
- Reduction in diameter of feces
- Bowel obstruction (rare).

Risk factors

The lifetime risk of developing colon cancer in the United States is about 7%. Certain factors increase a person's risk of developing the disease. These include:

1- Age. The risk of developing colorectal cancer increases with age. Most cases occur in the 60s and 70s, while cases before age 50 are uncommon unless a family history of early colon cancer is present.

2- Polyps of the colon, particularly adenomatous polyps, are a risk factor for colon cancer. The removal of colon polyps at the time of colonoscopy reduces the subsequent risk of colon cancer.

3- History of cancer. Individuals who have previously been diagnosed and treated for colon cancer are at risk for developing colon cancer in the future. Women who have had cancer of the ovary, uterus, or breast are at higher risk of developing colorectal cancer.

4- Family history of colon cancer, especially in a close relative before the age of 55 or multiple relatives

5- Long-standing ulcerative colitis or Crohn's disease of the colon, approximately 30% after 25 years if the entire colon is involved

6- Smoking. Smokers are more likely to die of colorectal cancer than non-smokers. An American Cancer Society study found that "Women who smoked were more than 40% more likely to die from colorectal cancer than women who never had smoked. Male smokers had more than a 30% increase in risk of dying from the disease compared to men who never had smoked.

7- Diet. Studies show that a diet high in red meat and low in fresh fruit, vegetables, poultry and fish increases the risk of colorectal cancer. In June 2005, a study by the European Prospective Investigation into Cancer and Nutrition suggested that diets high in red and processed meat, as well as those low in fiber, are associated with an increased risk of colorectal cancer. Individuals who frequently ate fish showed a decreased risk. However, other studies have cast doubt on the claim that diets high in fiber decrease the risk of colorectal cancer; rather, low-fiber diet was associated with other risk factors, leading to confounding. The nature of the relationship between dietary fiber and risk of colorectal cancer remains controversial.

8- Physical inactivity. People who are physically active are at lower risk of developing colorectal cancer.

9- Virus. Exposure to some viruses (such as particular strains of human papilloma virus) may be associated with colorectal cancer.

Diagnosis

Colorectal cancer can take many years to develop and early detection of colorectal cancer greatly improves the chances of a cure. Therefore, screening for the disease is recommended in individuals who are at increased risk. There are several different tests available for this purpose.