Colon, or colorectal, cancer is cancer that starts in the large intestine (colon) or the rectum (end of the colon). Most cases of colon cancer begin as small, non-cancerous (benign) clumps of cells called adenomatous polyps. Over time some of these polyps can become cancerous.

Risk factors

• Being over 50 years old
• Being African-American or eastern European descant
• Eating a diet high in red or processed meats
• Having had cancer elsewhere in the body
• Having colorectal polyps
• Having inflammatory bowel disease ( Crohn's disease or ulcerative colitis)
• Having a family history of colon cancer or polyps
• Heavy use of alcohol or smoking
• Engaging in a sedentary lifestyle 
• Having diabetes
• Having genetic syndromes such as familial adenomatous polyposis and hereditary non-polyposis colorectal cancer (Lynch syndrome)
• Being obese
• Having had radiation therapy directed at the abdomen to treat previous cancers

Symptoms

• A change in your bowel habits, including diarrhea or constipation
• Rectal bleeding or blood in your stool
• Persistent abdominal discomfort, such as cramps, gas, or pain
• A feeling that your bowel doesn't empty completely
• Weakness or fatigue
• Unexplained weight loss
• Screening and early detection

Regular colon cancer screening should begin at age 45 for people at average risk of colon cancer. The American College of Gastroenterology recommends African-Americans, who have an increased risk of colon cancer, begin screening at age 45. Several screening options exist—each with its own benefits and drawbacks. Talk about your options with your doctor, and together you can decide which test is appropriate for you.

Options for colon cancer screening:

• Colonoscopy
• Annual fecal occult blood testing
• Flexible sigmoidoscopy every five years
• Double-contrast barium enema every five years
• Stool DNA testing

More frequent or earlier screening may be recommended if you're at increased risk of colon cancer.

Diagnosis
The following techniques may be used to further evaluate symptoms and help diagnose colon cancer:

• Blood tests to better understand what may be causing your signs and symptoms, but there are no blood tests that can detect colon cancer
• Colonoscopy uses a long, flexible and slender tube attached to a video camera and monitor to view your entire colon and rectum. If any suspicious areas are found, your doctor can pass surgical tools through the tube to take tissue samples (biopsies) for analysis
• Using dye and X-rays to make a picture of your colon
• Using multiple CT images to create a picture of your colon

Treatment
The four main types of treatment for colorectal cancer are:

• Surgery
• Radiation therapy
• Chemotherapy
• Targeted therapies

Depending on the stage of your cancer, two or more types of treatment may be used at the same time, or used one after the other.