A malignant tumor arising from the inner wall of the end of the large intestine (rectum). In the US, it is the third leading cause of cancer in males and the fourth in females. Risk factors include heredity, colon polyps, and long-standing ulcerative colitis. Most rectal cancers develop from polyps in the colon. Removal of these polyps can prevent cancer. Colon polyps and early rectal cancer can have no symptoms, so regular screening is important. Diagnosis can be made by barium enema or by colonoscopy, with biopsy confirmation of cancer tissue. Surgery is the most common treatment.