Update on colorectal cancer.

An estimated 129,400 new cases of colorectal cancer occurred in the United States during 1999. The lifetime risk of developing this cancer is 2.5 to 5 percent in the general population but two to three times higher in persons who have a first-degree relative with colon cancer or an adenomatous polyp. Between 70 and 90 percent of colorectal cancers arise from adenomatous polyps, whereas only 10 to 30 percent arise from sessile adenomas. Tumors or polyps that develop proximal to the splenic flexure carry a poorer prognosis than those that arise more distally, in part because of delayed diagnosis secondary to later development of symptoms. The Dukes system is the classic staging method for colorectal cancer; the TNM staging system is more detailed and therefore more useful for surgical purposes. Although screening guidelines vary, most agree that colorectal cancer screening should begin at 50 years of age in patients without a personal or family history of colorectal cancer.