Colon cancer prevention update.

Abstract

C OLORECTAL cancer is second only to lung cancer in mortality in the United States. 1 A review of the epidemiology of colorectal cancer shows interesting findings in general trends, in geographic distribution, in migrant populations, in ethnic variation, and in location of cancer within the colon itself. 2-4 In the United States, colon cancer mortality has remained flat over the last 40 years. Incidence peaked during 1985, with increased screenings after President Reagan's diagnosis of colon cancer that same year ) Since 1985, incidence rates for colorectal cancer have been lower, but they show marked geographic and racial disparity. The highest rates of colorectal cancer occur in the United States, other Anglo-Saxon countries, and western and northern Europe. 4 Africa, China, Japan, and India have low incidence rates, especially in the nonurban areas. 4 Migrants from areas with low colorectal incidence to areas with higher incidence rates show an increase in colorectal cancer. 5'6 Similarly, migrants from areas with higher incidence rates to areas with lower incidence rates evidence a decrease in risk. 7 From 1973 to 1988, the National Cancer Institute Surveillance, Epidemiology, and End Results (SEER) Program reported an increase in age-adjusted colon cancer incidence rates of 30.6% for blacks and only 5.4% for whites. 2 Colorectal cancer occurs in the proximal colon, the distal colon, and the rectum. These divisions of the colon are significant for cancer development and, potentially, for cancer prevention. In areas of the world where the incidence of colorectal cancer is low, for example, colorectal cancer occurs predominantly in the proximal or transverse colon. 4 In geographic areas that have a high incidence of colorectal cancer, the cancer occurs more frequently in the distal colon. 8 Areas with a high incidence of colon cancer appear to have a high risk of rectal cancer. Aneuploidy, K-ras mutations, and allelic losses of chromosomes 5, 17, and 18 vary with location in the colon. 9 Rectal cancer occurs with higher frequency in men than in women, is positively associated with alcohol consumption, and is less common among persons of Jewish ancestry than is colon cancer, j~ There are also differences in age-specific sex ratios for various parts of the large bowel, as exemplified by older women who show an increase in right-sided colorectal cancer. 8 The reasons for these differences are not clear and may be attributed to variations in fecal contents, differences in the colonic and rectal mucosa, or differences in length of fecal contact. 10