INTRODUCTION
Cancer of the large bowel is a major health problem. Worldwide each year, over 900,000 new cases are diagnosed, and almost 500,000 people die from the disease (1). About two-thirds of the incident cases occur in developed countries, where colorectal cancer is the third most common cancer in men and second most common in women (2). In developing countries, it is the fifth most common cancer in both sexes.

Relatively few colorectal cancers occur in persons younger than 40. Rates increase rapidly with age thereafter, more markedly for colon than for rectal cancer (3). The burden of colorectal cancer is, therefore, expected to increase in the future as a result of population aging and increased life expectancy. This is particularly true for developing countries.

Descriptive Epidemiology of Colorectal Cancer

International Variations in Incidence

After allowing for differences between the age structures of populations, there are substantial variations in incidence internationally. In men, for the period 1993–1997, the highest rates, of 45 per 100,000 and above, occurred in Australia, New Zealand, parts of Japan (Miyagi), and parts of western Europe (Saarland, Germany; Bas-Rhin, France; northeast Italy) (3).

Rates in the range of 35 to 45 per 100,000 were observed in the rest of western Europe, the United States, Canada, Hong Kong, and in Israeli Jews. In eastern Europe incidence was somewhat lower—around 25 to 35 per 100,000. Incidence rates of less than 15 per 100,000 occurred in Africa, India, Thailand, and Vietnam, and parts of the Middle East. For women, the geographical pattern was similar, but the age-standardized rates were about 60% to 80% of those in men.

Time Trends in Incidence

In 1971, Haenszel and Correa noted that colon cancer incidence was slowly increasing (4). Since then, moderate increases in colorectal cancer incidence is observed in many populations, although the timing and the magnitude of the increases have differed between populations (5). Rates have risen both in populations that, in earlier decades, had intermediate or high rates of colorectal cancer—such as Sweden, Denmark, Spain, Italy, Australia, New Zealand, Britain, and the United States—and in those that had low rates—such as Japan (6–15). Although the general pattern is similar, the magnitude of the increase differed between populations, as did the timing. In most, the increase was either more pronounced in men than women or observed only in men. While these trends are in part an artifact of improvements in the efficiency of cancer registration and increased detection rates resulting from the introduction of newer diagnostic tools, this seems unlikely to be the full explanation. The different patterns in males and females indicate strong sex-specific cohort effects, most likely associated with changes in exposures to environmental and lifestyle risk factors for the disease.

Subsite of Tumor

Between 60% and 70% of large bowel cancers occur in the colon (16). In western European and U.S. data, tumors of the right (proximal) colon are overrepresented among women (6). This is partly a function of age, because right tumors are more common among older persons and there are greater numbers of women in the older age groups than men. Tumors of the right colon have been reported to have become more common over the past 30 years (17–20). However, these observations are difficult to interpret for a number of reasons including different data categorizations and methods of statistical analysis, selection bias, population aging, and increasing use of colonoscopy and flexible sigmoidoscopy (5). It is, therefore, not at all clear whether the underlying incidence of right colon tumors is truly increasing.

Rebecca A. Barnetson and Malcolm G. Dunlop
Colon Cancer Genetics Group, University of Edinburgh, School of Molecular and Clinical Medicine and MRC Human Genetics Unit, Western General Hospital, Edinburgh, U.K.

References