An estimated 875,000 cases of colorec-tal cancer occurred worldwide in 1996, representing about 8.5% of all new cancers {1531}. The age-standardized incidence (cases/100,000 population) varies greatly around the world, with up to 20fold differences between the high rates in developed countries of Europe, North and South America, Australia/New Zealand, and Asia and the still lower rates in some recently developed coun tries (Malaysia, Korea) and in developing countries of Africa, Asia and Polynesia. Significant differences also exist within continents, e.g. with higher incidences in western and northern Europe than in central and southern Europe {336}. Among immigrants and their descendants, incidence rates rapidly reach those of the adopted country, indicating that environmental factors are important. According to the U.S. SEER database, the incidence rate for adenocarcinoma of the colon is 33.7/100,000 and increased by 18% during the period from 1973 through 1987 while the incidence of rectal adenocarcinoma (12.8/100,000) and mucinous adenocarcinoma in the colon and rectum (0.3 and 0.8, respectively) remained relatively constant {1928}. During the last decade of the 20th century, incidence and mortality have decreased {566}. By contrast, the incidence in Japan, Korea and Singapore is rising rapidly {737}, probably due to the acquisition of a Western lifestyle. Incidence increases with age {2121}: carcinomas are rare before the age of 40 years except in individuals with genetic predisposition or predisposing conditions such as chronic inflammatory bowel disease.

Incidence rates in the 1973-87 SEER data for colonic and rectal adenocarci-

Fig. 6.03 Double contrast barium enema showing adenocarcinoma of colon. Between the proximal (top) and distal (bottom) segment of the colon the lumen is narrowed with an irregular surface, due to tumour infiltration.

noma for males were higher than those for females; whites had higher rates than blacks for rectal adenocarcinoma, but blacks had higher rates for colonic adenocarcinoma {1928}. During 1975-94, a decrease in incidence in whites was evident, while the incidence of proximal colon cancers in blacks still increased {1958}.

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