Purpose: In North America and other high-risk areas, there has been a proximal shift in the subsite distribution of colorectal cancer. We wanted to determine whether any similar change has occurred in Japan, and where the incidence of this disease has increased sharply.
Methods: Data from the Reports of the Japanese Society for Cancer of the Colon and Rectum were used to analyze the time trend of colorectal cancer in Japan between 1974 and 1994 according to the patients' age at diagnosis and sex, and the site of the tumor within the colon or rectum.
Results: The percentage of patients over the age of 70, especially females, increased. The increase in the percentage of right-sided colon cancer in colorectal cancer cases was accompanied by a continuous decline in the percentage of rectal cancer in both sexes at all ages. In general, the percentage of right-sided colon cancer in colon cancer cases was stable in men, but increased in women. The rate among patients older than 70 years increased in men, but predominated and remained stable in women. No proximal shift in colon cancer was found in either sex under the age of 69.
Conclusion: These findings indicated that a proximal shift in the subsite distribution of colorectal cancer has occurred in Japan. This rightward shift of colorectal cancer is due to the decreasing proportion of rectal cancer. Furthermore, the increasing proportion of older patients, especially females, may be another major determinant of the changing colon cancer subsite distribution.