Abstract
The frequencies of second primary cancers following colon and rectal cancers were estimated using the Osaka Cancer Registry's population-based data for Osaka, Japan. A series of 7,312 colon and 6,923 rectal cancer cases newly diagnosed in the period of 1966-1986 were followed up until the end of 1986. The average follow-up period was 3.6 years for colon cancer and 3.7 years for rectal cancer. Significantly elevated risks of second primary cancers following colon cancer were observed for cancers of the rectum (O/E = 2.0; 95% confidence interval (CI) = 1.1-3.4 among males, O/E = 4.3; 95% CI = 2.4-7.2 among females), corpus uteri (O/E = 8.2; 95% CI = 3.3-16.9), ovary (O/E = 4.3; 95% CI = 1.0-5.0), and female thyroid gland (O/E = 4.7; 95% CI = 1.7-8.8). These findings were more notable among right-sided colon cancer patients than left-sided colon cancer patients. The elevated risks of second primary cancers were particularly evident among patients younger than 50 years of age at the time of diagnosis of the initial cancer (colon cancer: O/E = 3.1 among males, 3.4 among females, rectal cancer: O/E = 1.7 among males, 1.3 among females). These findings suggest that younger colorectal cancer patients should undergo more careful checkups throughout their lives.