Trends in colorectal cancer incidence in Sweden 1959-93 by gender, localization, time period, and birth cohort

Objectives: This study examined invasive colorectal cancer incidence-rates in Sweden from 1959 through 1993 (n = 134,643 cases). Methods: Age-standardized rates were calculated using the Swedish population in 1970 as a reference. Results: In right-sided colon cancer (ascending and transverse colon including right and left flexures), male age-standardized rates rose from 8.0 to 15.0 (1.8 percent annually, 95 percent confidence interval [CI] = 1.3-2.4) and female rates increased from 9.1 to 14.4 (1.5 percent annually, CI = 1.0-2.0). For left-sided colon cancer (descending and sigmoid colon), the rates have been stable recently. For rectal cancer, the rates among men rose from 18.8 to 23.0 and among women from 10.7 to 14.7. For both men and women, the relative risk (RR) of right-sided colon cancer had been increasing in successive generations, until leveling-off in those born after 1930. The RR of left-sided colon cancer had been almost constant for cohorts born before 1930 but steadily decreasing in later-born cohorts. The RR of rectal cancer was slightly increasing in successive cohorts. Conclusions: Changes in lifestyle or carcinogenic exposures during early life probably explain Swedish colorectal cancer incidence-trends better than improved diagnostic activities.