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Abstract
BACKGROUNDDuring recent decades, colorectal cancer incidence rates have been rapidly increasing in Japan. To investigate trends in colorectal cancer incidence rates, we analyzed incidence data during 39 years between 1959 and 1997 in Miyagi Prefecture, Japan.METHODSUsing age-period-cohort models, we evaluated the effects of time period and cohort on colon and rectal cancer incidence. Model fitting was based on eleven 5-year age groups (30-34 to 80-84), eight 5-year time periods, and 18 overlapping birth cohorts of 10 years each.RESULTSThe analysis found a significant (p=0.04) and upward period effect on female colon cancer incidence, and a significant (p<0.01) and upward cohort effect on male colon cancer incidence. An upward period effect was also observed for male colon cancer incidence without significance. For rectal cancer incidence, a significant cohort effect was found among both males and females.CONCLUSIONSIn light of known risk factors of colorectal cancer, the effects of period and cohort might be related to the change in the prevalence of risk factors such as high intake of meat and animal fat, and obesity. The improved diagnostic procedures including the spread of cancer screening might be responsible for the period effect. Although the significant cohort effects may give a caution for a continuous increase of colorectal cancer incidence, the future trend may be influenced by the period-related factors. Successive monitoring of cancer incidence and prevalence of risk factors is required.
Highlights
During recent decades, colorectal cancer incidence rates have been rapidly increasing in Japan
An upward period effect was observed for male colon cancer incidence without significance
A significant cohort effect was found among both males and females
Summary
Using age-period-cohort models, we evaluated the effects of time period and cohort on colon and rectal cancer incidence. Incidence data of colon and rectal cancer were obtained from the MPCR. To investigate the effects of period and cohort on colon/rectum cancer incidence, we used age-period-cohort models.[7,8,9] The statistical method has already been described elsewhere.[10,11] In the present study, the age-period-cohort analysis was performed for ages. We reorganized the above original incidence data into eight 5-year time period groups including. 1958: 1958-62, 1963-67, 1968-72, 1973-77, 1978-82, 1983-87, 1988-1992, and 1993-1997 In this reorganization, we estimated the number of 1958-62 incidence cases by multiplying the number of 1959-1961 (3-year period) incidence cases and 5/3.
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