Abstract
Colorectal cancer (CRC) incidence rates have decreased among adults aged 50 and older while increasing in adults under age 50. Understanding these trends is challenging due to the multiple related time scales of age, period of diagnosis, and birth cohort. We analyzed rectal, distal colon, and proximal colon cancer incidence rates for individuals aged 20 and older from the Surveillance, Epidemiology, and End Results program for diagnosis years 1978-2017. We used a two-stage generalized linear model to describe age, period, and cohort effects for incidence. We first estimated birth cohort effects among people under 45. We used these results to specify prior distributions for cohort effects in a Bayesian model to estimate period effects among people 45 and older. There was no evidence of period effects for people under age 45. Risk for rectal and distal colon cancer increased for later birth cohorts. Compared to the 1943-1952 birth cohort, the 1983- 1992 birth cohort had 2.1 times the risk of rectal cancer, 1.8 times the risk of distal colon cancer and 1.3 times the risk of proximal colon cancer. For people over 45, period effects show declines in CRC risk that are attributable to screening.
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