Abstract
BACKGROUND: Incidence of early-onset colorectal cancer (CRC) has increased globally in recent decades. We examined early-onset CRC incidence trends worldwide for potential cohort effects, defined as changes associated with time of birth (eg, early-life exposure to carcinogens), and period effects, defined as changes associated with calendar periods (eg, screening programs). METHODS: We obtained long-term incidence data for early-onset CRC diagnosed in patients aged 20 to 49 years through the year 2012 for 35 countries in the Cancer Incidence in Five Continents database. We used a smoothing method to help compare cohort and period trends of early-onset CRC and used an age-period-cohort model to estimate cohort and period effects. RESULTS: Cohort effects had a more dominant role than period effects in the early-onset CRC incidence in Shanghai (China), the United Kingdom, Australia, New Zealand, Canada, the United States, and Osaka (Japan). The smoothed trends show the specific birth cohorts when early-onset CRC began to increase: the 1940s-1950s birth cohorts in the United States; the 1950s-1960s birth cohorts in other Western countries; the 1960s birth cohorts in Osaka; and the 1970s-1980s birth cohorts in Shanghai. Such increases occurred earlier for early-onset cancers of the rectum than of the colon. For the other countries, the results were less clear. CONCLUSIONS: Recent birth cohorts may have been exposed to risk factors different from earlier cohorts, contributing to increased early-onset CRC incidence in several developed countries or regions in the West and Asia. Such increases began in earlier birth cohorts in Western countries than in developed regions of Asia.