Age at Initiation of Lower Gastrointestinal Endoscopy and Colorectal Cancer Risk Among US Women

Importance: In the past 4 years, the American Cancer Society and the US Preventive Services Task Force updated recommendations to initiate colorectal cancer (CRC) screening at 45 years of age to address the increasing incidence of CRC among adults younger than 50 years. However, empirical evidence evaluating the potential benefits of screening in younger populations is scant. Objective: To examine the association between endoscopy initiation at different ages and risk of CRC among US women. Design, Setting, and Participants: This prospective cohort study used data from the Nurses' Health Study II, which included US female health professionals followed up from 1991 through 2017. Data analysis was performed from August 2020 to June 2021. Exposure: Age at initiation of sigmoidoscopy or colonoscopy for screening (routine screening or because of family history) or symptoms. Main Outcomes and Measures: Incident CRC, confirmed by medical records, pathology reports, and the National Death Index. Cumulative incidence of CRC in each group was estimated with age as the time scale, and the absolute risk reduction associated with endoscopy initiation at different ages through 60 years was calculated. Cox proportional hazards regression was used to calculate hazard ratios (HRs) and 95% CIs, stratified by age and calendar year of questionnaire cycle and adjusted for CRC risk factors in the multivariable models. Results: Among 111801 women aged 26 to 46 years (median, 36 years) at enrollment, 519 incident CRC cases were documented over 26 years, encompassing 2509358 person-years of follow-up. In the multivariable analysis, compared with no endoscopy, undergoing endoscopy was associated with a significantly lower risk of incident CRC for age at initiation before 45 years (HR, 0.37; 95% CI, 0.26-0.53), 45 to 49 years (HR, 0.43; 95% CI, 0.29-0.62), 50 to 54 years (HR, 0.47; 95% CI, 0.35-0.62), and 55 years or older (HR, 0.46; 95% CI, 0.30-0.69). The absolute reduction in the estimated cumulative incidence of CRC through 60 years of age was 72 per 100000 persons for initiation of endoscopy at 45 to 49 years of age vs 50 to 54 years of age. Compared with no endoscopy, initiation of endoscopy before 50 years of age was also associated with a reduced risk of CRC diagnosed before 55 years of age (<45 years: HR, 0.45 [95% CI, 0.29-0.70]; 45-49 years: HR, 0.43 [95% CI, 0.24-0.76]). Conclusions and Relevance: In this cohort study, compared with no endoscopy, initiation of endoscopy before 50 years of age was associated with a reduced risk of CRC, including CRC diagnosed before 55 years of age. Screening before 50 years of age was associated with greater absolute reduction in CRC risk compared with initiation of CRC screening at 50 years of age or later.