Abstract 1193: The impact of rapid changes in reproductive factors on the increased incidence of breast cancer in Korea

Introduction The age-standardized incidence rates of Korean breast cancer have significantly increased from 26.1 in 1999 to 63.9 in 2014, which coincides with changes in lifestyle, especially reproductive factors. We examined the contribution of temporal changes in reproductive factors to the increased incidence of breast cancer in Korea. Methods We estimated the means of age at menarche, number of pregnancy, age at first pregnancy and duration of breastfeeding by birth cohort using the nationwide Korean National Health and Nutrition Examination Survey 2007-2015 (n=20,522 women aged between 20 and 69 years with no history of cancer). Cohorts were divided into a ten-year interval of birth year from 1940s to 1970s. Analyses took into account the complex sample design by incorporating weights, stratification and clustering. The percent increase in breast cancer incidence due to a specific factor was calculated as 100 x (RR d - 1) (RR, relative risk for a specific factor; and d, difference in the mean of a risk factor among birth cohorts). Results Weighted proportion of birth cohorts was 12.1% in 1940s (mean age=64.9y), 24.1% in 1950s (56.1y), 31.9% in 1960s (46.8y), and 31.9% in 1970s (36.9y). We observed notable changes in reproductive factors over time: younger menarche, lower number of pregnancies, older age at first pregnancy and shorter duration of breastfeeding. A decrease in the number of pregnancies from 5.0 in 1940s to 2.7 in 1970s cohorts contributed to an 18.3% increase in breast cancer incidence. Shorter duration of lactation, earlier onset of menarche and older age at first pregnancy also contributed to 16.5%, 10.7%, and 7.1% increase in the incidence rates, respectively. Conclusion We found that significant changes in reproductive factors in Korean women had appreciable impacts on the increase in breast cancer in Korea. Further study is warranted to evaluate the burden of breast cancer due to other modifiable risk factors such as adiposity, alcohol drinking and physical activity. Citation Format: Seho Park, Hyunsoon Cho, Jee Ye Kim, Hyung Seok Park, Seung Il Kim, Young Up Cho, Byeong-Woo Park, Graham A. Colditz, Yikyung Park. The impact of rapid changes in reproductive factors on the increased incidence of breast cancer in Korea [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1193.