Fraction of cancers attributable to modifiable reproductive factors and hormone use in 2020 in Korea

Abstract Background Cancer is a significant health concern for women globally, with reproductive factors playing a key role. The combined estrogen-progesterone contraceptives (COC) and hormone replacement therapy (HRT) have been identified as carcinogens by the International Agency for Research on Cancer (IARC). No breastfeeding has been linked to cancer risk among women with experience of pregnancy, according to the World Cancer Research Fund (WCRF). In 2009, National Cancer Center Korea estimated the population attributable fraction (PAF) of cancer incidence attributable to these factors. As of 2024, updated data is necessary to assess PAF. Methods Cancer sites and modifiable reproductive factors for PAF estimation were selected according to WCRF and IARC. Data on exposure prevalence was obtained from the Korea National Health and Nutrition Examination Survey (KNHANES) in 2005, assuming 15 years of latency. Relative risk (RR) and 95% confidence intervals (CIs) were estimated using KNHANES, Korean Genome and Epidemiology Study (KoGES), and National Health Examinees data from the National Health Insurance Services (NHIS) linked with Korea Central Cancer Registry. Cox proportional hazards model was used to estimate RRs, adjusted by age groups, alcohol use, tobacco use, and body mass index group. Pooled RRs with 95% confidence intervals (CIs) were estimated by a meta-analysis with random-effects models. PAF was calculated by Levin's formula. Results The total PAF of cancer incidence due to modifiable reproductive factors and hormone use was 0.97% in 2020. No breastfeeding attributed to 3.57% of breast cancer and 4.08% of ovarian cancer. COC was responsible for 0.77% of breast cancer but also prevented 1.34 % of endometrial and 2.27 % of ovarian cancer. HRT accounted for 0.61 % and 0.9% of breast and endometrium cancer, respectively. Conclusions This study suggests that hormone use and modifiable reproductive factors are associated with cancer incidence up to 0.97% in Korea.