Population attributable risks of modifiable reproductive factors for breast and ovarian cancers in Korea

Background: Breast and ovarian cancers are predominant female cancers with increasing prevalence. The purpose of this study was to estimate the population attributable risks (PARs) of breast and ovarian cancer occurrence based on the relative risks (RRs) of modifiable reproductive factors and population-specific exposure prevalence. Methods: The PAR was calculated by using the 1990 standardized prevalence rates, the 2010 national cancer incidence with a 20 year lag period, the meta-analyzed RRs from studies conducted in the Korean population for breast cancer, and the meta-analyzed RRs from a Korean epithelial ovarian cancer study and a prior meta-analysis, and ovarian cancer cohort results up to 2012. For oral contraceptive and hormone replacement therapy use, we did not consider lag period. Results: The summary PARs for modifiable reproductive factors were 16.7 % (95 % CI 15.8-17.6) for breast cancer (2404 cases) and 81.9 % (95 % CI 55.0-100.0) for ovarian cancer (1579 cases). The modifiable reproductive factors included pregnancy/age at first birth (8.0 %), total period of breastfeeding (3.1 %), oral contraceptive use (5.3 %), and hormone replacement therapy use (0.3 %) for breast cancer and included breastfeeding experience (2.9 %), pregnancy (1.2 %), tubal ligation (24.5 %), and oral contraceptive use (53.3 %) for ovarian cancer. Conclusions: Despite inherent uncertainties in the risk factors for breast and ovarian cancers, we suggest that appropriate long-term control of modifiable reproductive factors could reduce breast and ovarian cancer incidences and their related burdens by 16.7 % and 81.9 %, respectively.