Hormonal and body-size factors in relation to breast cancer risk: a prospective study of 11,889 women in a low-incidence area.

Purpose: This prospective cohort study of 11,889 women was conducted to determine significant factors associated with the risk of breast cancer among Chinese women in Taiwan, a low-incidence area.
Methods: In-person interviews were completed for subjects to solicit information on hormonal factors. Measurements of height, weight, and waist-and- hip circumferences were performed by well-trained assistants using standardized techniques. Cox proportional hazards models were employed to estimate relative risks (RRs) and 95% confidence intervals (CIs).
Results: During an average follow-up time of 10.3 years (range: 1 to 11 years) with an accumulation of 134,063 person-years, 104 incident breast cancer cases were identified through data linkage with national cancer registry profile. There was a significant elevation in breast-cancer risk with increasing duration of the interval between age at menarche and age at first full-term pregnancy (FFTP). Additionally, central adiposity reflected by hip circumference was a significant predictor of breast cancer in this Chinese female population.
Conclusions: The findings of this study indicated common mechanisms responsible for the higher incidence of breast cancer in Western populations may also explain the risk of breast cancer development in Taiwan, a low-incidence area.