420 BREAST SIZE AND PREMENOPAUSAL BREAST CANCER INCIDENCE: A PROSPECTIVE ANALYSIS OF THE NURSES' HEALTH STUDY II

Objectives We conducted a prospective analysis of the relation between breast size measured by self-reported bra cup size and breast cancer risk among a cohort of premenopausal women. Design Prospective cohort study Setting The Nurses9 Health Study II is an ongoing, prospective cohort of 116,671 American female registered nurses. The study was initiated in 1989 and enrolled women between 25 - 42 years of age living within 14 states in the U.S. Participants Bra cup size and breast cancer risk was assessed among 88,787 premenopausal women aged 29 to 47 in 1993. Bra cup size at age 20 was assessed by self-report in 1993. Women were excluded if at baseline they were postmenopausal, reported previous cancer, or did not report bra cup size. Censoring occurred if a women experienced breast cancer, reached menopause or died. Main Outcome Measures New cases of invasive breast cancer were self-reported and confirmed by review of pathology reports. Hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated with a Cox proportional hazards model adjusting for potential risk factors for breast cancer. All statistical tests were two-sided. Results During 621,097 total years of follow-up, 893 women developed incident invasive breast cancer. For women with a body mass index (BMI) below 25, those with a bra cup size of “B” (covariate adjusted HR=1.22, 95% CI 1.00-1.49) and “D or larger” (covariate adjusted HR=1.77, 95% CI 1.13-2.77) had an increased risk of breast cancer relative to “A or smaller” (Ptrend = 0.01). There was no important association among women with a BMI of 25 or higher. Stratifying by BMI at age 18 using a cut point of 21 gave similar results. Conclusion Larger bra cup size at a younger age is associated with an increased risk of premenopausal breast cancer, though this association is limited to leaner women.