Risk factors for breast cancer in elderly women.

As the population ages, an increasing fraction of women diagnosed with breast cancer will be elderly. Heterogeneity of breast cancer risk factors between pre- and postmenopausal women is recognized, but few studies have examined elderly women specifically. The authors describe the age-specific influence of risk factors for postmenopausal breast cancer, with emphasis on women aged 75 or more years. Among 36,658 members of the Iowa Women's Health Study (1986-2001), a population-based cohort study, 428 incident breast cancers were diagnosed in women aged 55-64 years, 1,297 in women aged 65-74 years, and 561 in women aged 75-84 years. Women with a body mass index (weight (kg)/height (m)(2)) in the highest versus the lowest quartile were at increased risk of breast cancer at age 75 or more years (adjusted hazard ratio = 1.44, 95% confidence interval (CI): 1.12, 1.84). Family history of breast cancer (hazard ratio = 1.54, 95% CI: 1.24, 1.93 for a first-degree family history vs. none) and an older age at menopause (p(trend) = 0.07) conferred increased risk for women aged 75 or more years, and a high number of livebirths was protective (hazard ratio = 0.67, 95% CI: 0.51, 0.88 for five or more births compared with one or two). Obesity, a modifiable risk factor, remained positively associated with breast cancer for all age groups of postmenopausal women.