SEX STEROID-HORMONES, BONE-MINERAL DENSITY, AND RISK OF BREAST-CANCER

Increased bone mineral density (BMD), as a marker of higher integrated estrogen exposure over time, could be an important risk factor for po stmenopausal breast cancer. In the Study of Osteoporotic Fractures 806 5 non-black women age 65 years and older were followed for an average of 3.2 years. There were 121 incident breast cancer cases. The age adj usted incidence rate/1000 person years of breast cancer was substantia lly higher among women with high BMD at several measured bone sites. T here was approximately a 2-fold higher risk of breast cancer for women in the upper as compared to the lower 25th percentile of BMD. Conside rable controversy exists about the association of hormone replacement therapy (HRT) and increased risk of breast cancer. In this paper we mo deled the effects of selection for HRT, presuming that women with lowe r BMD would be more likely to be on HRT, then estimated the observed v ersus potential risk of breast cancer among HRT users as compared to n onusers. The model suggests that the potential risk of breast cancer a ssociated with HRT could be greatly underestimated and that postmenopa usal women with high BMD who are placed on HRT could have a substantia lly increased risk of breast cancer. This model of increased risk of b reast cancer associated with BMD and HRT needs to be evaluated within clinical trials and larger observational studies that include measures of BMD.