Relationship between bone mass, invasive breast cancer incidence and raloxifene therapy in postmenopausal women with low bone mass or osteoporosis

Objective: To evaluate the relationship between bone mass and risk of breast cancer and to determine the effect of raloxifene therapy on breast cancer incidence in women categorized by bone mass into low bone mass and osteoporosis subgroups. Design: In this post hoc analysis, data were analyzed from the Multiple Outcomes of Raloxifene Evaluation (MORE) trial, enrolling postmenopausal women with low bone mass (N = 7705), and the Continuing Outcomes Relevant to Evista (CORE) trial, a follow-up to MORE enrolling 4011 MORE participants. Total follow-up was for up to 8 years. Women with a total hip bone mineral density (BMD) T-score < -1 to > -2.5 or T-score <= -2.5 (referent, NHANES III database) were classified as having low bone mass or osteoporosis, respectively. Women with a pre-existing vertebral fracture were considered as having osteoporosis irrespective of BMD T-score. Analyses were performed for invasive breast cancers and invasive estrogen-receptor (ER) positive breast cancers. Results: Women with low bone mass (N = 3829) had a twofold higher incidence of invasive ER-positive breast cancer than those with osteoporosis (N = 3836) (HR 2.13, 95% CI 1.12-4.03). The incidence of all invasive breast cancers did not differ significantly between the bone mass groups. The incidences of invasive and invasive ER-positive breast cancers were 65-78% lower in women assigned raloxifene versus placebo in both the low bone mass and osteoporosis groups (p < 0.05). Conclusions: In this post hoc analysis of postmenopausal women participating in MORE and CORE, bone mass was a predictor of invasive ER-positive breast cancer. Raloxifene treatment reduced the risk of invasive and invasive ER-positive breast cancers in women with low bone mass and those with osteoporosis. Since participants were older postmenopausal women with low bone mass, whether these findings can be generalized to other postmenopausal women is unclear.